Skeleton

From Britannica 11th Edition (1911)

Skeleton. In most animals, and indeed in plants, the shape could not be maintained without a thickening and hardening of certain parts to form a support for the whole. These hardened parts are called the skeleton (Gr. vrcO¦Aco, I dry), because they dry up and remain after the rest of the body has disappeared. In animals the skeleton is usually, and in higher animals always, rendered more rigid and permanent by the deposit in it of lime salts, thus leading to the formation of bone. Sometimes, as in most of the lower or invertebrate animals, the skeleton is on the surface and thus acts as a protection as well as a framework. This is known as an exoskeleton. In the higher or vertebrate animals there is an internal or endoskeleton and the exoskeleton is either greatly modified or disappears.

The following descriptive account is divided into (I) axial, or skeleton of the trunk, (2) appendicular or skeleton of the limbs, (3) skull, (4) visceral skeleton, or those parts which originally form the gill supports of water breathing vertebrates, (5) the exoskeleton, which is considered under the heading Skin And Exoskeleton. These divisions, although they seem logical, cannot in practice be strictly adhered to, especially in the case of the visceral skeleton, because doing so would involve, among other things, separating the description of the upper jaw from that of the rest of the skull. For the microscopical structure of bone see Connective Tissues.

xxv. 6 a Axial. The axial skeleton, from a strictly scientific point of view, should comprise a good deal of the skull as well as the spinal column, ribs and breast bone, but, as the skull (q.v.) is dealt with in a separate article, the three latter structures alone are dealt with here.

The Spine, Spinal or Vertebral Column, chine or backbone in man consists of a number of superimposed bones which are named vertebrae, because they can move or turn somewhat on Spine. each other. It lies in the middle of the back of the neck and trunk; has the cranium at its summit; the ribs at its sides, which in their turn support the upper limbs; whilst the pelvis, with the lower limbs, is jointed to its lower end. The spine consists in an adult of twenty-six bones, in a young child of thirty-three, certain of the bones in the spine of the child becoming ankylosed or blended with each other in the adult. These blended bones lose their mobility and are called false vertebrae; whilst those which retain their mobility are the true vertebrae. The bones of the spine are arranged in groups, which are named from their position - vertebrae of the neck or cervical; of the chest, thoracic, formerly called dorsal; of the loins, lumbar; of the pelvis, sacral; and of the tail, coccygeal or caudal; and the number of vertebrae in each group may be expressed in a formula. In man the formula is as follows: - C7Th12L5S5Coca =33 bones, as seen in the child; but the five sacral vertebrae fuse together into a single bone - the sacrum - and the four coccygeal into the single coccyx. Hence the sacrum and coccyx of the adult are the false, whilst the lumbar, dorsal and cervical are the true vertebrae.

The vertebrae are irregularly-shaped bones, but as a rule have certain characters in common. Each possesses a body and an arch, which enclose a ring, with certain processes and notches. The body, or centrum, is a short cylinder, which by its upper and lower surfaces is connected by means of fibrocartilage with the bodies of the vertebrae immediately above and below. The collective series of vertebral bodies forms the great column of the spine. The arch, also called neural arch, because it encloses the spinal marrow or nervous axis, springs from the back of the centrum, and consists of two symmetrical halves united behind in the middle line. Each half has an anterior part or pedicle, and a posterior part or lamina. The rings collectively form the spinal canal. The processes usually spring from the arch. The spinous process projects backward from the junction of the two laminae, and the collective series of these processes gives to the entire column the spiny character from which has arisen the term spine, applied to it. The transverse processes project out ward, one from each side of the arch. _ The articular processes project, two upward and two downward, and are for connecting adjacent vertebrae together. The notches, situated on the upper and lower borders of the pedicles, form in the articulated spine the intervertebral foramina through which the nerves pass out of the spinal canal.

The vertebrae in each group have characters which specially distinguish them. In man and all mammals, with few exceptions, whatever be the length of the neck, the cervical vertebrae Cervical are seven in number. In man the body of a cervical vertebra is comparatively small, and its upper surface is transversely concave; the arch has long and obliquely sloping laminae; the ring is large and triangular; the spine is short, bifid, and horizontal; the transverse process consists of two bars of bone, the anterior springing from the side of the body, the posterior from the arch, and uniting externally to enclose a foramen (vertebrarterial) through which, as a rule, the vertebral artery passes; the articular processes are flat and oblique, and the upper pair of notches are deeper than the lower. The first, second and seventh cervical vertebrae havo characters which specially distinguish them. The first, or atlas, has no body or spine: its ring is very large, and on each side of the ring is a thick mass of bone, the by V i V: FIG. I. - The Axial Skeleton.

C 7 The cervical vertebrae. D12 The thoracic.

L 5 The lumbar.

S5 The sacral.

Coc 4 The coccygeal.

CC The series of twelve ribs on one side.

Ps The presternum.

Ms The meso-sternum. Xs The xiphisternum. The dotted line VV represents the vertical axis of the spine.

[AXIAL

which it articulates with the occipital bone above and the second vertebra below. The second vertebra, axis, or Vertebra dentata, has its body surmounted by a thick, tooth-like odontoid process, which is regarded as the body of the atlas displaced from its proper vertebra and fused with the axis. This process forms a pivot round which the atlas and head move in turning the head from one side to the other; the spine is large, thick and deeply bifid. The seventh, called Vertebra prominens, is distinguished by its long prominent spine, which is not bifid, and by the small size of the foramen at the root of the transverse process. In the human spine the distinguishing character of all the cervical vertebrae is the foramen at the root of the transverse process.

The thoracic vertebrae, formerly called dorsal, are twelve i. ? number ic in the human spine. They are intermediate in size and position to the cervical and lumbar vertebrae, and are all distinguished by having one or two smooth surfaces on each side of the body for articulation with the head of one or two ribs. The arch is short and with imbricated laminae; the ring is nearly circular; the spine is oblique, elongated and bayonetshaped; the transverse processes are directed back and out, not bifid, and with an articular surface in front for the tubercle of a rib; and the articular processes are flat and nearly vertical. The first, twelfth, eleventh, tenth and sometimes the ninth, thoracic vertebrae are distinguished from the rest. The first is in shape like the seventh cervical, but has no foramen at the root of the transverse process, and has two articular facets on each side of the body; the ninth has sometimes only one facet at the side of the body; the tenth, eleventh, and twelfth have invariably only a single facet on the side of the body, but the eleventh and twelfth have stunted transverse processes, and the twelfth has its lower articular processes shaped like those of a lumbar vertebra.

The lumbar vertebrae in man are five in number. They are the lowest of the true vertebrae, and also the largest, especially in the centrum. The arch has short and deep laminae; the ring is triangular; the spine is massive and hatchet-shaped; the transverse processes are long and pointed; the articular are thick and strong, the superior pair concave, the inferior convex, and the inferior notches, as in the thoracic vertebrae, are deeper than the superior. In the lumbar vertebrae and in the lower thoracic an accessory process projects from the base of each transverse process, and a mammillary tubercle from each superior articular process. The fifth lumbar vertebra has its body much deeper in front than behind and its spine is less massive.

The sacrum is composed of five originally separate vertebrae fused into a single bone. It forms Sacrum, the upper and back wall of the pelvis, is triangular in form, and possesses two surfaces, two borders, a base, and an apex. The anterior or pelvic surface is concave, and is marked by four transverse lines, which indicate its original subdivision into five bones, and by four pairs of foramina, through which are transmitted the anterior sacral nerves. Its posterior surface is convex; in the middle line are four spines„ because in the last sacral vertebra the spinal canal is not closed behind. On each side of these are two rows of tubercles, the inner of which are the conjoined articular and mammillary processes, the outer the transverse processes of the originally distinct vertebrae. t Between these rows four pairs of foramina are found transmitting' the posterior sacral nerves from the sacral canal, which extends through the bone from base to near the apex, and forms the lower end of the spinal canal. By its borders the sacrum is articulated with the haunch-bones - by its base with the last lumbar vertebra, by its apex with the coccyx. The human sacrum is broader in proportion to its length than in other mammals; this great breadth gives solidity to the lower part of the spine, and, conjoined with the size of the lateral articular surfaces, it permits a more perfect junction with the haunch-bones, and is correlated with the erect position. Owing to the need in woman for a wide pelvis, the sacrum is broader than in man. (For details see A. M. Paterson, " The Human Sacrum," Sci. Trans. R. Dublin Soc. vol. v. ser. 2.) The coccyx consists of four or five vertebrae in the human spine though the last one is sometimes suppressed. It is the rudimentary tail, but instead of projecting back, as in mammals Coccyx. generally, is curved forward, and is not visible externally, an arrangement which is also found in the anthropoid apes and in Hoffmann's sloth. Not only is the tail itself rudimentary in man, but the vertebrae of which it is composed are small, and represent merely the bodies and transverse processes of the true vertebrae. As there are no arches, the ring is not formed, and the spinal canal does not extend, therefore, beyond the fourth piece of the sacrum. The first coccygeal vertebra, in addition to a body, possesses two processes or horns, which are the superior articular processes.

The human spine is more uniform in length in persons of the same race than might be supposed from the individual differences in stature, the variation in the height of the body in adults being due chiefly to differences in the length of the lower limbs. The average length of the spine is 28 in.; its widest part is at the base of the sacrum, from which it tapers down to the tip of the coccyx. It diminishes also in breadth from the base of the sacrum upwards to the region of the neck. Owing to the projection of the spines behind and the transverse processes on each side, it presents an irregular outline on those aspects; but in front it is more uniformly rounded, owing to the convex form of the antero-lateral surfaces of the bodies of its respective vertebrae. In its general contour two series of curves may be seen, an antero-posterior and a lateral. The antero-posterior is the more important. In the infant at the time of birth the sacrococcygeal part of the spine is concave forward, but the rest of the spine, except a slight forward concavity in the series of thoracic vertebrae, is almost straight. When the infant begins to sit up in the arms of its nurse, a convexity forward in the region of the neck appears, and subsequently, as the child learns to walk, a convexity forward in the region of the loins. Hence in the adult spine a series of convexo-concave curves are found, which are alternate and mutually dependent, and are associated with the erect attitude of man. A lateral curve, convex to the right, opposite the third, fourth, and fifth thoracic vertebrae, with compensatory curve convex to the left immediately above and below, is due apparently to the much greater use of the muscles of the right arm over those of the left, drawing the spine in that region somewhat to the right. In disease of the spine its natural curvatures are much increased, and the deformity known as humpback is produced. As the spine forms the central part of the axial skeleton, it acts as a column to support not only the weight of the body, but of all that can be carried on the head, back and in the upper limbs: by its transverse and spinous processes it serves also to give attachment to numerous muscles, and the transverse processes of its thoracic vertebrae are also for articulation with the ribs.

The Thorax, Pectus, or Chest is a cavity or enclosure the walls of which are in part formed of bone and cartilage. Its skeleton consists of the sternum in front, the twelve thoracic vertebrae behind, and the twelve ribs, with their corre sponding cartilages, on each side.

The sternum or breast bone is an elongated bone which inclines downward and forward in the front wall of the chest. It consists of three parts - an upper, called manubrium or presternum; a middle, the gladiolus or mesosternum; and a lower, the ensiform process or xiphisternum. Its anterior and posterior surfaces are marked by transverse lines, which indicate not only the subdivision of the entire bone into three parts, but that of the mesosternum into four originally distinct segments. Each lateral border of the bone is marked by seven depressed surfaces for articulation with the seven upper ribs: at each side of the upper border of the presternum is a sinuous depression, where the clavicle, a bone of the upper limb, articulates with this bone of the axial skeleton. The xiphisternum remains cartilaginous up to a late period of life, and from its pointed form has been named the ensiform cartilage.

The ribs or costae, twenty-four in number, twelve on each side of the thorax, consist not only of the bony ribs, but of a bar of cartilage continuous with the anterior end of each bone, called a costal cartilage, so that they furnish examples of a cartilaginous skeleton in the adult human body; in aged persons these cartilages usually become converted into bone. The upper seven ribs are connected by their costal cartilages to the side of the sternum, and are called sternal or true ribs; the lower five do not reach the sternum, and are named a-sternal or false, and of these the two lowest, from being comparatively unattached in front, are called free or floating. Another and perhaps more useful classification is to speak of the first seven ribs as vertebro-sternal, the next three as vertebro-costal, and the last two as vertebral. All the ribs are From Arthur Thomson, Cunningham's Text-Book of Anatomy. FIG. 2. - Vertebral Column as seen from behind.

articulated behind to the thoracic vertebrae, and as they are symmetrical on the two sides of the body, the ribs in any given animal are always twice as numerous as the thoracic vertebrae in that animal. They form a series of osseocartilaginous arches, which From Arthur Thomson, Cunningham's FIG. 3. - The Thorax as seen from the Front.

extend more or less perfectly around the sides of the chest. A rib is an elongated bone, and as a rule possesses a head, a neck, a tubercle and a shaft. The head usually has two articular surfaces, and is connected to the side of the body of two adjacent thoracic vertebrae; the neck is a constricted part of the bone, uniting the head to the shaft; the tubercle, close to the junction of the shaft and neck, is the part which articulates with the transverse process of the vertebra. The shaft is compressed, possesses an inner and outer surface, and an upper and lower border, but from the shaft being somewhat twisted on itself, the direction of the surfaces and borders is not uniform throughout the length of the bone. The ribs slope from their attachments to the spine, at first outward, downward and backward, then downward and forward, and where the curve changes from the backward to the SoP.. forward direction an angle is formed on the rib. The angle and the tubercle are at the same place in the first rib and in each succeeding rib the angle is a little farther from the tubercle SpP than in the last.

The first, tenth, eleventh and twelfth ribs articulate each with only one vertebra so that there is only one surface on the head. The surface of the first rib which is not in contact with the lung is directed upward, forward and outward while that of the second rib is much more outward; the eleventh and twelfth ribs are rudimentary, have neither neck nor tubercle, and are pointed anteriorly. The ribs are by no means uniform in length: they increase from the first to the seventh or eighth, and then diminish to the twelfth; the first and twelfth are therefore the shortest ribs. The first and second costal cartilages are almost horizontal, but the others are directed upward and inward.

In its general form the chest may be likened to a barrel which is wider below than above. It is rounded at the sides and flattened in front and behind, so that a man can lie either on his back or his belly. Its upper opening slopes downward and forward, is small in size, and allows the passage of the windpipe, gullet, large veins and nerves into the chest, and of several large arteries out of the chest into the neck. The base or lower boundary of the cavity is much larger than the upper, slopes downward and backward, and is occupied by the diaphragm, a muscle which separates the chest from the cavity of the abdomen. The transverse diameter is greater than the anteroposterior, and the antero-posterior is greater laterally, where the lungs are lodged, than in the mesial plane, which is occupied by the heart.

Embryology

The first appearance of any stiffening of the embryo is the formation of the notochord, which in the higher vertebrates is a temporary structure and is not converted into cartilage or bone. It also differs from the bony skeleton in that it is derived from the entoderm or inner of the three layers of the embryo while the bony skeleton is formed from the mesoderm or middle layer and, just as the entoderm is an older layer of the embryo than the mesoderm, so the notochord or entodermal skeleton precedes, both in embryology and in phylogeny or comparative anatomy, the bony mesodermal skeleton.

In the accompanying figure (fig. 4) the notochord is seen in section fully formed and lying between the entoderm and the neural canal. Its first formation is at an earlier period than this, before the neural groove has closed into a canal, and it appears at first as an upward groove from the most dorsal part of the entoderm in what will later on be the cervical region of the embryo. The groove, by the union of its edges, becomes a tube, sometimes spoken of as the chordal tube, but the cavity of this is soon obliterated by the growth of its cells, so that a solid elastic rod is formed which grows forward as far as the pituitary region of the skull and backward to where the end of the coccyx will be.

While the development of the notochord is going on the mesoderm on each side of it is dividing itself into a series of masses called mesodermic somites (see fig. 4, PS) or protovertebrae. This process begins in the cervical region and proceeds forward and backward until thirty-eight pairs have been formed for the neck and trunk and probably four extra ones for the occipital region of the skull. Each of these somites consists of three parts: that nearest the surface ectoderm is the cutaneous lamella (fig. 4, CL). Deep to this and separated in the earlier-formed somites by a space is the muscle layer (fig. 4, ML) while deepest of all and nearest the nerve cord and notochord is the scleratogenous layer (fig. 4, SL). It is this layer which gradually meets its fellow of the opposite side and encloses the nerve cord and the notochord in continuous tubes of mesodermal tissue, thus forming the membranous vertebral column, which is perforated for'the exit of the spinal nerves, but the intervals between the successive mesodermic somites are still marked by the tissue being rather denser there. The next stage is that of chondrification or the conversion into cartilage of each segment of the membranous vertebral column surrounding the notochord. In this way the bodies of the cartilaginous vertebrae are formed and each of these is segmental, that is, it corresponds to a muscle segment and a spinal nerve. The cartilaginous neural arch, however, which surrounds the nerve cord is intersegmental and is formed in the denser fibrous tissue which separates each somite from the next. This also applies to the cartilaginous ribs which appear in the fibrous intervals (myo SB NC CC GC SG SG Spinal ganglion.

SL Scleratogenous layer of protovertebral somite.

SoM Somatic mesoderm. SoP Somatopleure.

SpM Splanchnic mesoderm. SpP Splanchnopleure.

commata) between the muscle plates (myotomes), and so it is easy to realize that each typical rib must articulate with the bodies of two adjacent vertebrae, but with the neural arch, through its transverse process, of only one.

The intersegmental tissue between the bodies of the vertebrae becomes the intervertebral discs and in the centre of these a pulpy From Alfred H. Young and Arthur Robinson, Cunningham's FIG. 4. - Transverse Section of a Ferret Embryo, showing further differentiation of the mesoderm.

CC Central canal.

CL Cutaneous lamella of protovertebral somite.

CO Coelom.

EC Ectoderm.

EN Entoderm.

GC Germinal cell.

ML Muscular layer of mesodermic somite. N Notochord.

NC Neural crest. PA Primitive aorta.

PS Mesodermic somite.

SB Spongioblast.

SC Spinal cord.

[AXIAL

mass is found which contains some remnants of the notochord. Elsewhere this structure is pressed out of existence and there is no further use for it when the cartilaginous vertebrae are once formed. One other series of structures must be mentioned though they do not 8 14 27 FIG. 5. - Ossification of Vertebrae.

Cervical Vertebra. I Centre for body.

2 Superior epiphysial plate.

3 Anterior bar of transverse process developed by lateral extension from pedicle.

4 Neuro-central synchondrosis.

5 Inferior epiphysial plate.

Lumbar Vertebra. 6 Body.

7 Superior epiphysial plate.

8 Epiphysis for mammillary process.

9 Epiphysis for transverse process. Io Epiphysis for spine.

I Neuro-central synchondrosis.

12 Inferior epiphysial plate.

Dorsal Vertebra. '13' Centre for body.

14 Superior epiphysial plate, appears about puberty; unites at 25th year.

15 Neuro-central synchondrosis does not ossify till 5th or 6th year.

16 Appears at puberty; unites at 25th year.

17 Appears at puberty; unites at 25th year.

18 Appears about 6th week.

19 Centre for transverse process and neural arch; appears about 8th 31 week.

20 Synchondroses close about 3rd year.

play any great part in human development. In the intersegmental tissue ventral to each of the intervertebral disks a transverse rod of cells, known as a hypochordal bar, is formed which connects the heads of two opposite ribs. In man the greater number of these either disappear or form the middle fasciculus of the stellate ligament which joins the head of the rib to the intervertebral disk, but in the case of the atlas the rod 'chondrifies to form the anterior (ventral) arch which is therefore intersegmental, while the segmental body of the atlas, through which the notochord is passing, joins the axis to form the odontoid process. These hypochordal bars are interesting as the last remnant in man of the haemal arch of the vertebrae of fishes (see subsection on comparative anatomy). In the cervical region the ribs are very short and form the ventral boundary of the foramen for the vertebral artery. They are so short that little movement occurs between them and the rest of the vertebra, hence no joints are formed and the rib element becomes fused with the centrum and transverse process, leaving the vertebrarterial canal between. Sometimes in the seventh cervical vertebra the rib element is much longer and then of course more movement occurs, and instead of fusing with the rest of the vertebra it remains as a separate cervical rib with definite joints.

The sternum is developed according to G. Ruge by a fusion of the ventral ends of the ribs on each side thus forming two parallel longitudinal bars which chondrify and eventually fuse together in the mid line. The anterior seven or sometimes eight ribs reach the sternum, but the ventral ends of the ninth and sometimes the eighth probably remain as the xiphisternum, indeed a fibrous band is sometimes seen joining the caudal end of that structure to the ninth rib. The fusion of the two parallel bars begins at their cephalic ends and sometimes is interrupted toward the caudal end, thus leading to 15 -6 cleft or perforate sternum. At the cephalic end of each sternal bar, close to the place where the clavicles articulate, is an imperfectly separated patch of cartilage which usually fuses completely with the presternum, though sometimes it remains distinct and may later acquire a separate centre of ossification and so form a separate episternal bone on each side. If the sternum is to be regarded as the fused ventral ends of the thoracic ribs, the episternal elements are probably the remnants of the ventral ends of the seventh cervical ribs. The question of the morphological meaning of the sternum and surrounding parts cannot be settled entirely by a study of their development even when combined with what we know of their comparative anatomy or phylogeny. Professor A. M. Paterson (The Human Sternum, London, 1904) takes a different view from the foregoing and regards the sternum as derived from the shoulder girdle. To this point of view we shall return in the section on comparative anatomy.

The last stage in the development of the axial skeleton is the ossification of the cartilage; bony centres appear first in each half of the neural arches of the vertebrae and a little later (tenth week) double centres are deposited in the centra though these are so close together and fuse so rapidly that their double nature is often only indicated by their oval or dumb-bell-like appearance. The bone in the two halves of the neural arch spreads and fuses in the mid dorsal line, and later on joins the ossified centrum ventral to the facet for the rib. This point of junction remains as a narrow strip of cartilage for a long time and is known as the neuro-central suture or synchondrosis. The head of the rib therefore articulates with the developmental neural arch instead of the centrum. About the age of puberty secondary centres or epiphyses appear at the tips of the transverse and spinous processes and as thin plates just above and below the body (see fig. 5-2 and 3). These are fully united by the twenty-fifth year. In the lower two cervical vertebrae there is often a separate centre for the part corresponding to the rib, while the lumbar have an extra epiphysis for the mammillary process. The atlas has one centre for each side of the dorsal part of the arch and one (probably two fused) for the ventral part, which has already been referred to as a hypochordal bar. In the axis, in addition to the ordinary centres, there is one for each side of the odontoid process and one for the tip (see fig. 520, 21, 22). The sacral vertebrae have the usual centres, except that the anterior part of the lateral mass (costal element) has a separate centre and that there are two extra centres on each side of the whole sacrum where it articulates with the ilium (see fig. 6).

The ribs ossify by one primary centre appearing about the sixth week and by secondary ones for the tubercle and head. The sternum is ossified by centres which do not appear opposite the attachment of the ribs but alternately with them, so that although the original From Arthur Thomson, Cunningham's FIG. 6. - Ossification of Sacrum - a,a, Centres for bodies; b,b, Epiphysial plates on bodies; c,c, Centres for costal elements; d,d, Centres for neural arches; e,e, Lateral epiphyses.

cartilaginous structure is probably intersegmental the bony segments are segmental like those of the vertebral centra. As seven ribs articulate with the sternum six centres of ossification between them might be looked for, but there is so little room between the points of attachment of the sixth and seventh ribs that centres do not occur 25 24 From Arthur Thomson, Cunningham's 21 Centre for summit of odontoid process; appears 3rd to 5th year, fuses 8th to 12th year.

Atlas. 26 Posterior arch and lateral masses developed from a single centre on either side, which appears about 7th week.

27 Anterior arch and portion of superior articular surface developed from single or double centre, appearing during 1st year.

Dorsal Vertebra. 28 Epiphysis for transverse process; appears about puberty, unites about 25th year.

30 Centre for neural arch on either side; appears about 6th or 7th week, the laminae unite from birth to 15th month.

Centre for body; appears about 6th week, unites with neural arch from 5th to 6th year.

here as a rule. Consequently five centres are found; those for the two higher segments being single while the lower ones are often double. Later on in life a centre for the xiphisternum appears.

From Arthur Thomson, Cunningham's Text-Book of Anatomy. FIG. 7. - Ossification of the Sternum. In this figure the second as well as the third segment of the body possesses two centres.

1 Appears about 5th or 6th month; III. segment unites month. with II. about puberty; IV.

3 Appear about 8th or 9th 4 Appears about 3rdyear or For further details see C. S. McMurrich, The Development of the Human Body (London, 1906). This includes bibliography, but G. Ruge's paper on the development of the sternum (Morph. Jahrb. vi. 1880) is of special importance.

Comparative Anatomy. - Just as in development the;notochord forms the earliest structure for stiffening the embryo, so in the animal kingdom it appears before the true backbone or vertebral column is evolved. This is so important that the older phylum of Vertebrata has now been expanded into that of Chordata to include all animals which either permanently or temporarily possess a notochord. In the subphylum Adelochorda, which includes the wormlike Balanoglossus, as well as the colonial forms Rhabdopleura and Cephalodiscus, an entodermal structure, apparently corresponding to the notochord of higher forms, is found in the dorsal wall of the pharynx. In the subphylum Urochorda or Tunicata, to which the ascidians or sea-squirts belong, the notochord is present in the tail region only and as a rule disappears after the metamorphosis from the larval to the adult form. In the Acrania, which are represented by Amphioxus (the lancelet). and are sometimes classed as the lowest division of the subphylum Vertebrata, the notochord is permanent and extends the whole length of the animal. Both this and the nerve cord dorsal to it are enclosed in tubes of mesodermal connective tissue which are continuous with the fibrous myocommata between the myotomes. Here then is a notochord and a membranous vertebral column resembling a stage in man's development. In the Cyclostomata (hags and lampreys) the notochord and its sheath persist through life, but in the adult lamprey (Petromyzon) cartilaginous neural arches are developed. In cartilaginous ganoid fishes like the sturgeon, the notochord is persistent and has a strong fibrous sheath into which the cartilage from the neural arches encroaches while in the elasmobranch fishes (sharks and rays) the cartilaginous centra are formed and grow into the notochord, thus causing its partial absorption. The growth is more marked peripherally than centrally, and so each centrum when removed is seen to be deeply concave toward both the head and tail; such a vertebra is spoken of as amphicoelous and with one exception is always found in fishes which have centra. In the body fish (Teleostei) and mudfish (Dipnoi) the vertebrae are ossified.

If a vertebra from the tail of a bony fish like the herring be examined, it will be seen to have a ventral (haemal) arch surrounding the caudal blood-vessels and corresponding to the dorsal or neural arch which is also present. In the anterior or visceral part of the body the haemal arch is split and its two sides spread out deep to the muscles and lying between them and the coelom to form the ribs. In the elasmobranchs on the other hand the ribs lie among the muscles as they do in higher vertebrates, and the fact that both kinds of ribs are coexistent in the same segments in the interesting and archaic Nilotic fish Polypterus bichir shows that they are developed independently of one another. The sternum is never found in fishes with the possible exception of the comb-toothed shark (Notidanus). Among the Amphibia the tailed forms (Urodela) have amphicoelous vertebrae in embryonic life and so have some of the adult salamanders, but usually the intercentral remnants of the notochord are pressed out of existence by the forward growth of the centrum behind it, so that in the adult each vertebra is only concave behind (opisthocoelous). In the Anura (frogs and toads), on the other hand, the centra are usually concave forward (procoelous) and some of the posterior ones become fused into a long delicate bone, the urostyle. The ribs of urodeles have forked vertebral ends and are thus attached to the centrum as well as to the neural arch of a vertebra; this forking is supposed to be homologous with the double ribs of Polypterus already referred to. The sternum as a constant structure first appears in amphibians and is more closely connected with the shoulder girdle than with the ribs, the ventral ends of which, except in the salamander Necturus, are rudimentary. It is not certain whether it is the homologue of the sternum of the fish Notidanus, but the subject is discussed by T. J. Parker and A. M. Paterson (The Human Sternum, London, 1904, p. 50), and still requires further research. If the sternum be regarded as a segmental structure or series of segmental structures corresponding to the centra of the vertebrae there is no reason why it should not develop independently of the intersegmental ribs and, when the ribs are suppressed, gain a secondary connexion with the shoulder girdle In Reptilia the centra of the vertebrae are usually procoelous, though there are a few examples, such as the archaic Tuatera lizard (Sphenodon), in which the amphicoelous arrangement persists. There are several cervical vertebrae instead of one, which is all the amphibians have. The odontoid bone is usually separate both from the atlas and axis while, between the atlas and the skull, there are rudiments of an extra intervertebral dorsal structure or pro-atlas in some forms such as the crocodile and Sphenodon lizard. Two sacral vertebrae (i.e. vertebrae articulating with the ilium) are generally present instead of the one of the Amphibia, but they are not fused together as in mammals. In the tail region haemal arches are often found enclosing the caudal artery and vein as they are also in urodele amphibians; in some species these are separate and are then spoken of as chevron bones. In the Crocodilia intervertebral disks first appear. Ribs are present in the cervical, thoracic and lumbar regions, and in the Chelonia (tortoises) the cervical ones blend with the vertebrae as they do in higher forms. In crocodiles a definite vertebrarterial canal is established in the cervical region which henceforward becomes permanent. The shafts of the ribs are sometimes all in one piece as in snakes or they may be developed by three separate centres as in Sphenodon with intervening joints. In these cases dorsal, intermediate and ventral elements to each shaft are present. In Crocodilia and Sphenodon there are spurs from each thoracic rib which overlap the next rib behind and are known as uncinate processes; they are developed in connexion with the origin of the external oblique muscle of the abdomen and are very constant in birds. The ventral elements of some of the hinder ribs are found in the Crocodilia lying loose in the myocommata of the rectus and obliquus internus (inscriptiones tendineae) and are known as abdominal ribs, while the sacral vertebrae articulate with the ilium through the intervention of short rods of bone, sometimes called pleurapophyses, which are no doubt sacral ribs. The sternum of reptiles is a broad plate of cartilage which may be calcified but is seldom converted into true bone; it always articulates with the coracoids (see section Appendicular) anteriorly and with a variable. number of ribs laterally and posteriorly. It should not be confounded with the dagger-shaped interclavicle which, like the clavicles, is a membrane bone and overlaps the sternum ventrally. It is also probable that the interclavicle is morphologically quite distinct from the episternum, of which vestiges are present in man and are referred to above in the section on embryology (see fig. 27). In birds the characteristics are largely reptilian with some specialized adaptations to their bipedal locomotion and power of flight. One effect of this is that the two true sacral vertebrae become secondarily fused with the adjacent lumbar, caudal and even thoracic, and theseagain fuse with the ilium so that the posterior part of a bird's trunk is very rigid. The neck, on the other hand, is very movable and the centra articulate by means of saddle-shaped joints which give the maximum of movement combined with strength (see JoINTs). The caudal vertebrae are fused into a flattened bone, the pygostyle, to support the tail feathers. In the fossil bird Archaeopteryx the centra are amphicoelous and the long tail has separate caudal vertebrae. The ribs are few and consist of dorsal (vertebral) and ventral (sternal) parts; the former almost always have uncinate processes. Free cervical ribs are often present and Archaeopteryx possessed abdominal ribs. The sternum is very large and in flying birds (Carinatae) has a median keel (carina) projecting from it, while the non-flying, ostrich-like birds (Ratitae) have no such structure.

In Mammalia the centra articulate by means of the intervertebral disks and it is only in this class that the epiphysial plates appear though these are absent in the Monotremata (duck-mole, &c.) and Sirenia (sea-cows). The cervical vertebrae are with a few exceptions (two-toed and three-toed sloths and the manatee or sea-cow) always seven in number, and some, usually all, of them have a vertebrarterial canal in the transverse process. In some of the Cetacea they are fused together. In the Ornithorhynchus the odontoid is a separate bone, as it is in many reptiles, but this part includes the facets by means of which the axis and atlas articulate. The thoracic vertebrae vary from ten in some of the whales and the peba armadillo to twenty-four in the two-toed sloth, though thirteen or fourteen is the commonest number. In the anterior part of the thoracic region the spines point backward, while in the posterior thoracic and lumbar regions they have a forward direction. There is always one spine in the posterior thoracic region, which is vertical, and the vertebra which bears this is known as the anticlinal vertebra. The ?

FIG. 9. - Side View of the First Lumbar Vertebra of a Dog (Canis familiaris). s Spinous process.

lumbar vertebrae vary from two in the Ornithorhynchus and some of the armadillos to twenty-one in the dolphin, the average number being probably six. Both the mammillary and accessory tubercles (metaand ana-pophyses) are in some forms greatly enlarged. It is usually held that the former are morphologically muscular processes while the latter represent the transverse processes of the thoracic vertebrae. In the American edentates additional articular processes (zygapophyses) are developed, so that these animals are sometimes divided from the old-world edentates and spoken of as Xenarthra.

Lying ventral to the intervertebral disks in many mammals small paired ossicles are occasionally found; these are called intercentra and are ossifications in the hypochordal bar (see subsection on embryology). They probably represent the places where the chevron bones or haemal arches would be attached and are the serial homologues of the anterior arch of the atlas (see fig. Io). Boulenger has pointed out that these intercentra, either as paired or median ossicles, are often found in lizards (P.Z.S., 1891, p. 114). The sacrum consists of true sacral vertebrae, which directly articulate with the sacrum, and false, which are caudal vertebrae fused with the others to form a single bone. There is also reason to believe that vertebrae which are originally lumbar become secondarily included in the sacrum because in the development of man the pelvis is at first attached to the thirtieth vertebra, but gradually shifts forward until it reaches the twenty-fifth, twenty-sixth and twenty-seventh; the twentyfifth or first sacral vertebra has, however, a frequent tendency to revert to the lumbar type and sometimes may do so on one side but not on the other. A. Paterson, on the other hand, brings forward evidence to prove that the human sacrum undergoes a backward rather than a forward shifting (Scientif. Trans. R. Dublin Society, vol. v., ser. II, p. 123). Taking the vertebrae which fuse together as an arbitrary definition of the sacrum, we find that the number may vary from one in Cercopithecus parts to thirteen in some of the armadillos, and, if the Cetacea are included, seventeen in the bottle-nosed dolphin, Tursiops. Four seems to be about the average of sacral vertebrae in the mammalian class and of these one or two are true sacral. In some of the Edentata the posterior sacral vertebrae are fused with the ischium, in other words the great sacro-sciatic ligament is ossified. The lateral centres of ossification which form the articular surface for the ilium probably represent rib elements. The caudal or tail vertebrae vary from none at all in the bat Megaderma to forty-nine in the pangolin (Manis macrura). The anterior ones are remarkable for usually having chevron bones (shaped like a V) on the ventral surface of the intercentral articulation. These protect the caudal vessels and give attachment to the ventral tail muscles. The ribs in mammals correspond in number to the thoracic vertebrae. In monotremes the three parts of the rib (dorsal, intermediate and ventral) already noticed in the reptiles are found, but usually the intermediate part is suppressed. The ventral part generally remains cartilaginous as it does in man though sometimes it ossifies as in the armadillos. In the typical pronograde mammals the shape of the ribs differs from that of the higher Primates and man: they are so curved that the dorso-ventral diameter of the thorax is greater than the transverse while in the higher Primates the thorax is broader from side to side than it is dorso-ventrally. In this respect the bats agree with man and the lemurs with the pronograde mammals.

In some whales the first rib articulates by an apparently double head with two verte brae; this is probably the result of a cervical rib joining it a little way from the vertebral column, and the result is homologous with those cases in man in which a cervical rib joins the first thoracic as it sometimes does. In the toothed whales, of which the porpoise is an example, the more posterior ribs lose their heads and necks and only articulate with the transverse processes. The sternum of mammals typically consists of from seven to nine narrow segments or sternebrae, the first of which (presternum) is often broader than those behind. As a rule the second rib articulates with the interval between the first and second pieces, but sometimes, as in the gibbon, it is the third rib which does so. When this is the case, as it sometimes is in man, the first two sternebrae have probably fused (see A. Keith, Journ. Anat. and Phys. xxx. 275). The segmental character of the separate sternebrae contrasts strongly with the intersegmental of the ribs. When the pectoralis major muscle is largely developed, as in the mole and bats, the sternum, especially the presternum, develops a keel as in birds. In the toothed whales there is usually a cleft or perforation throughout life between the two lateral halves of the sternum. In the whalebone whales the mesosternum is suppressed and consequently only the first ribs reach the ster num; this is of great interest when the oblique position of the diaphragm (see art. Diaphragm) in these animals is remembered, and makes one suspect that the development of the sternum in mammals is dependent on and subservient to the attachment of the diaphragm. The broadened thorax of the anthropomorpha is accompanied by a broadened sternum and the sternebrae of the mesosternum fuse together early, though in the orang they not only remain separate but each half of them reFIG. 12. - Sternum and strongly ossified mains separate until the Sternal Ribs of Great Armadillo (Priodon animal is half-grown. gigas). ps, Presternum; xs, xiphisternum. The episternum is re presented b y small ossicles which occasionally occur in man, while in the Ornithorhynchus and the tapir there is a separate bone in front (cephalad) of the presternum which in the former animal is distinct at first from the interclavicle, and this probably represents the episternum, though it was called by W. K. Parker by the noncommittal name of proosteon. For further details and literature see S. H. Reynolds, The Vertebrate Skeleton (Cambridge, 1897); W. H. Flower and H. Gadow, FIG. 8. - Anterior Surface of Sixth Cervical Vertebra of Dog.

After F. G. Parsons, " On Anatomy of Atherura Africana," Proc. Zool. Soc., 1894.

FIG. io. - The Intercentra of the Lower Part of the Vertebral Column. a, a, a, Intercentra.

FIG. I I. - Anterior Surface of Fourth Caudal Vertebra of Porpoise (Phocaena communis). s Spinous process.

Osteology of the Mammalia (London, 1885); R. Wiedersheim, Comparative Anatomy of Vertebrates, adapted and translated by W. N. Parker (London, 1907); R. Wiedersheim and G. Howes, The Structure of Man (London, 1897); C. Gegenbaur, Vergleich. Anat. der Wirbeltiere, Band i. (Leipzig, 1901).

Appendicular. The bony framework of the two appendages or extremities, as the upper and lower limbs are called, is built up on the same plan in both. Each consists of a limb girdle (shoulder and hip girdles) connecting it with the axial skeleton, a proximal single bone segment (humerus, femur), a distal double bone segment {radius, ulna; tibia, fibula), the hand and foot segments (carpus, metacarpus; tarsus, metatarsus) and the digits (phalanges). It should be understood that in the following descriptions the terms internal and external are used in relation to the mid-line of the body and not to that of the limb.

The upper limb in man may be subdivided into a proximal part or shoulder, a distal part or hand, and an intermediate shaft, which consists of an upper arm or brachium, and a forearm or ante-brachium. In each of these subdivisions certain bones are found: in the shoulder, the clavicle and scapula; in the upper arm, the humerus; in the forearm, the radius and ulna, the bone of the upper arm in man being longer than the bones v of the forearm; in the hand, the carpal and metacarpal bones and the phalanges. The scapula and clavicle together form an imperfect bony arch, the Scapular Arch or Shoulder Girdle; the shaft and hand form a free V A thoracic vertebra. Cl The clavicle. divergent A p C A rib. M The meniscus at p e n d a g e. The St The sternum. its sternal end. shoulder girdle is Sc The scapula. H The humerus. the direct medium Cr The coracoid. of connexion be tween the axial skeleton and the divergent part of the limb; its anterior segment, the clavicle, articulates with the upper end of the sternum, whilst its posterior segment, the scapula, approaches, but does not reach, the dorsal spines.

The clavicle, or collar bone (fig. 14), is an elongated bone which extends from the upper end of the sternum horizontally outward, to articulate with the acromion process of the scapula.

It presents a strong sigmoidal curve, which is associated with the transverse and horizontal direction of the axis of the human shoulder. It is slender in the female, but powerful in muscular males; its sternal end thick and somewhat triangular; its acromial end, flattened from above downward, has an oval articular surface for the acromion. Its shaft has four surfaces for the attachment of muscles; and strong ligaments connecting it with the coracoid, is attached to the under surface, near the outer end, whilst near the inner a strong ligament passes between it and the first rib.

The scapula, or shoulder blade (fig. 14), is the most important bone of the shoulder girdle, and is present in all mammals. It lies. at the upper and back part of the wall of the chest, reaching from the second to the seventh rib. Its form is plate-like and triangular, with three surfaces, three borders, and three angles. Its costal or ventral surface is in relation to the ribs, from which it is separated by certain muscles: one, called subscapularis, arises from the surface itself, which is often termed subscapular fossa. The dorsum or back of the scapula is traversed from behind forward by a prominent spine, which lies in the proper axis of the scapula, and subdivides this aspect of the bone into a surface above the spine, the supra-spinous fossa, and one below the spine, the infra-spinous fossa. The spine arches forward to end in a broad flattened process, the acromion, which has an oval articular surface for the clavicle; both spine and acromion are largely developed in the human scapula in correlation with the great size of the trapezius and deltoid muscles, which are concerned in the elevation and abduction of the upper limb. The borders of the scapula, directed upward, backward, and downward, give attachment to several muscles. The angles are inferior, antero-superior, and postero-superior. The antero-superior is the most important; it is truncated, and has a large, shallow, oval, smooth surface, the glenoid fossa, for articulation with the humerus, to form the shoulder joint. Overhanging the glenoid fossa is a curved beak-like process, the coracoid, which is of -importance as corresponding with the separate coracoid bone of monotremes, birds and reptiles. The line of demarcation between it and the scapula proper is marked on the upper border of the scapula by the supra-scapular notch The humerus, or bone of the upper arm (fig. 14), is a long bone, and consists of a shaft and two extremities. The upper extremity possesses a convex spheroidal smooth surface, the head, for articulation with the glenoid fossa of the scapula; i t is surrounded by a narrow constricted neck, and where the neck and shaft become continuous with each other, two processes or tuberosities are found, to which are attached the rotator muscles arising from the scapular fossae. Between the tuberosities is a groove in which the long tendon of the biceps rests. A line drawn through the head of the humerus perpendicular to the middle of its articular surface, forms with the axis of the shaft of the bone an angle of 40°. The shaft of the humerus is triangular in section above, but flattened and expanded below; about midway down the outer surface is a rough ridge for the insertion of the deltoid muscle, and on the inner surface another rough mark for the insertion of the coraco brachialis. A shallow groove winds round the back of the bone, in which the musculo-spiral nerve is lodged. The lower extremity of the humerus consists of an articular and a non-articular portion. The articular has a small head or capitellum externally for the radius, and a pulley or trochlea internally for the movements of the ulna in flexion and extension of the limb. The non-articular part has a projection both on its inner and outer aspect; these are known as the internal and external condyles, and of these the internal is the more prominent; each is surmounted by a supracondylar ridge, and the internal condyle and ridge attach the muscles passing to the flexor surface of the forearm, while the external are for those passing to the extensor surface.

A small, downwardly directed, hooklike process of bone is occasionally found above the internal condyle and is the vestige of the supracondylar foramen found in so many of the lower animals (see below Comparative Anatomy). Before describing the two bones of the forearm, the range of movement which can take place between them should be noticed. In one position, which is called supine, they lie parallel to each other, the radius being the more external bone, and the palm of the hand being directed forward; in the other or prone position the radius crosses obliquely in front of the ulna, and the palm of the hand is directed backward. Not only the bones of the forearm, but those of the hand are supposed to be in the supine position when they are described.

The radius (fig. 14) is the outer bone of the forearm, and like all long bones possesses a shaft and two extremities. The upper extremity or head has a shallow, smooth cup for moving on the capitellum of the humerus; the outer margin of the cup is also smooth, for articulation with the ulna and orbicular ligament; below the cup is a constricted neck, and immediately below the neck a tuberosity for the insertion of the biceps. The shaft of the bone possesses three surfaces for the attachment of muscles, and a sharp inner border for the interosseous membrane. The lower end of the bone is much broader than the upper, and is marked posteriorly by grooves for the lodgment of tendons passing to the back, of the hand: from its outer border a pointed styloid process projects downward; its inner border has a smooth shallow fossa (the sigmoid cavity of the radius) for articulation with the ulna, and its broad lower surface is smooth and concave, for articulation with the scaphoid and semilunar bones of the wrist.

The Ulna (fig. 14) is also a long bone. Its upper end is subdivided into two strong processes by a deep fossa, the greater sigmoid cavity, which possesses a smooth surface for articulation with Ulna. the trochlea of the humerus. The anterior or coronoid process is rough in front for the insertion of the brachialis anticus, whilst the posterior or olecranon process gives insertion to the large triceps muscle of the upper arm. Immediately below the outer border of the great sigmoid cavity is the small sigmoid cavity fOr articulation with the side of the, head of the;radius.,, Theshafl Of St FIG. 13. - Diagrammatic Section to represent the Relations of the Shoulder Girdle to the Trunk.

Opposite the eight carpal bones. Opposite the five metacarpal bones. Pollex, or thumb. Index.

R U C Mc P the bone has three surfaces for the attachment of muscles, and a sharp outer border for the interosseous membrane. The lower end, much smaller than the upper, has a pointed styloid process and a smooth articular surface, the outer portion of which is for the lower end of the radius, the lower part for moving on a cartilage of the wrist joint called the triangular fibro-cartilage.

The hand consists of the carpus or wrist, of the metacarpus or palm, and of the free digits, the thumb and four fingers. Anatomists. describe it with the palm turned to the front, and with its axis in line with the axis of the forearm.

The carpal or wrist bones (fig. 14) are eight in number and small in size: they are arranged in two rows, a proximal, - i.e. a row. next the forearm, - consisting of the scaphoid, semilunar, cuneiform and pisiform; and a distal, - i.e. a row next the bones of the palm, - consisting of a trapezium, trapezoid, os magnum and unciform; the bones in each row being named in the order they are met with, from the radial or outer to the ulnar or inner side of the wrist. It is unnecessary to give a separate description of each bone. Except the pisiform or pea-shaped bone, which articulates with the front of the cuneiform, each carpal bone is short and irregularly cuboidal in shape; its anterior (or palmar) surface and its posterior (or dorsal) being rough, for the attachment of ligaments; its superior and inferior surfaces being invariably smooth, for articulation with adjacent bones; whilst the inner and outer surfaces are also smooth, for articulation, except the outer surfaces of the scaphoid and trapezium (the two external bones of the carpus), and the inner surfaces of the cuneiform and unciform (the two internal bones). Occasionally extra bones are found, but they are apparently the remnants of cartilaginous elements found in the hand of the early embryo (see G. Thilenius, Morph. Arbeiten, v., 1896).

The metacarpal bones, or bones of the palm of the hand, are five in number (fig. 14). They are miniature long bones, and each possesses a shaft and two extremities. The metacarpal of the thumb is the shortest, and diverges outward from the rest; its carpal extremity is saddle-shaped, for articulation with the trapezium; its shaft is somewhat compressed, and its phalangeal end is smooth and rounded, for the first phalanx of the thumb. The four other metacarpal bones belong to the four fingers; they are almost parallel to each other, and diminish in size from the second to the fifth. Their carpal ends articulate with the trapezoid, os magnum and unciform: their shafts are three-sided: their phalangeal ends articulate with the proximal phalanges of the fingers.

The number of digits in the hand is five. They are distinguished by the names of pollex or thumb, index, medius, annularis and. minimus. Their skeleton consists of fourteen bones, named phalanges, of which the thumb has two, and each of the four fingers three. The phalanx next the metacarpal bone is the proximal, that which carries the nail, the terminal or ungual phalanx, whilst the intermediate bone is the middle phalanx. Each is a miniature long bone, with two articular extremities and an intermediate shaft, except the terminal phalanges, which have an articular surface only at their proximal ends, the distal end being rounded and rough, to afford a surface for the lodgment of the nail.

The Inferior or Pelvic Extremity, or Lower Limb, consists of a proximal part or haunch, a distal part or foot, and an intermediate shaft subdivided into thigh and leg. Each part has its appropriate skeleton (the thigh-bone in man being longer li than the leg-bones). The bone of the haunch (os innomina turn) forms an arch or pelvic girdle, which articulates behind with the side of the sacrum, and arches forward to articulate with the opposite haunch-bone at the pubic symphysis. It 1t is the direct medium of connexion between the axial skeleton and the shaft and foot, which form a free divergent appendage.

The os innominatum, or haunch-bone, is a large irregular platelike bone, which forms the lateral and inferior boundary of the cavity of the pelvis. In early life it consists of three bones - ilium, ischium and pubis - which unite about the twenty-fifth year into a single bone. These bones converge, and join to form a deep fossa or cup, the acetabulum or cotyloid cavity, on the outer Pelvic surface of the bone, which lodges the head of the thigh. bone at the hip-joint. One-fifth of this cup is formed by the pubes, and about two-fifths each by the ischium and ilium. At the bottom of the acetabulum is a depression, to the sides of which the ligamentum teres of the hip-joint is attached. From the acetabulum the ilium extends upward and backward, the ischium downward and backward, the pubis forward, inward and downward. Below the acetabulum is a large hole, the obturator or thyroid foramen, which is bounded by the ischium and pubes; behind and above the acetabulum is the deep sciatic notch, which is bounded by the ischium and ilium, and below this is the small sciatic notch. The ilium (fig. 16) in man is a broad plate-like bone, the lower end of which aids in forming the acetabulum, while the upper end forms the iliac crest, which, in man, in conformity with the general expansion of the bone, is elongated into the sinuous crest of the ilium. This crest is of great importance, for it affords attachment to the broad muscles which form the wall of the abdominal cavity. One surface of the ilium is external, and marked by curved lines which subdivide it into areas for the origin of the muscles of the buttock; another surface is anterior, and hollowed out to give origin to the iliacus muscle; the third, or internal, surface articulates posteriorly with the sacrum, whilst anteriorly it forms a part of the wall of the true pelvis. The external is separated from the anterior surface by a border which joins the anterior end of the crest, where it forms a process, the anterior superior spine. About the middle of this border is the anterior inferior spine. Between the external and internal surfaces is a border on which are found the posterior superior and inferior spines; between the anterior and internal surfaces is the ilio-pectineal line, which forms part of the line of separation between the true and false pelvis.

The pubis (fig. 16) is also a threesided, prismatic, rod-like bone, the fundamental form of which is obscured by the modi fication in shape of its inner end. In human anatomy it is customary to regard it as consisting of a body and of two branches, an upper and a lower ramus. The upper ramus runs downward, forward and inward from the acetabulum to the body of the pubis, which is a plate of bone placed nearly horizontally in the upright position of the subject and articulating with its FIG. 16. - The Appendicular fellow of the opposite side at the Skeleton of the Left Lower symphysis pubis (see Joints). ProLimb.

pubic spine, an important landmark Pb Pubis, the three parts of in surgery, and to this the ilio-pecthe innominate bone. tineal line, already mentioned, may F Femur.

used to be called the horizontal Tr Opposite the seven tarsal' ramus), and runs backward and bones.

outward from the body to meet the C Os calcis, forming promi- ramus of the ischium and so form the nence of heel.

subpubic arch. Mt Opposite the five meta The ischium (fig. 16), like the ilium tarsal bones.

and pubis, has the fundamental form H Hallux or great toe. of a three-sided prismatic II. Second.

whilst the lower forms the large V. Fifth or little toe. The prominence, or tuber ischii. The dotted line HH repre surfaces of the bone are internal or sents the horizontal pelvic, antero-external, and postero- plane, whilst the dotted external. The pelvic and posteroline V is in line with external surfaces are separated from the vertical axis of the each other by a sharp border, on spine.

which is seen the ischial spine. The pelvic and antero-external surfaces are separated by a border, which forms a part of the boundary of the obturator foramen; but the margin between the anteroand postero-external surfaces is feebly marked. The tuberosity, a thick, rough and strong process,. gives origin to several powerful muscles: on it the body rests in the sitting posture; a flattened ramus ascends from it to join the lower ramus of the pubis, and completes both the pubic arch and the - margin of the obturator foramen.

p FIG. 15. - Diagrammatic section to represent the relations of the Pelvic Girdle to the Trunk.

By the articulation of the two innominate bones with each other in front at the pubic symphysis, and with the sides of the sacrum behind, the osseous walls of the cavity of the Pelvis are formed. This cavity is subdivided into a false and a true pelvis. The false pelvis lies between the expanded wing-like portions of the two ilia. The true pelvis lies below the two ilio-pectineal lines and the base of the sacrum, which surround the upper orifice or brim of the true pelvis, or pelvic inlet; whilst its lower orifice or outlet is bounded behind by the coccyx, laterally by the ischial tuberosities, and in front by the pubic arch. In the erect attitude the pelvis is so inclined that the plane of the brim forms with the horizontal plane an angle of from 60° to 65°. The axis of the cavity is curved, and is represented by a line dropped perpendicularly from the planes of the brim, the cavity and the outlet; at the brim it is directed downward and backward, at the outlet downward and a little forward. Owing to the inclination of the pelvis, the base of the sacrum is nearly 4 in. higher than the upper border of the pubic symphysis. The female pelvis is distinguished from the male by certain sexual characters. The bones are more slender, the ridges and processes for muscular attachment more feeble, the breadth and capacity greater, the depth less, giving the greater breadth to the hips of a woman; the inlet more nearly circular, the pubic arch wider, the distance between the tuberosities greater, and the acetabulum smaller in the female than in the male. The greater capacity of the woman's over the man's pelvis is to afford greater room for the expansion of the uterus during pregnancy, and for the expulsion of the child at the time of birth.

The femur or thigh-bone (fig. 16) is the longest bone in the body, and consists of a shaft and two extremities. The upper extremity, or head has a smooth hemispherical surface, in which an oval roughened fossa, for the attachment of the liga mentum teres of the hip, is found; from the head a strong elongated neck passes downward and outward to join the upper end of the shaft; the place of junction is marked by two processes or trochanters; to the external or great trochanter are attached many muscles; the internal or lesser trochanter gives attachment to the psoas and iliacus. A line drawn through the axis of the head and neck forms with a vertical line drawn through the shaft an angle of 30°; in a woman this angle is a little less obtuse than in a man, and the obliquity of the shaft of the femur is slightly greater in the former than in the latter. The shaft is almost cylindrical about its centre, but expanded above and below; its front and sides give origin to the extensor muscles of the leg; behind there is a rough ridge, which, though called linea aspera, is really a narrow surface and not a line; it gives attachment to several muscles. The lower end of the bone presents a large smooth articular surface for the knee-joint, the anterior portion of which forms a trochlea or pulley for the movements of the patella, whilst the lower and posterior part is subdivided into two convex condyles by a deep fossa which gives attachment to the crucial ligaments of the knee. The inner and outer surfaces of this end of the bone are rough, for the attachment of muscles and the lateral ligaments of the knee.

The femur constitutes usually about 0.275 of the individual stature; but this proportion is not constant, as this bone forms a larger element in the stature of a tall than of a short man. The human femur presents also a concave popliteal surface, thus differing from that of Pithecanthropus, whose popliteal surface is convex. In the bones of some races the dorsal ridge of the thigh-bone (linea aspera) projects as a prominent crest causing the bones to appear " pilastered," a condition the amount of which is indicated by the increased relative length of the sagittal of the coronal diameter of the bone. Pilasteting, though characteristic of lower and primitive races of man, is never found in the anthropoias. The upper third of the femur in some races is sagittally flattened, a condition which is called platymeria. Its degree is indicated by the excess of the coronal over the sagittal diameter in this region.

The patella or knee-pan (fig. 16)16) is a small triangular flattened bone developed in the tendon of the great extensor muscles of the. leg. Its anterior surface and sides are rough, for the attachment of the fibres of that tendon; its posterior surface is smooth, and enters into the formation of the kneejoint.

Between the two bones of the leg there are no movements of pronation and supination as between the two bones of the forearm. The tibia and fibula are fixed in position; the fibula is always external, the tibia internal.

The tibia or shin-bone (fig. 16) is the larger and more important of the two bones of the leg; the femur moves and rests upon its. upper end, and down it the weight of the body in the erect position is transmitted to the foot. Except the femur, it is the longest bone of the skeleton,'and consists of a shaft and two extremities. The upper extremity is broad, and is expanded into two tuberosities, the external of which has a small articular facet inferiorly, for the head of the fibula; superiorly, the tuberosities have two smooth surfaces, for articulation with the condyles of the femur; they are separated by an intermediate rough surface, from which a short spine (really a series of elevations) projects, which gives attachments to the interarticular crucial ligaments and semilunar cartilages of the knee, and lies opposite the intercondylar fossa of the femur. The shaft of the bone is three-sided; its inner sole or plantar surface in relation to the ground; the dorsum or back of the foot directed upward; the axis of the foot at about a right angle to the axis of the leg; and the great toe or hallux, which is the corresponding digit to the thumb, at the inner border of the foot. The human foot, therefore, is a pentadactylous, plantigrade foot.

The bones of the tarsus or ankle (fig. 16, Tr), are seven in number, and are arranged in three transverse rows - a proximal, next the bones of the leg, consisting of the astragalus and os calcis, a middle, of the scaphoid and a distal next the metatarsus, consisting of the cuboid, ectomesoand ento-cuneiform. If the tarsal bones be looked at along with those of the metatarsus and toes, the bones of the foot may be arranged in two longitudinal columns - an outer, consisting of the os calcis, cuboid and the metatarsal bones and phalanges of the fourth and fifth toes; an inner column consisting of the astragalus, scaphoid, three cuneiform and the metatarsal bones and phalanges of the first, second and third toes. The tarsal, like the carpal bones, are short and, with the exception of the cuneiforms which are wedge-shaped, irregularly cuboidal; the dorsal and plantar surfaces are as a rule rough for ligaments, but as the astragalus is locked in between the bones of the leg and the os calcis, its dorsal and plantar surfaces, as well as the dorsum of the os calcis, are smooth for articulation; similarly, its lateral surfaces are smooth for articulation with the two malleoli. The posterior surface of the os calcis projects backward to form the prominence of the heel. With this exception, the bones have their anterior and posterior surfaces smooth for articulation. Their lateral surfaces are also articular, except the outer surface of the os calcis and cuboid, which form the outer border; and the inner surface of the os calcis, scaphoid and ento-cuneiform, which form the inner border of the tarsus. Supernumerary bones are occasionally found as in the hand.

The metatarsal bones and the phalanges of the toes agree in number and general form with the metacarpal bones and the phalanges in the hand. The bones of the great toe or hallux are more massive than those of the other digits, and this digit, unlike the thumb or pollex, does not diverge from the other digits. but lies almost parallel to them.

The development of the appendicular skeleton takes surface is subcutaneous, and forms the shin; its outer and posterior surfaces are for the origin of muscles; the anterior border forms the sharp ridge of the shin, and terminates superiorly in a tubercle for the insertion of the extensor tendon of the leg; the outer border of the bone gives attachment to the interosseous membrane of the leg. The lower end of the bone, smaller than the upper, is prolonged into a broad process, internal malleolus, which forms the inner prominence of the ankle: its under surface is smooth for articulation with the astragalus; externally it articulates with the lower end of the fibula.

The tibia in most civilized races is triangular in the section of its shaft, but in many savage and prehistoric races it is two-edged.. The condition is named platycnemia, and is indicated by the proportional excess of the sagittal over the coronal diameter. The foetal tibia has its head slightly bent backward with regard to the shaft, a condition which usually disappears in the adult, but which is shown in the prehistoric tibae found in the cave of Spy. In races that squat on their heels the front margin of the lower end of the tibia is marked by a small articular facet for the neck of the astragalus.

The fibula, or splint-bone of the leg (fig. 16), is a slender long bone with a shaft and two extremities. The upper end or head articulates with the outer tuberosity of the tibia. The shaft is four sided, and roughened for the origins of the muscles.

Separating the anterior from the internal surface is a slender ridge for the attachment of the interosseous membrane. The lower end has a strong process (external malleolus) projecting downward to form the outer prominence of the ankle, and a smooth inner surface for articulation with the astragalus, above which is a rough surface for the attachment of ligaments which bind together the tibia and fibula.

The foot consists of the tarsus, the metatarsus and the five free digits or toes. The human foot is placed in the prone position, with the +?'? ct ' 'c FIG. 17. - Bones of the right Human Foot. T Tarsus.

c 3 External cuneiform. The digits are indicated by Roman numerals, counting from the tibial to the fibular side.

place in the core of mesenchyme in the centre of each limb.' This substance first becomes changed into cartilage, except perhaps in the case of the clavicle, though there is at present some doubt as to how much of this bone is chondrified before ossification reaches it.

The present belief is that, although a deposit of lime salts constituting the process of calcification may and frequently does occur in cartilage, true ossification or the orderly disposal of that deposit into bony tissue can only take place through the intervention of osteoblasts and osteoclasts, and as these cells are not formed in cartilage they must make their way in from the surrounding fibrous tissue which constitutes the perichondrium.

The factors which determine the general shape and proportionate size of each limb bone are at work while the cartilage is being formed, because each future bone has a good cartilaginous model laid down before ossification begins. Calcification usually begins at one point in each bone, unless that bone be a compound one formed by the fusion of two or more elements which were distinct in lower vertebrate types, as is the case with the os innominatum.

It is interesting to notice that this centre of calcification, which will later on be the centre of ossification, is usually in the middle of the shaft of a long bone, or, when a cuboidal block of cartilage is dealt with, as in the case of the carpal and tarsal bones, in that place which is farthest away from the periphery, and which is likely to be least well nourished. There seems, too, to be a general tendency for larger masses of cartilage to begin calcifying before smaller ones. Contrasting these facts with the behaviour of tumours, which contain cartilage and which are liable to undergo a process of calcareous degeneration, the present writer is led to suspect that the calcification which precedes ossification in cartilage may be a degenerative change brought about by ill-nutrition. However this may be, there is little doubt that the calcification, once established, acts as an attraction for blood-vessels, which probably bring with them osteoblasts, and the subsequent ossification is a process which needs and receives a plenteous supply of nourishment. After a long bone has reached a certain size it very often has extra centres of ossification developed at its ends as well as at places where important muscles have raised lever-like knobs of cartilage on the model. These extra centres are called epiphyses, and it is convenient to distinguish three varieties of these: (a) pressure epiphyses at the joint ends of long bones; (b) traction epiphyses, where muscles pull; and (c) atavistic epiphyses, the mechanical causes of which are more remote, but which represent structures of greater import in the lowlier vertebrates. With regard to the pressure epiphyses, they form a cap which protects the epiphysial line, or plate of cartilage, by means of which the bone increases in length, but they are certainly not essential to the growth of a bone, because they often do not appear until the bone has been growing for a long time, while in birds they are not found at all. The traction epiphyses are, in the opinion of the writer, originally pieces of cartilage which have the same nature as sesamoid cartilages developed in the play of a tendon, where it presses against a neighbouring cartilaginous model of a bone, and which, instead of remaining separate structures throughout life, as is the case with the patella, fuse early with the model against which they are pulled, and so form a knob. For practical purposes the coracoid process of man may be regarded as an example of an atavistic epiphysis or perhaps of two atavistic epiphyses. (For further details on this subject see the writer's papers on epiphyses, Jour. Anat. and Phys. vol. xxxvii. p. 315; vol. xxxviii. p. 248; vol. xxxix. p. 402.) Turning now to the development of the individual bones of the axial skeleton, the clavicle, as has been mentioned, is partly fibrous, and partly cartilaginous, but the exact proportions are still imperfectly Sternal epiphysis ossifies about Primary centre appears about zoth year; fuses about 25th year. 5th or 6th month of foetal life.

known; its primary centre is the earliest of all in the body to appear, while its sternal epiphysis does not come till the bone is fully grown, and so can have no effect on the growth of the bone. It is probably .one of the atavistic class, and is often regarded as the vestige of the precoracoid (see subsection on comparative anatomy), though it may represent the inter-clavicle, which, as has been pointed out in the article on the axial skeleton, is quite distinct from the episternum. It sometimes fails to appear at all.

The centres for the scapula are shown in the accompanying figures (fig. 19). G. B. Howes regarded the subcoracoid centre as the atavistic epiphysis representing the coracoid bone of lower verte ' By mesenchyme is meant that part of the mesoderm, or middle layer of the embryo, in which the cells are irregularly scattered in a matrix, and are not arranged in definite rows or sheets as in the coelomic membrane.

brates, while the human coracoid he looked upon as the equivalent of the epicoracoid. The epiphyses in the vertebral border are atavistic and represent the supra-scapular element (see section below on Comparative Anatomy). In the humerus the centre for the shaft appears about the eighth week of foetal life, which is the usual time for primary centres. The head, trochlea and capitellum have pressure epiphyses, while those for the tuberosities and condyles are of the traction variety.

The ulna is a very interesting bone because there is no pressure epiphysis for its upper end. The upper epiphysis shown in fig. 21 does not encroach upon the articular surface, but is a pure traction epiphysis developed in the triceps tendon and serially homologous with the patella (a sesamoid bone) in the lower limb.

In the radius there are two terminal pressure epiphyses and one traction for the insertion of the biceps.

The carpus ossifies after birth, one centre for each bone occurring in the following order: os magnum, II to 12 months; unciform, 12 Appears about 1 6-17 yrs.; fuses Acromial centres about zo yrs.

appear 15-16 yrs.; fuse about 25 yrs. Secondary centres for Primary centre coracoid appears appears about about end 1st year; 2nd m. foetal life. fuses about r8 yrs.

From Arthur Thomson, Cunningham's Scapula at end of First Year. Scapula about the Age of Puberty. FIG. 19. - Ossification of the Scapula.

to 14 months; cuneiform, 3 years; semilunar, 5 to 6 years; trapezium, 6 years; scaphoid, 6 years; trapezoid, 6 to 7 years; pisiform, 10 to 12 years.

Up to the third month of foetal life a separate cartilage for the os centrale (see subsection on comparative anatomy) is found, but this later on fuses with the scaphoid. It will be noticed that, broadly' speaking, the larger cartilaginous masses ossify before the smaller.

The metacarpal bones have one centre each for the shaft and one epiphysis for the head, except that for the thumb which has one centre for the shaft and one epiphysis for the proximal end. The phalanges develop in the same way that the metacarpal bone of the thumb does.

The special centres for the crest of the ilium are probably a serial repetition of those for the vertebral border of the scapula (see fig. 19); that for the anterior inferior spine is a purely human traction epiphysis connected with the use of the straight head of the rectus femoris in the upright position. The centre for the pubic symphysis probably represents the epipubis of amphibians, while that for the tuberosity of the ischium is the hypoischium of reptiles (see subsection on comparative anatomy). The most anterior of the epiphyses in the acetabulum is the os acetabuli of lower mammals, while the occasional one for the spine of the pubis is often looked on as the vestige of the marsupial bone of monotremes and marsupials. It will thus be seen that many of the secondary centres of the os innominatum are of the nature of atavistic epiphyses.

The femur has two pressure epiphyses, one for the head and another for the lower end, and two traction for the great and small trochanters.

The cartilaginous patella does not appear until the third month of foetal life, that is well after the quadriceps extensor cruris, in the tendon of which it is formed, is defined. Its ossification begins in the third year. The patella is usually looked upon as the largest and most typical example of a sesamoid bone in the body.

The tibia has a pressure epiphysis at either end, but that for the upper comes down in front so as to include a good deal of the tubercle. In almost any other mammal, and often in man himself, may be ,Subcoracoid centre appears ro yrs.; fuses 16-17 yrs.

seen that this down-growth is a traction epiphysis developed in the quadriceps tendon below the patella and joining the main upper epiphysis before uniting with the diaphysis or shaft.

The fibula has two pressure epiphyses, the lower of which appears 2 3 At birth. About 5 years. About 12 years. From Arthur Thomson, Cunningham's Text-Book of Anatomy. FIG. 20. - Ossification of 1 Appears early in 2nd month foetal life. 8 2 For tuberosity, appears 2 to 3 years.

3 For head, appears within first 6 months. 9 4 For internal condyle, appears about 5 years. 10 5 For capitellum, appears 2 to 3 years.

6 Appears about 12 years. 11 7 Centres for head and great tuberosity coalesce about 5 years. 12 first. The general rule with the long bones of the extremities is that the epiphysis nearest the elbow or farthest from the knee is the first to appear and the last to join. The writer accounts for the neglect of this rule in the case of the fibula by the fact that the lower cartilaginous end is larger than the upper (see fig. 26).

In the tarsus the cartilages are at an early stage arranged in three Fuses with shaft about 16 years Fuses with shaft 20-23 years At birth. About 12 years. About 16 years. From Arthur 'Thomson, Cunningham's Text-Book of Anatomy. FIG. 21. - The Ossification of the Ulna.

rows in just the same way that those of the hand are, but in the proximal row the middle one (intermedium), corresponding to the semilunar in the hand, fuses with the one on the tibial side to form the astragalus, though sometimes a vestige of it seems to persist The calcaneum has a very defirf to traction epiphysis developed in the insertion of the tendo Achillis behind.

The development of the metatarsal hones and phalanges of the foot is the same as that of the hand.

For further details and literature see J. P. M`Murrich's Development of the Hunan Body (London, 1906) and D. J. Cunningham's Text-Book of Anatomy (Edinburgh, 1906).

It is only when the class of pisces is reached that paired appendages are found, and there are two main theories to account for their first occurrence. The one which is at present most favoured is that in some ancestral fishes two folds ran 12 along the ventro-lateral part of the body, like the bilge keels of a boat, and that these joined one another in the mid-ventral line behind the cloacal orifice to form the median caudal fin. Into these folds the segments of the body including myotomes and myocommata, extended. Later on parts of these ridges were suppressed, but in the pectoral and pelvic regions they were retained to form the paired fins. This theory was first foreshadowed by Goodsir, and has been elaborated by Balfour, Dohrn and many others. It is supported by the fact that in some elasmobranch embryos the whole length of the folds can be traced.

The second theory is that the limbs are elaborated gills; this was proposed by C. Gegenbaur, and has lately been supported by Graham Kerr. It is probable that the limb girdles are of later evolution than the skeleton of the fins themselves.

In the elasmobranch fishes (sharks and rays) there is a crescentic Unites with shaft 20-25 years At birth. About 12 years. About 16 years. From Arthur Thomson, Cunningham's Text-Book of Anatomy. FIG. 22. - The Ossification of the Radius.

bar of cartilage (pectoral girdle), concave upward, which girdles the ventral and lateral parts of the body; it is divided into a dorsal part (scapula) and a ventral part (precoracoid and coracoid) by a 8 Appears about ro years Appears about 6 years Appears ° about years w a a?l w Lases with shaft 18-20 years Appears about - years as a little bone at the back of the astragalus, known as the trigonum. The centre for the calcaneum appears in the sixth month of foetal life, that for the astragalus in the seventh, the cuboid about birth, the external, middle and internal cuneiforms in the first and second years, while the navicular is the last to appear in the third year. It will be noticed that, although ossification occurs in the bigger cartilaginous masses earliest, e.g. calcaneum astragalus and cuboid, the large navicular is the last cartilage to ossify, and this is an exception to the general rule which is probably caused by some factor which we do not at present understand.

Centre for small tuberosity fuses with other centres about 7 years. Appears about I I or 12 years.

facet for the articulation of the fin. This of course is the glenoid cavity. In some forms, e.g. the shark Heptanchus, there Pectoral girdle. i s a perforation in the ventral part of the bar on each g side, which possibly indicates the division between the precoracoid and coracoid elements.

In many of the bony fish (Teleostei) the outline is obscured by a At birth. About 12 or 13 years.

From Arthur Thomson, Cunningham's Text-Book of Anatomy. FIG. 23. - Ossification of the Innominate Bone.

series of bones which connect the girdle with the skull and may be the precursors of the clavicle.

In the Amphibia the dorsally-placed scapula (fig. 27, S) has more dorsally still a cartilaginous plate, the supra-scapula (fig. 27, S.S), which may be calcified. The precoracoid (fig. 27, P.C) and coracoid (C) are quite distinct, the former being in front (cephalad) and overlaid by a dermal bone, the clavicle (C1). The attachment of the coracoids to the sternum has been noticed in section Axial of this article. Uniting the ventral ends of the precoracoid and coracoid is the epicoracoid on each side (fig. 27, E.C).

In the Reptilia the same general plan is evident, but in the lizards Fuses with shaft about 18-1g years Appears about early part of first year Appears about 2 - 3 years Appears about 52-13 years Usually appears in Usually appears the 9th month of before birth foetal life Fuses with shaft about 20-22 years At birth. About 12 years. About 16 years. From Arthur Thomson, Cunningham's Text-Book of Anatomy. FIG. 24. - Ossification of Femur.

the ventral ends of the two clavicles are united by a median daggerlike dermal bone, the interclavicle (fig. 27, I.C), which lies on a plane superficial to the sternum and epicoracoids.

In birds the scapula has the shape of a sabre blade, and there is a rudimentary acromion process, though this is also indicated in some reptiles. The preand epi-coracoids are aborted, but the coracoids are very strong. The clavicles and interclavicle unite into a Vshaped bar which forms the furcula or " merrythought." In the Mammalia the Monotremata (Ornithorhynchus and Echidna) retain the reptilian arrangement of large coracoids and epicoracoids articulating with the sternum, while the clavicles and inter Fuses with shaft about 20-24 years May appear Appears independently before birth about r r years Appears about a/ years Fuses about r8th year At birth. About 12 years. About 16 years.

From Arthur Thomson, Cunningham's Text-Book of Anatomy. FIG. 25. - Ossification of the Tibia.

clavicle are also largely developed; the scapula too is more birdlike in shape than mammalian. In the higher mammals the scapula develops a spine and usually an acromial process, and has a triangular outline. As long as the forelimb is used for support, the vertebral border is the shortest of the three, and the long axis of the bone runs from this border to the glenoid cavity; but when the extremity is used for prehension, as in the Primates, or for flight, as in the Chiroptera, the vertebral border elongates and the distance from it to the glenoid cavity decreases so that the long axis is now parallel with that of the body instead of being transverse.

Above the monotremes too the coracoid becomes a mere knob for muscles, and no longer articu lates with the sternum. There is thus a sudden transition from the way in which the forepart of the body is propped up on the forelimbs when the coracoid is functional (as in reptiles) to the way in which it is suspended like a suspension bridge between the two scapulae in pronograde mammals, the serratus magnus muscles forming the chains of the bridge (see fig. 28).

The clavicle is often entirely suppressed in mammals; this is the case in most of the Ursidae, all the Pinnipedia, Manis among edentates, the Cetacea, Sirenia, all Ungulata and some of the Rodentia. It is complete in all the Primates, Chiroptera, Insectivora (except Potamogale), many of the Rodentia, most Edentata, and all the Marsupialia except Perameles. In the Monotremata it is fused with a welldeveloped interclavicle, but in other mammals the interclavicle is either suppressed or possibly represented by the sternal epiphysis of the clavicle of the Primates. The precoracoid as a distinct structure entirely disappears, though vestiges of it may remain in the cartilaginous parts of the clavicle.

The chief modifications of the humerus are the development of the pectoral ridge, which is large whenever the pectoral muscles are strong, and is represented in man by the outer lip of the bicipital groove and the supracondylar foramina. In the tuatera lizard (Sphenodon) Appears about 15 years; fuses 22-25 years Appears about later end of 2nd m. of foetal life Appears about 15 years; fuses 22-25 years Appears about 12 years Appears about 18 years Appears about 18 years Appears about Appears about 4th m. of foetal 5-6 m. foetal life life Unite about so years Appears about 15 years: fuses 22 - 25 years Fuses with shaft about 18 years -2 4 ?

Fuses with shaft about 20-24 years Fuses with shaft about rg years At About About birth. 12 years. 16 years.

From Arthur Thomson, Cunningham's Text-Book of Anatomy. FIG. 26. - Ossification of Fibula.

Appears about 3 - 4 years Appears about 2nd year there are two of these, one on the outer side for the musculospiral nerve, and one on the inner for the median nerve; in other living and fossil reptiles one or other of these may be present. The three bars hounding these two foramina in Sphenodon are sometimes regarded as indications that the humerus contains vestiges of three fin rays in its evolution from the fin of the fish. In the mammals the internal supracondylar (entepicondylar) foramen is most erratic in its appearance and disappearance, very few E.S. orders being without some family or genus which shows it. In some mammals, e.g. dog, a supratrochlear foramen is present just above the trochlea; it transmits nothing. Epiphyses are found in this, as in other long bones, in amphibians, reptiles and R mammals, but not in birds.

In the tailless amphibians (Anura) the radius and ulna are fused, while in the Urodela and reptiles they are always distinct. In some lizards (Iguana, Sphenodon, &c.) the olecranon epiphysis remains a distinct sesamoid bone just as the patella does, and this is also the case in some bats. In the pronograde mammals the radius is in a position of permanent pronation, and is a much more important bone than the ulna, which is sometimes R suppressed, so that little more than the olecranon process remains (e.g. horse, giraffe). In the lower Primates the ulna articulates directly with the cuneiform and (some times) pisiform bones, and is not shut off from the carpus by a meniscus as in man.

The carpus of the higher vertebrates may be reduced from a generalized type by the fusion or suppression of certain of its elements. A perfect generalized type is not known to exist in any vertebrate, though it is very closely approached by the following names, beginning at the outer or radial side of the wrist: (I) Radiate marginate (fig. 29, R.M); (2) Radiate (R); (3) Intermedium (I); (4) Ulnare (U); (5) Ulnare marginate (U.M). In the middle row there are two: (I) Centrale radiale (C.R); (2) Centrale ulnare (C.U)." In the distal row there are again five bones, which are spoken of as the first, second, third, fourth and fifth distalia. Sphenodon has all these bones except the radiale marginate.

In many of the urodele amphibians, e.g. the salamander and newt (Molge), the carpus is very generalized, the only elements wanting being the radiale marginate, ulnare marginate, centrale ulnare and distale V. In the tailless forms (Anura), however, it is more specialized, although the radiale marginate is sometimes present and by some morphologists is spoken of as the prepollex. When only four distalia are present it is doubtful whether the fifth is suppressed, or whether it has fused with the fourth.

l In the giant salamander of Japan (Megalo-batrachus) three centralia are sometimes found, so that possibly the generalized carpus should have three instead of two of these elements in the middle row.

In the Reptilia the carpus is often very generalized, as in Sphenodon and Chelydra (see fig. 30).

In the birds the radiale and ulnare are distinct, but the distal bones are fused with the metacarpus to form a cargo-metacarpus. In Mammalia various examples of fusion and suppression occur. All that space will here allow is to attempt to show how the human carpus is derived from the generalized type. In man the radiale, radiale marginate, and centrale radiale fuse to form the scaphoid; the semilunar is the intermedium; the cuneiform the ulnare; and the pisiform the ulnare marginate.

The trapezium and trapezoid are distalia I. and II.; the os magnum distale III. fused with the centrale ulnare; while distalia IV. and V. have either fused to form the unciform, or, as some believe, distale V. has been suppressed.

In some mammals the radiale marginate is very large, e.g. mole and elephant, and is regarded as a stage in the evolution of a digit on the radial side of the pollex, hence named the prepollex. In the Cape jumping hare (Pedetes) this digit is two-jointed and bears a rudimentary nail. Feebler indications of another digit on the ulnar side of the carpus, called the post-minimus, are sometimes seen in relation with the pisiform, which is therefore no longer regarded as a sesamoid bone, but, with the radiale marginate, as a stage in the progress from a pentadactylous_to a,heptadactylous manus. The centrale radiale Rad. Uln. FIG. 30. - Dorsal Surface of a generalized carpus.

serpentina). After Gegenof the Right Manus of a Water Tortoise (Chelydra U Ulna.

occurs; the pollex is the first to go, then the minimus, index and annularis one after another, so that an animal like the horse, which persists as a distinct bone throughout life in many monkeys, as also does the radiale marginate.

In the suppression of digits in vertebrates a regular sequence has only one digit, has lost all except the medius.

In the mammals the number of the phalanges usually corresponds with that of man, though in the lower vertebrates they are often much more numerous.

When the extremity is modified to form a paddle, as in Ichthyosaurus and the Cetacea, the phalanges are often greatly increased in number.

In the elasmobranch fishes the pelvic girdle is a repetition of the pectoral though it is not quite so well marked. The acetabulum corresponds to the glenoid cavity, and the part of the girdle dorsal to this is the ilium; the ventral part, uniting with its fellow in the mid-line, is the ischio-pubis, the two elements of which are sometimes separated by a small foramen for the passage of a nerve. When this is the case the anterior (cephalic) part is the pubis, and is in series with the precoracoid, while the ischium (caudad) repeats the coracoid.

In Amphibia the connexion between the ilium and sacrum becomes established, and some of the extinct Labyrinthodontia have separate pubic and ischial symphyses, though in existing forms the ischium and pubis are generally fused.

In the Urodela there is usually a bifid cartilage just in front (cephalad) of the pubes, in the mid-line, which is called the epipubis (see subsection on embryology).

A B t i representing the change of Mechanism in in the Reptilian (A) the Mammalian (B) e.

R FIG. 27. - Diagrammatic Representation of a Generalized Form of Shoulder Girdle.

G Glenoid cavity. E.S Epior omo Cl Clavicle. sternum (dotted I.0 Interclavicle. deep to inter P.0 Precoracoid. clavicle).

primitive reptile Sphenodon. In such a type the bones are arranged in three rows; proximal, nearest the forearm, middle and distal. There are five bones in the proximal row, which bear the tail; the ischia usually meet in a ventral ischial symphysis, from which a cartilage or bone projects backward to support the anterior lip of the cloacal orifice; this is the hypoischium, a structure which is traceable throughout the Vertebrata to man (see fig. 31).

The hypoischium and epipubis are parts of a cartilaginous pelvic sternum, the former representing are xiphisternum and the latter the episternum of the shoulder girdle (see F. G. Parsons, " Epiphyses of the Pelvis," J. Anat. and Phys. vol. xxxvii. p. 315). The pubis may or may not form a symphysis; occasionally it is double and then a preand postpubis are recognized.

In birds the ilium extends forward and backward, and is fused FIG. 31. - Pelvis of Sphenodon with the vertebral column, as has Lizard. been noticed in section Axial of A. Pubic symphysis. this article. The ischia and pubes B Ischial symphysis. do not form a symphysis except in C Epipubis. the struthious birds (ostrich and D Hypoischium. rhea). The acetabulum is always (The dotted part is cartilaperforate.

ginous, the white and darkly In mammals the ilium projects shaded parts bony.) forward toward the head, and an ischio-pubic symphysis is common, though sometimes it is only pubic as in man. In Echidna among the monotremes the acetabulum is perforate as in birds. In the monotremes and marsupials part of the external oblique muscle is ossified to form the marsupial bones; these are sometimes regarded as part of the epipubis, though it is more probable that they are merely adaptive strengthenings of the external oblique to support the traction of the pouch. A cotyloid bone (os acetabuli) is usually present, at all events in early life, and it often shuts out the pubis from taking any part in the formation of the acetabulum.

The femur is comparatively a very stable bone. Sometimes, especially in the odd-toed ungulates (Perissodactyla), the gluteal. ridge forms a large third trochanter, while in most mammals, though not in ungulates, there are two sesamoid bones, called fabellae, developed in the gastrocnemius just above the condyles.

The patella first appears in the reptiles, though it is not present in all of them. Most of the Lacertilia show it as a small sesamoid. structure in the quadriceps extensor tendon. It is present in all birds and mammals, with the exception of some bats. In most marsupials it remains cartilaginous throughout life.

The tibia and fibula fuse in the Anura and also in some mammals (e.g. rodents). The fibula is often nearly or quite suppressed in birds and mammals, while in birds the tibia fuses with the Tibia; proximal row of tarsal bones, so that the ankle joint is obliterated and a tibio-tarsus formed. In the marsupials the upper - end of the fibula is large and may articulate with the femur in certain positions of the knee, but, as a whole, it reaches its maximum development in the Carnivora in the aquatic suborder of which (Pinnipedia) it is as large as the tibia. It is curious that the only epiphysis which occurs in the long bones of birds is in the head of the tibia of the Gallinaceae.

In the tarsus the bones are arranged on the same generalized plan as in the carpus; the proximal row consists of tibiale marginale,. tibiale, intermedium, fibulare and fibulare marginale; the middle row as far as we know only contains one centrale, while the distal row has five distalia. It is more difficult to trace the fate of these structures in existing vertebrates than it is with the carpal bones. In man the astragalus probably contains the tibiale, tibiale marginale and intermedium, the latter structure possibly accounting for the occasional os trigonum, already mentioned in the subsection on embryology. The fibulare and fibulare marginale probably form the calcaneum, though it is unlikely that the epiphysis at the back of that bone represents any integral part of a generalized tarsus. The centrale persists as the navicular, while the three cuneiform represent tarsalia I., II. and III. and the cuboid tarsalia IV. and V., unless V. is suppressed as some believe. Vestiges of a prehallux are found in the Cape jumping hare and other rodents, though they are usually more closely connected with the navicular and internal cuneiform than with the bones of the proximal row. The large size of the hallux in man is an adaptation to the erect position.

Most of the remarks already made about the metacarpals and phalanges of the hand apply equally to the foot, though there is a greater tendency to reduction of digits in the hind limb than in the fore.

For further details and literature see S. H. Reynolds, The Vertebrate Skeleton (Cambridge, 1897); W. Flower and H. Gadow, Osteology of the Mammalia (London, 1885); R. Wiedersheim, Comparative Anatomy of Vertebrates, adapted by W. N. Parker, (London 1907); C. Gegenbaur, Vergleich Anat. der Wirbeltiere (Bd. i.) (Leipzig, 1901).

Visceral. In the lower vertebrates as well as in the embryo of man, a number of cartilaginous or bony arches encircle the mouth and pharynx (anterior part of the food tube), just as hoops encircle a barrel. There is little doubt that, when they first appeared in the history of evolution, all these bars supported gills and bounded gill slits, but in all existing types the first arch has been modified to surround the mouth and to act as both upper and lower jaws, gaining in different animals a more or less complete connexion with the cranium or brain-containing part of the skull. The first of these visceral arches, therefore, is known as the oral or jaw arch and, as has been shown, the muscles in connexion with it are supplied by the fifth nerve (see Muscular System; and Nerve: Cranial). The second visceral arch is the hyoid and is accompanied by the seventh or facial nerve. The third visceral or first branchial arch of most writers has the ninth or glosso-pharyngeal for its nerve supply, while the arches behind this are supplied by the vagus or tenth nerve.

It will be seen, on reading the subsections devoted to embryology and comparative anatomy, that in man the maxilla, palate, internal pterygoid plate, malar and tympanic bones as well as the ear ossicles, mandible, hyoid bone and thyroid cartilage are developed in connexion with this visceral skeleton. Of these the ear ossicles are described in the article EAR, the thyroid cartilage in that on the Respiratory System, while the other bones, with the exception of the hyoid, are treated under the head of Skull. It therefore only remains to describe here the hyoid bone of man.

The hyoid bone, so called from its likeness to the Greek letter v, lies in the upper part of the neck in close connexion with the root of the tongue and just above the thyroid cartilage of the larynx. It consists of a body across the mid-ventral line and a great and small cornu on each side (see fig. I).

The body (basihyal) is rectangular with its long axis placed horizontally; behind it is markedly concave both from above down From Gray's Anatomy, Descriptive and Surgical. FIG. 32. - Hyoid Bone, anterior surface (enlarged).

ward and from side to side. In front it attaches several muscles, but behind it is smooth and is separated from the thyrohyoid membrane by a bursa. From its upper border this membrane runs downward to the thyroid cartilage. The great cornua (thyrohyals) are attached to each side of the body by cartilage until middle life and afterwards by bony union. They curve upward and backward round the side of the pharynx and are laterally compressed. To their inner surfaces the thyrohyoid membrane is attached, while their knob-like ends are connected with the superior cornua of the thyroid cartilage by the lateral thyrohyoid ligaments.

The small cornua (ceratohyals) are conical structures about a quarter of an inch long attached to the upper part of the body at its junction with the great cornua. It is only in late life that they become united with the body by bony union, if they ever do so. At their apices they are connected with the tips of the styloid processes by the long stylohyoid ligaments (epihyals).

In the early embryo (see Mouth and Salivary Glands) the mandibular processes grow forward on each side of the slit-like stomatodaeum or primitive mouth, and at length join one another in the mid-ventral line. From the proximal part of each of these another process, the maxillary, grows forward (ventrad), only more slowly, to blend with the fronto-nasal process. In each of these processes cartilage is formed in the lower vertebrates, which in the case of the mandible (lower jaw) reaches to the mid-ventral line and ody Le forms what is known as Meckel's cartilage; but in the maxillary process the stage of chondrification is suppressed in man and other mammals, and the palato-quadrate cartilaginous bar which is so evident in embryo fishes and amphibians is not formed. It will thus be seen that both the maxillary and mandibular bars are derivatives of the first visceral arch. In the maxillary process a membrane bone is formed which blends with the sphenoid to form the internal pterygoid plate, while in front (ventrad) of this the upper jaw (maxilla) is developed in membrane by several centres. Of these, according to the usual description, (I) forms the body of the bone on the outer side of the infraorbital canal; (2) forms the body of the bone on the inner side of that canal; (3) forms the nasal process and the socket for the canine tooth; (4) makes the posterior threequarters of the palatine process; while (5) and (6) form the premaxilla, each of the latter contributing a socket for one of the two incisor teeth. When these premaxillary sutures fail to unite, the deformity known as " cleft palate " is produced and this may occur either between the lateral incisor and the canine or between the central and lateral incisor teeth. The recent researches of Professor E. Fawcett point to the conclusion that these centres are not really as numerous as is generally thought. He regards (I) and (2) as a single centre which grows up round the infraorbital canal, while the premaxilla he finds need not necessarily have two centres. The maxillary antrum is first developed as an outgrowth from the cartilaginous olfactory capsule into the membranous maxilla, though the cartilage soon disappears. The palate bone is developed by one centre which is formed in what will be the vertical plate of that bone in the membrane, behind the centre or centres for the body of the maxilla and at a little later date (see E. Fawcett, Journ. Anat. and Phys. vol. 40, p. 400).

The mandibular or Meckel's cartilage is continued up into the tympanum where it joins the proximal end of the cartilage of the second or hyoid arch, and it is from this junction (hyomandibular plate) that, according to H. Gadow, Anat. Anzeiger, Bd. 1 9, p. 396, the malleus and incus bones of the middle ear are developed (see EAR). Between the slender process of the malleus and the region of the inferior dental foramen, the cartilage later on disappears and its fibrous sheath forms the long internal lateral or sphenomandibular ligament (see fig. 33, L.I.L).

Hitherto each half of the lower jaw has been considered to be composed of several distinct skeletal elements, homologous with the elements found in the jaws of lower vertebrates. This view is still held by Professor K. von Bardeleben, who contends that there are present in the lower jaws of man and mammals six separate elements, the os mentale, coronoid, condyloid, angular, marginal and dentary. The researches of B. Henneberg, Professor E. Fawcett and of Dr A. Lowe, however, are so complete and correspond so closely that one cannot help believing that the human lower jaw, at all events, is ossified from one centre only on each side, which appears in membrane near the symphysis and extends into a small part of Meckel's cartilage near the incisor tooth germs. From this centre, which represents the dentary of lower vertebrates, the whole adult bony jaw is formed and the greater part of Meckel's cartilage disappears by a process of resorption. But, although this bone is mainly membranous, patches of cartilage appear in the coronoid and condylar processes as well as near the symphysis and perhaps at the angle. These, however, do not ossify by separate centres, but are invaded by the main dentary ossification already described. It seems evident, therefore, that in man the process of ossification is slurred over although some of the original elements of the lower vertebrates are repeated as temporary cartilaginous masses, e.g. coronary, condylar and angular. (See E. Fawcett, " Thesis for the Degree of Doctor of Medicine," University Library, Edinburgh, 1906; also A. Lowe, " Development of Lower Jaw in Man," Proc. Anat. Soc. of the University of Aberdeen, 1905,5, p. 59. In the latter paper the literature is reviewed.) At birth the two halves of the mandible are separate as they are throughout life in many mammals (e.g. rodents), but in man they join together about the end of the first year.

It has been stated that within the tympanum the dorsal or proximal ends of the first and second visceral arches unite to form the hyomandibular plate from which, following H. Gadow, the malleus and incus are derived. The stapes is also probably formed from the proximal end of the second or hyoid arch (see fig. 33, St), and just ventral to this the cartilage of the arch fuses with that of the periotic capsule, where it is later on ossified as the tympanohyal element of the temporal bone (fig. 33, T.H). From this point the cartilage becomes free from the skull and runs round the pharynx until it meets its fellow of the opposite side in the mid-ventral line. That part of the cartilage which is nearest the skull remains as the stylohyal element (fig. 33, S.H) and this later on ossifies to form the styloid process which fuses with the tympanohyal between twenty and twenty-five. For some distance beyond the stylohyal element the cartilage degenerates into fibrous tissue forming the stylohyoid ligament; this represents the epihyal element, and occasionally instead of degenerating it ossifies to form an abnormal bone (fig. 33, E.H). Near the middle line the cartilage persists as the ceratohyal element or lesser cornu of the hyoid bone (fig. 33, C.H), while the most ventral part, where it fuses with its fellow of the opposite side as well as with the ventral part of the third arch, is the basihyal or body of the hyoid bone (fig. 33, B.H).

The dorsal part of the cartilage of the third arch is wanting, but its lateral part forms the thyrohyal or great cornu of the hyoid bone (fig. 33, Th.H), while its ventral part fuses with its fellow of the opposite side as well as with the ventral part of the second arch to form the body of the hyoid bone. The fourth and fifth arches only develop cartilage in their ventro-lateral parts and fuse to form the thyroid cartilage of the larynx (fig. 33, Th.C) (see Respiratory System).

For further details see J. P. McMurrich, Development of the Human Body (1906); A. Keith, Human Embryology and Morphology (1905); H. Gadow, " Modifications of the first and second Visceral Arches," Phil. Trans. vol. 179 (1888), and " The Evolution of the Auditory Ossicles," Anat. Anzeiger, Bd. xix. (1901).

In the Amphioxus the pharynx is stiffened by chitinous bars which lie between the gill slits, but it is unlikely that FIG. 33. - Diagram to show the fate of the Visceral Arches in man and (with modifications) other mammals. Membrane bones white. Cartilage and cartilage bones black. Cartilage which has degenerated into ligaments dotted.

S.H Stylohyal (styloid process). E.H Occasional epihyal cartil age or bone in stylohyoid ligament.

these are really homologous with the visceral skeleton of higher forms, though, in serving the same purpose, they are certainly analogous.

Among the Cyclostomata (hags and lampreys) there is an arrangement known as the " branchial basket," which has a more superficial position than the visceral arches of fish and probably corresponds to the extra-branchials of those vertebrates. The oral and hyoid arches are very rudimentary and probably have degenerated in consequence of the suctorial mode of nourishment. In the Elasmobranchii (sharks and rays) the visceral skeleton is entirely cartilaginous. In the more primitive types such as the comb-toothed shark (Notidanus) the oral and hyoid arches are quite distinct. The oral arch consists of the upper jaw, or palato-quadrate cartilage, and the lower jaw, or Meckel's cartilage; these articulate with one another posteriorly and also with the skull. Behind these and distinct from them is the hyoid arch. Such a type of suspensorium or jaw articulation is called autostylic. In the rays, on the other hand, the oral arch is connected with the skull by the proximal segment of the hyoid arch, which, since it connects both the hyoid and mandibular (oral) arches with the skull, is called the hyomandibular cartilage. This type of suspensorium is termed hyostylic. Below the hyomandibular cartilage the hyoid arch has two other 1st. Arch .?Ca? H l }B.H. 2nd. Arch 3rd. Arch 4th. Arch sth. Arch Th.H.

Mand. Mandible surrounding Meckel's cartilage (black). L.I.L Long internal lateral liga ment.

FIG. 34. - Longitudinal and Vertical Section of the Skull of a Dog (Canis familiaris), with mandible and hyoid arch.

segments, the ceratohyal laterally and the basihyal ventrally where it fuses with its fellow of the opposite side. Sometimes an epihyal intervenes between the hyomandibular and the ceratohyal. Behind the hyoid arch are usually five branchial arches, though in Heptanchus there are as many as seven. These are divided into a number of segments, and outside these there is often another series of arches called extra-branchials which are probably homologous with the branchial basket of the Cyclostomata.

The chimaeroid fishes are called Holocephali because in them the palato-quadrate bar is fused with the rest of the skull. In the bony ganoids and teleosteans (Teleostomi) the palato-quadrate bar ossifies to form the palatine, ecto-, mesoand meta-pterygoids and quadrate bones from before backward, while outside these is another row of dermal bones formed by the premaxilla, maxilla and jugal or malar. In the lower jaw, Meckel's cartilage is ossified at its proximal end to form the articular bone, but distally it remains and is partly encased by the dentary, and more posteriorly by the angular, both of MT ET Na ME CE Fr Pa IP SO ExO BO Per BS Pt AS OS which are membrane bones. The jaw joint therefore is between the quadrate and the articular. In comparing this description with the section on human embryology it will be seen that certain bones, like the palate and pterygoids, which in the fish are ossifications in cartilage, become in the higher vertebrates membrane bones, and so it is clear that too great stress must not be laid on the histological history of a bone in determining its morphological significance.

The branchial arches of the Teleostomi closely resemble those of the Elasmobranchii except that they are ossified and that the extrabranchials have disappeared.

In the Dipnoi (mudfish) the suspensorium is autostylic, and either five or six branchial arches are present. In the Amphibia, too, the suspensorium is autostylic, the palato-quadrate bar remains largely cartilaginous, though its posterior part is often ossified to form the quadrate. The membranous premaxilla, maxilla, palatine, pterygoid, quadratojugal and squamosal bones are developed in connexion with it, though it is interesting to notice that the pterygoid is sometimes partly cartilaginous and the quadrato-jugal is absent in the tailed forms (Urodela). In the lower jaw a splenial element has appeared, and in the frog a cartilaginous mento-meckellian bone develops close to the symphysis. In the larval stages there are rudiments of four branchial arches behind the hyoid, but in the adult these are reduced in the Anura and their ventral ends are united into a broad basilingual plate.

In the Reptilia the site of the palato-quadrate bar is surrounded by the same series of bones that are found in the Amphibia, but in lizards and chelonians a Para-quadrate bone is found which, according to E. Gaupp, is the precursor of the tympanic ring of mammals. In the crocodiles the maxilla and palate grow inwards to meet one another and so form a hard palate. The mandible has dentary, splenial, angular, surangular, articular and coronoid ossifications and in some cases a mento-meckellian as well. The quadrate bone with which it still articulates is becoming included in the wall of the tympanic cavity, and, according to H. Gadow, it is this bone and not the para-quadrate which will become the tympanic of mammals. The hyoid arch is sometimes suppressed in snakes, but in Sphenodon its continuity with the columella or stapes can be demonstrated.

The branchial skeleton is reduced with the cessation of branchial respiration and only the ventral parts of two arches can be seen; these unite to form a plate with the hyoid (basihyobranchial) and with this the glottis is closely connected. In birds the morphology of the visceral skeleton is on the reptilian plan, and, although the modifications are numerous, they are not of special interest in elucidating the problems of human morphology.

kt, In the Mammalia the premaxilla, maxilla, palate and pterygoid bones can be seen in connexion with the region where the palatoquadrate cartilage lay in the lower Vertebrata (see fig. 34). The premaxilla bears the incisor teeth, and except in man the suture between it and the maxilla is evident on the face if a young enough animal be looked at. The maxilla bears the rest of the teeth and articulates laterally with the jugal or malar, which in its turn articulates posteriorly with the zygomatic process of the squamosal, so that a zygomatic arch, peculiar to mammals, is formed. Both the maxilla and palate form the hard palate as in crocodiles, though the pterygoid bone does not do so but fuses with the sphenoid to form the internal pterygoid plate (see fig. 34, Pt). The mandible no longer articulates with the quadrate but forms a new articulation, by means of the condyle, with the glenoid cavity of the squamosal, and many modern morphologists, including the writer, are inclined to agree with H. Gadow that the quadrate has probably become the tympanic bone. In many mammals (e.g. Carnivora) this bone swells out to form the bulla tympani. The derivation of the auditory ossicles has been discussed in the section on embryology as well as in the article EAR. The presence of a chain of ossicles is peculiar to the Mammalia.

In many of the lower mammals (e.g. Ungulata and Carnivora) the hyoid arch is much more completely ossified than it is in man, tympano-, stylo-, epi-, cerato- and basihyal elements all being bony (see fig. 34). It is of interest to notice that in the hares and rabbits the body of the hyoid has occasionally been found in two pieces, indicating its derivation from the second and third visceral arches. The fourth and fifth arches, which form the thyroid cartilage in mammals, are considered in the article Respiratory System.

,For further details see S. H. Reynolds, The Vertebrate Skeleton (Cambridge, 1897); W. Flower, Osteology of the Mammalia (London, 1885); R. Wiedersheim, Comparative Anatomy of Vertebrates, adapted and translated by W. N. Parker (London, 1907); C. Gegenbaur, Vergleich. Anat. der Wirbeltiere, Bd. i.