Muscular_System

Muscular System (Anatomy 1). (For Physiology, see Muscle And Nerve.)

The muscular tissue (Lat. musculus, from a fancied resemblance of certain muscles to a little mouse) is of three kinds: (1) voluntary or striped muscle; (2) involuntary or unstriped muscle, found in the skin, walls of hollow viscera, coats of blood and lymphatic vessels, &c.; ( 3) heart muscle. The microscopical differences of these different kinds are discussed in the article on Connective Tissues. Here only the voluntary muscles, which are under the control of the will, are to be considered.

## Characteristics

The voluntary muscles form the red flesh of an animal, and are the structures by which one part of the body is moved at will upon another. Each muscle is said to have an origin and an insertion, the former being that attachment which is usually more fixed, the latter that which is more movable. This distinction, however, although convenient, is an arbitrary one, and an example may make this clear. If we take the pectoralis major, which is attached to the front of the chest on the one hand and to the upper part of the arm bone on the other, the effect of its contraction will obviously be to draw the arm towards the chest, so that its origin under ordinary circumstances is said to be from the chest while its insertion is into the arm; but if, in climbing a tree, the hand grasps a branch above, the muscular contraction will draw the chest towards the arm, and the latter will then become the origin. 

Generally, but not always, a muscle is partly fleshy and partly tendinous; the fleshy-contractile part is attached at one or both ends to cords or sheets of white fibrous tissue, which in some cases pass round pullies and so change the direction of the muscle's action. The other end of these cords or tendons is usually attached to the periosteum of bones, with which it blends. In some cases, when a tendon passes round a bony pulley, a sesamoid bone is developed in it which diminishes the effects of friction. A good example of this is the patella in the tendon of the rectus femoris (fig. r, P.).

## Muscular Actions

Every muscle is supplied with blood vessels and lymphatics (fig. i, v, a, 1), and also with one or more nerves. The nerve supply is very important both from a medical and a morphological point of view. The approximate attachments are also important, because unless they are realized the action of the muscle cannot be understood, but the exact attachments are perhaps laid too great stress on in the anatomical teaching of medical students. The study of the actions of muscles is, of course, a physiological one, but teaching the subject has been handed over to the anatomists, and the results have been in some respects unfortunate. Until very recently the anatomist studied only the dead body, and his one idea of demonstrating the action of a muscle was to expose and then to pull it, and whatever happened he said was the action of that muscle. It is now generally recognized that no movement is so simple that only one muscle is concerned in it, and that what a muscle may do and what it really does do are not necessarily the same thing. As far as the deeper muscles are concerned, we still have only the anatomical method to depend upon, but with the superficial muscles it should be checked by causing a living person to perform certain movements and then studying which muscles take part in them.

For a modern study of muscular actions, see C. E. Beevor's Croonian Lectures for 1903 (London, 1904).

## Muscle Shapes

Muscles have various shapes: they may be fusiform, as in fig. 1, conical, riband-like, or flattened into triangular or quadrilateral sheets. They may also be attached to skin, cartilage or fascia instead of to bone, while certain muscles surround openings which they constrict and are called sphincters. The names of the muscles have gradually grown up, and no settled plan has been used in giving them. Sometimes, as in the coraco-brachialis and thyro-hyoid, the name describes the origin and insertion of the muscle, and, no doubt, for the student of human anatomy this is the most satisfactory plan, since by learning the name the approximate attachments are also learnt. Sometimes the name only indicates some peculiarity in the shape of the muscle and gives no clue to its position in the body or its attachments; examples of this are biceps, semitendinosus and pyriformis.  Sometimes, as in the flexor carpi ulnaris and corrugator supercilii, the use of the muscle is shown. At other times the position in the body is indicated, but not the attachments, as in the tibialis anticus and peroneus longus, while, at other times, as in the case of the pectineus, the name is only misleading. Fortunately the names of the describers themselves are very seldom applied to muscles; among the few examples are Horner's Muscle and the muscular band of Treitz.

The German anatomists at the Basel conference lately proposed a uniform Latin and Greek nomenclature, which, though not altogether satisfactory, is gaining ground on the European continent. As there are some four hundred muscles on each side of the body it will be impossible here to attempt more than a mere sketch of them; for the details the anatomical textbooks must be consulted.

## Muscles Of The Head And Face

(See fig. 2). - The scalp is moved by a large flat muscle called the occipito frontalis, which has two muscular bellies, the occipitalis and frontalis, and an intervening epicranial aponeurosis; this muscle moves the scalp and causes the transverse wrinkles in the forehead. The anterior, posterior and superior auricular muscles are present but are almost functionless in man. The orbicularis palpebrarum forms a sphincter round the eyelids, which it closes, though there is little doubt that parts of the muscle can act separately and cause various expressions. The side of the nose has several muscles, the - actions of which are indicated by their names; they are the compressor, two dilatores and the depressor alae nasi, while the levator labii superioris et alae nasi sometimes goes to the nose. Raising the upper lip, in addition to the last named, are the levator labii superioris proprius and the levator anguli oris, while the zygomaticus major draws the angle of the mouth outward. The lower lip is depressed by the depressor labii inferioris and depressor anguli oris, while the orbicularis oris acts as a sphincter to the mouth.

The buccinator muscle in the substance of the cheeks rises from the upper and lower jaws and runs forward to blend with the orbicularis oris. All the foregoing are known as muscles of expression and all are supplied by the seventh or facial nerve. The temporal muscle  at the side of the cranium (fig. 3) and the masseter (fig. 2), which rises from the zygoma, close the mouth, since both are inserted into the ramus of the mandible; while, rising from the pterygoid plates, are the external and internal pterygoid muscles (fig. 3), the former of which pulls forward the condyle, and so the whole mandible, while the latter helps to close the mouth by acting on the angle of the lower jaw. This group of muscles forms the masticatory set, all of which are supplied by the third division of the fifth nerve. For the muscles of the orbit, see Eye; for those of the soft palate and pharynx, see Pharynx; and for those of the tongue, see Tongue.

## Muscles Of The Neck

(See fig. 4). - Just below the mandible is the digastric, which, as its name shows, has two bellies and a central tendon; the anterior belly, supplied by the fifth nerve, is attached to the mandible near the symphysis, the posterior supplied by the seventh of the mastoid process, while the central tendon is bound to the hyoid bone. Stretching across from one side of the lower jaw to the other and forming a floor to the mouth is the mylo-hyoid muscle;  posteriorly this reaches the hyoid bone, and in the mid-line has a tendinous raphe separating the two halves of the muscle. Rising from the manubrium sterni and inner part of the clavicle is the sterno-cleido-mastoid, which is inserted into the mastoid process and superior curved lines of the occipital bone; when it contracts it makes the face look over the opposite shoulder, and it is supplied by the spinal accessory nerve as well as by branches from the cervical plexus. It is an important surgical landmark, and forms a diagonal across the quadrilateral outline of the side of the neck, dividing it into an anterior triangle with its apex downward and a posterior with its apex upward. In the anterior triangle the relative positions of the hyoid bone, thyroid cartilage and sternum should be realized, and then the hyo-glossus, thyro-hyoid, sterno-hyoid and sterno-thyroid muscles are explained by their names. The omo-hyoid  muscle rises from the upper border of the scapula and runs both triangles to the hyoid bone Where it passes deep to the sterno-mastoid it has a central tendon which is bound to the first rib by a loop of cervical fascia. Rising from the styloid process are three muscles, the stylo-glossus, stylo-hyoid and stylo-pharyngeus, the names of which indicate their attachments. Covering these muscles of the anterior triangle is a thin sheet, close to the skin, called the platysma, the upper fibres of which run back from the mouth over the cheek and are named the risorius (fig. z); this sheet is one of the few remnants in man of the skin musculature or panniculus carnosus of lower Mammals. With regard to the nerve supply of the anterior triangle muscles, all those which go to the tongue are supplied by the hypoglossal or twelfth cranial nerve, while the muscles below the hyoid bone are apparently supplied from this nerve but really from the upper cervical nerves. 

The posterior triangle is formed by the sterno-mastoid in front, the trapezius behind, and the clavicle below; in its floor from above downward part of the following muscles are seen: complexus, splenius, levator anguli scapulae, scalenus medius and scalenus anticus. Sometimes a small piece of the scalenus posticus is caught sight of behind the scalenus medius. The splenius rotates the head to its own side, the levator anguli scapulae  raises the upper angle of the scapula, while the three scalenes run from the transverse processes of the cervical vertebrae and fix or raise the upper ribs. The trapezius (fig. 5) arises from the spines of the thoracic vertebrae and the ligamentum nuchae, and is inserted into the outer third of the clavicle and the spine of the scapula; it is used in shrugging the shoulders and in drawing the upper part of the scapula toward the mid-dorsal line. Its nerve supply is the spinal accessory and third and fourth cervical nerves. When the superficial muscles and complexus are removed from the back of the neck, the sub-occipital triangle is seen beneath the occipital bone. Externally it is bounded by the superior oblique, running from the transverse process of the atlas to the lateral part of the occipital bone, internally by the rectus capitis posticus major, passing from the spine of the axis to the lateral part of the occipital bone, and inferiorly by the inferior oblique joining the spine of the axis to the transverse process of the atlas. These muscles move the head on the atlas and the atlas on the axis. They are supplied by the posterior branch of the first cervical nerve.

## Muscles Of The Trunk

  


The trapezius has already been described as a superficial muscle of the upper part of the back; in the loin region the latissimus dorsi (fig. 5) is the superficial muscle, its origin being from the lower thoracic spines, lower ribs and lumbar fascia, and it is inserted into the upper part of the arm bone or humerus. When the trapezius is cut, the rhomboid muscles (major and minor) passing from the upper thoracic spines to the vertebral border of the scapula are seen, and deep to these is the serratus posticus superior passing from nearly the same spines to the upper ribs. On reflecting the latissimus dorsi the serratus posticus inferior  is seen running from the lower thoracic spines to the lower ribs. When these muscles are removed the great mass of the erector spinae  is exposed, familiar to every one as the upper cut of the sirloin or ribs of beef; it runs all the way up the dorsal side of the vertebral column from the pelvis to the occiput, the complexus already mentioned being its extension to the head. It is longitudinally segmented into many different bundles to which special names are given, and it is attached to the various vertebrae and ribs as it goes up, thus straightening the spinal column. Deep to the erector spinae are found shorter bundles passing from one vertebra to another and forming the semispinalis and multifidus spinae muscles. The latissimus dorsi and rhomboids are supplied by branches of the brachial plexus of nerves, while the deeper muscles get their nerves from the posterior primary divisions of the spinal nerves. On the anterior part of the thoracic region the pectoralis major runs from the clavicle, sternum and ribs, to the humerus (fig. 6); deep to this is the pectoralis minor, passing from the upper ribs to the coracoid process. The serratus magnus is a large muscle rising by serrations from the upper eight ribs, and running back to the vertebral border of the scapula, which it draws forward as in the fencer's lunge. Between the ribs are the external and internal intercostal muscles; the former beginning at the tubercle and ending at the junctions of the ribs with their cartilages, while the latter only begin at the angle of the ribs but are prolonged on to the sternum, so that an interchondral as well as an intercostal part of each muscle is recognized. The fibres of the external intercostals run downward and forward, those of the internal downward and backward. The abdominal walls are formed of three sheets of muscle, of which the most superficial or external oblique (fig. 6) is attached to the outer surfaces of the lower ribs; its fibres run downward and forward to the pelvis and mid-line of the abdomen, the middle one or internal oblique is on the same plane as the ribs, and its fibres run downward and backward, while the transversalis  is attached to the deep surfaces of the ribs, and its fibres run horizontally forward. Below, all these muscles are attached to the crest of the ilium and to Poupart's ligament, which is really the lower free edge of the external oblique, while, behind, the two deeper ones, at all events, blend with the fascia lumborum. As they approach the mid-ventral line they become aponeurotic and form the sheath of the rectus. The rectus abdominis (fig. 6) is a flat muscular band which runs up on each side of the linea alba or mid-ventral line of the abdomen from the pubis to the ribs and sternum. This muscle has certain tendinous intersections or lineae transversae, the positions of which are noticed in the article Anatomy (Superficial and Artistic), and the morphology of which is referred to later. In front of the lowest part of the rectus is sometimes a small triangular muscle called the pyramidalis. The quadratus lumborum is a muscle at the back of the abdominal wall which runs between the last rib and the crest of the ilium. In front of the bodies of the vertebrae is a prevertebral or hypaxial musculature, of which the rectus capitis anticus major and minor muscles and longus colli in the neck and the psoas in the loins form the chief parts, the latter being familiar as the undercut of the sirloin of beef, while the pelvis is closed below by a muscular floor formed by the levator ani and coccygeus muscles. The diaphragm is explained in a separate article.

## Muscles Of The Upper Extremity

Thedeltoid (see figs.7 and 8) is the muscle which forms the shoulder cap and is used in abducting the arm to a right angle with the trunk; it runs from the clavicle, acromial process and spine of the scapula, to the middle of the humerus, and is supplied by the circumflex nerve. Several short rotating muscles pass from the scapula to the upper end of the humerus; these are the subscapularis passing in front of the shoulder joint, the supraspinatus above the joint, and the infraspinatus and Ceres minor behind. The teres major (fig. 5) comes from near the lower angle of the scapula, and is inserted with the latissimus dorsi into the front of the surgical neck of the humerus. The coracobrachialis (fig. 7) passes from the coracoid process to the middle of the humerus in front of the shoulder joint, while the brachialis anticus passes in front of the elbow from the humerus to the coronoid process of the ulna. Passing in front of both shoulder and elbow is the biceps (fig. 7), the long head of which rises from the top of the glenoid cavity inside the joint, while the short head comes from the coracoid process. The insertion is into the tubercle of the radius. 

These three muscles are all supplied by the same (musculo-cutaneous) nerve. At the back of the arm is the triceps (fig. 8) which passes behind both shoulder and elbow joints and is the great extensor muscle of them; its long head rises from just below the glenoid cavity of the scapula, while the inner and outer heads come from the back of the humerus. It is inserted into the olecranon process of the ulna and is supplied by the musculo-spinal nerve. The muscles of the front of the forearm form superficial and deep sets (see fig. 7). Most of the superficial muscles come from the internal condyle of the humerus. From without inward they are the pronator radii teres going to the radius, the flexor carpi radialis to the base of the index metacarpal bone, the palmaris longus to the palmar fascia, the flexor sublimis digitorum to the middle phalanges of the fingers, and the flexor carpi ulnaris to the pisiform bone. 

The important points of practical interest about these muscles are noticed in the article Anatomy (Superficial and Artistic). In addition to these the brachio-radialis is a flexor of the forearm, though it arises from the outer supracondylar ridge of the humerus. It is supplied by the musculo-spiral nerve, the flexor carpi ulnaris by the ulnar, the rest by the median. The deep muscles of the front of the forearm consist of the flexor longus pollicis running from the radius to the terminal k phalanx of the thumb, while passing across from the lower third of the ulna to the same amount of the radius. 

These three muscles are supplied by the anterior interosseous branch of the median nerve, but the flexor profundus digitorum has an extra twig from the ulnar. The extensor muscles at the back of the forearm are also divided into superficial and deep sets (see fig. 8). 

The former rise from the region of the external condyle of the humerus, and consist of the extensor carpi radialis longior and brevior inserted into the index and medius metacarpal bones, the extensor communis digitorum to the middle phalanges of the fingers, the extensor minimi digiti, the extensor carpi ulnaris passing to the metatarsal bone of the minimus, and the supinator brevis wrapping round the neck of the radius to which it is inserted. The aconeus which runs from the external condyle to the olecranon process is really a part of the triceps. The deep muscles rise from the posterior surfaces of the radius and ulna, and are the extensor ossis metacarpi pollicis, the name of which gives its insertion, the extensor brevis pollicis to the proximal phalanx, and the extensor longus pollicis to the distal phalanx of the thumb, the extensor indicis joins the extensor communis slip to the index finger; all these posterior muscles are supplied by the posterior interosseous nerve. In front and behind the wrist the tendons are bound down by the anterior and posterior annular ligaments, while on the flexor surface of each finger is a strong fibrous sheath or theca  for the flexor tendons. The ball of the thumb is occupied by short muscles called the thenar group, while hypothenar muscles are found in the ball of the little finger. 

The four lumbrical muscles (fig. 9, d)  run from the flexor profundus digitorum tendons to those of the extensor communis between the heads of the metacarpal bones, while, rising from the shafts of these bones, are the three palmar  and four dorsal interosseous muscles (fig. 9, e) which also are inserted into the extensor tendons. The two outer lumbricals and the thenar muscles are supplied by the median nerve; all the other hand muscles by the ulnar.

## Muscles Of The Lower Extremity

On the front of the thigh the quadriceps extensor muscles are the most important: there are four of these, the rectus femoris (fig. I) with its straight and reflected heads rising from just above the acetabulum, the crureus, deep to this, from the front of the femur, and the vastus externus and internus  wrapping round the femur on each side from the linea aspera. All these are inserted into the patella, or rather the patella is a sesamoid bone developed where their common tendon passes round the lower end of the femur when the knee is bent. The distal part of this tendon, which passes from the patella to the tubercle of the tibia, is the ligamentum patellae. The sartorius is a long riband-like muscle running from the anterior superior spine of the ilium to the inner surface of the tibia, obliquely across the front of the thigh. It forms the outer boundary of Scarpa's triangle, the inner limit of which is the adductor longus and the base Poupart's ligament. The floor is formed by the iliacus from the iliac fossa of the pelvis, which joins the psoas, to be inserted with it into the lesser trochanter, and by the pectineus running from the upper ramus of the pubis to just below the insertion of the last muscles. The adductor muscles, longus, brevis and magnus, all rise from the subpubic arch, and are inserted into the linea aspera of the femur, so that they draw the femur toward the middle line. The gracilis (fig. 10) is part of the adductor mass, though its insertion is into the upper part of the tibia. The extensor muscles of the front of the thigh are supplied by the anterior crural nerve, but the adductor group on the inner side from the obturator. The pectineus is often supplied from both sources. 

On the back of the thigh the gluteus maximus (figs. 5 and 10) plays an important part in determining man's outline (see Anatomy: Superficial and Artistic). It rises from the sacral region, and is inserted into the upper part of the femur and the deep fascia of the thigh, which is very thick and is known as the fascia lata;  the muscle is a great extensor of the hip and raises the body from the stooping position. The gluteus medius rises from the ilium, above the hip joint, and passes to the great trochanter; it abducts the hip and enables the body to be balanced on one leg, as in taking a step forward. The gluteus minimus is covered by the last muscle, and passes from the ilium to the front of the great trochanter, thus rotating the hip joint inward. Some of its anterior fibres are sometimes separate from the rest, and are then called the scansorius (see Joints). When the gluteus maximus is removed, a number of short externally rotating muscles are seen, rising from the pelvis and inserted into the great trochanter (fig. to); these are, from above downward, the pyriformis, gemellus superior, obturator internus, gemellus inferior  and quadratus femoris. They are all supplied by special branches of the sacral plexus. On cutting the quadratus femoris a good deal of the obturator externus can be seen, coming from the outer surface of the obturator membrane and passing to the digital fossa of the great trochanter. Unlike the rest of this group, it is supplied by the obturator nerve. Coming from the anterior part of the crest of the ilium is the tensor fasciae femoris, which is inserted into the fascia lata, as is part of the gluteus maximus, and the thickened band of fascia which runs down the outer side of the thigh from these to the head of the tibia is known as the ilio tibial band. The tensor fasciae femoris, gluteus medius and minimus, are supplied by the superior gluteal nerve, the gluteus maximus by the inferior gluteal. At the back of the thigh are the hamstrings rising from the tuberosity of the ischium (fig. 10); these are the semimembranosus and semitendinosus, passing to the inner part of the upper end of the tibia and forming the internal hamstrings, and the biceps femoris or external hamstring, which has an extra head from the shaft of the femur and is inserted into the head of the fibula. These muscles are supplied by the great sciatic nerve and extend the hip joint while they flex the knee. In the leg, as distinguished from the thigh, are three groups of muscles, anterior, external and posterior. The anterior group (fig. 11) all come from the front of the tibia and fibula, and consist of the extensor longus digitorum, extending the middle and distal phalanges of the four outer toes, the extensor proprius hallucis, extending the big toe, and the peroneus tertius, a purely human muscle inserted into the base of the fifth metatarsal bone. All these are supplied by the anterior tibial nerve.

The external group comprises the peroneus longus and brevis, rising from the outer surface of the fibula and inserted into the tarsus (fig. 11), the longus tendon passing across the sole to the base of the first metatarsal bone, the brevis to the base of the fifth metatarsal. These are supplied by the musculo-cutaneous nerve.

The posterior group is divided into a superficial and a deep set. The superficial is composed of the gastrocnemius, the two heads of which rise from the two condyles of the femur, the soleus, which rises from the upper parts of the back of the tibia and fibula, the plantaris, which comes from just above the external condyle of the femur, and the popliteus which, although on a deeper plane, really belongs to this group and rises by a tendon from the outer condyle while its fleshy part is inserted into the upper part of the back of the tibia. The gastrocnemius and soleus unite to form the tendo Achillis, which is attached to the posterior part of the calcaneum, while the plantaris runs separately as a very thin tendon to the same place. These muscles are supplied by the internal popliteal nerve. The deep set is formed by three muscles which rise from the posterior surfaces of the tibia and fibula, the flexor longus digitorum, the tibialis posticus,(short head) Tibial nerve Biceps (along with peroneal nerve) Gastrocnemius and the flexor longus hallucis from within outward. Their tendons all pass into the sole, that of the flexor longus digitorum being inserted into the terminal phalanges of the four outer toes, the flexor longus hallucis into the terminal phalanx of the big toe, while the tibialis posticus sends expansions to most of the tarsal bones. 

The nerve supply of this group is the posterior tibial. On the dorsum of the foot is the extensor brevis digitorum (fig. 11), which helps to extend the four inner toes, while in the sole are four layers of short muscles, the most superficial of which consists of the abductor hallucis, the flexor brevis digitorum, and the abductor minimi digiti, the names of which indicate their attachments. The second layer is formed by muscles which are attached to the flexor longus digitorum tendon; they are the accessorius, running forward to the tendon from the lower surface of the calcaneum, and the four lumbricales, which rise from the tendon after it has split for the four toes and pass between the toes to be inserted into the tendons of the extensor longus digitorum on the dorsum. The third layer comprises the flexor brevis hallucis, adductor obliquus and adductor transversus hallucis and the flexor brevis minimi digiti. The fourth layer contains the three plantar and four dorsal interosseous muscles, rising from the metatarsal bones and inserted into the proximal phalanges and extensor tendons in such a way that the plantar muscles draw the toes towards the line of the second toe while the dorsal draw them away from that line. Of these sole muscles the flexor brevis digitorum, flexor brevis hallucis, abductor hallucis and the innermost lumbrical are supplied by the internal plantar nerve, while all the rest are supplied by the external plantar.

  


  


## Embryology

The development of the muscular system is partly known from the results of direct observation, and partly inferred from the study of the part of the nervous system whence the innervation is derived. The unstriped muscle is formed from the mesenchyme cells of the somatic and splanchnic layers of the mesoderm (see Embryology), but never, as far as we know, from the mesodermic somites. The heart muscle is also developed from mesenchymal cells, though the changes producing its feebly striped fibres are more complicated. The skeletal or real striped muscles are derived either from the mesodermic somites or from the branchial arches. 

As the mesodermic somites are placed on each side of the neural canal in the early embryo, it is obvious that the greater part of the trunk musculature spreads gradually round the body from the dorsal to the ventral side and consists of a series of plates called myotomes (fig. 12). The muscle fibres in these plates run in the long axis of the embryo, and are at first separated from those of the two neighbouring plates by thin fibrous intervals called myocommata. In some cases these myocommata persist and even become ossified, as in the ribs, but more usually they disappear early, and the myotomes then unite with one another to form a great muscular sheet. In the whole length of the trunk a longitudinal cleavage at right angles to the surface occurs, splitting the musculature into a dorsal and ventral part, supplied respectively by the dorsal and ventral primary divisions of the spinal nerves. From the dorsal part the various muscles of the erector spinae series are derived by further longitudinal cleavages either tangential or at right angles to the surface, while the ventral part is again longitudinally split into mesial and lateral portions. A transverse section of the trunk at this stage, therefore, would show the cut ends of three longitudinal strips of muscle: (I) a mesial ventral, from which the rectus, pyramidalis sterno-hyoid, omohyoid and sterno-thyroid muscles are derived; (2) a lateral ventral, forming the flat muscles of the abdomen, intercostals and part of the sternomastoid and trapezius; and (3) the dorsal portion already noticed. 

The mesial ventral part is remarkable for the persistence of remnants of myocommata in it, forming the lineae transversae of the rectus and the central tendon of the omo-hyoid. The lateral part in the abdominal region splits tangentially into three layers, Upper portion of anterior annular ligament Lower portion of anterior annula ligament Tendon the external and internal oblique and the transversalis, the fibres of which become differently directed. In the thoracic region the intercostals probably indicate a further tangential splitting of the middle or internal oblique layer, because the external oblique is continued headward superficially to the ribs and the transversalis deeply to them. The more cephalic part of the external oblique layer probably disappears by a process of pressure or crowding out owing to the encroachment of the serratus magnus, a muscle which its nerve supply indicates is derived from the lower cervical myotomes. The deeper parts of the lateral mass of muscles spread to the ventral surface of the bodies of the vertebrae, and form the hypaxial muscles - such as the psoas, longus colli and recti capitis antici. The nerve supply indicates that the lowest myotomes taking part in the formation of the abdominal walls are those supplied by the first and second lumbar nerves, and are represented by the cremaster muscle in the scrotum. In the perineum, however, the third and fourth sacral myotomes are represented, and these muscles are differentiated largely from the primitive sphincter which surrounds the cloacal orifice, though partly from vestigial tail muscles (see P. Thompson, Journ. Anat. and Phys., vol. xxxv; and R. H. Paramore, Lancet, May 21, 1910). 

In the head no distinct myotomes have been demonstrated in the mammalian embryo, but as they are present in more lowly vertebrates, it is probable that their development has been slurred over, a process often found in the embryology of the higher forms. Probably nine cephalic myotomes originally existed, of which the first gives rise to the eye muscles supplied by the third nerve, the second to the superior oblique muscle supplied by the fourth nerve, and the third to the external rectus supplied by the sixth nerve. The fourth, fifth and sixth myotomes are suppressed, but the seventh, eighth and ninth possibly form the muscles of the tongue supplied by the twelfth cranial nerve.

Turning now to the branchial arches, the first branchiomere is innervated by the fifth cranial nerve, and to it belong the masseter, temporal, pterygoids, anterior belly of the digastric, mylo-hyoid, tensor tympani and tensor palati, while from the second branchiomere, supplied by the seventh or facial nerve, all the facial muscles of expression and the stylo-hyoid and posterior belly of the digastric are derived, as well as the platysma, which is one of the few remnants of the panniculus carnosus or skin musculature of the lower mammals. From the third branchiomere, the nerve of which is the ninth or glossopharyngeal, the stylo-pharyngeus and upper part of the pharyngeal constrictors are formed, while the fourth and fifth gill arches give rise to the muscles of the larynx and the lower part of the constrictors supplied by the vagus or tenth nerve. 

It is possible that parts of the sterno-mastoid and trapezius are also branchial in their origin, since they are supplied by the spinal accessory or eleventh nerve, but this is unsettled. The limb musculature is usually regarded as a sleeve-like outpushing of the external oblique stratum of the lateral ventral musculature of the trunk, and it is believed that parts of several myotomes are in this way pushed out in the growth of the limb bud. This process actually occurs - in the lower vertebrates, and the nerve supplies provide strong presumptive evidence that this is the real phylogenetic history of the higher forms, though direct observation shows that the limb muscles of mammals are formed from the central mesoderm of the limb and at first are quite distinct from the myotomes of the trunk. A possible explanation of the difficulty is that this is another example of the slurring over of stages in phylogeny, but this is one of many, obscure morphological points. The muscles of each limb are divided into a dorsal and ventral series, supplied by dorsal and ventral secondary divisions of the nerves in the limb plexuses, and these correspond to the original position of the limbs as they grow out from the embryo, so that in the upper extremity the back of the arm, forearm and dorsum of the hand are dorsal, while in the lower the dorsal surface is the front of the thigh and leg and the dorsum of the foot.

For further details see Development of the Human Body, by J. P. McMurrich (London, 1906), and the writings of L. Bolk, Morphol. Jahrb. vols. xxi-xxv.

## Comparative Anatomy

In the acrania (e.g. amphioxus) the simple arrangement of myotomes and myocommata seen in the early human embryo is permanent. The myotomes or muscle plates are < shaped, with their apices pointing towards the head end, each being supplied by its own spinal nerve. In the fishes this arrangement is largely persistent, but each limb of the < is bent on itself, so that the myotomes have now the shape of a 3, the central angle of which corresponds to the lateral line of the fish. In the abdominal region, however, the myotomes fuse and rudiments of the recti and obliqui abdominis muscles of higher types are seen. In other regions too, such as the fins of fish and the tongue of the Cyclostomata (lamprey), specialized muscular bundles are separated off and are coincident with the acquirement of movements of these parts in different directions. 

In the Amphibia the limb musculature becomes much more complex as the joints are formed, and many of the muscles can be homologized with those of mammals, though this is by no means always the case, while, in the abdominal region, a superficial delamination occurs, so that in many forms a superficial and deep rectus abdominis occurs as well as a cutaneus abdominis delaminated from the external oblique. It is probable that this delamination is the precursor of the panniculus carnosus or skin musculature of mammals. The branchial musculature also becomes much more complex, and the mylo-hyoid muscle, derived from the first branchial arch and lying beneath the floor of the mouth, is very noticeable and of great importance in breathing.

In the reptiles further differentiation of the muscles is seen, and with the acquirement of costal respiration the external and internal intercostals are formed by a delamination of the internal oblique stratum. In the dorsal region several of the longitudinal muscles which together make up the erector spinae are distinct, and a very definite sphincter cloacae is formed round and cloacal aperture. In mammals certain muscles vary in their attachments or presence and absence in different orders, sub-orders and families, so that, were it not for the large amount of technical knowledge required in recognizing them, they might be useful from a classificatory point of view. There is, however, a greater gap between the musculature of Man and that of the other Primates than there is between many different orders, and this is usually traceable either directly or indirectly to the assumption of the erect position.

The chief causes which produce changes of musculature are: (I) splitting, (2) fusion, (3) suppression either partial or complete, (4) shifting of origin, (5) shifting of insertion, (6) new formation, (7) transference of part of one muscle to another. In many of these cases the nerve supply gives an important clue to the change which has been effected. Splitting of a muscular mass is often the result of one part of a muscle being used separately, and a good example of this is the deep flexor mass of the forearm. In the lower mammals this mass rises from the flexor surface of the radius and ulna, and supplies tendons to the terminal phalanges of all five digits, but in man the thumb is used separately, and, in response to this, that part of the mass which goes to the thumb is completely split off into a separate muscle, the flexor longus pollicis. The process, however, is going farther, for we have acquired the habit of using our index finger alone for many purposes, and the index slip of the flexor profundus digitorum is in us almost as distinct a muscle as the flexor longus pollicis. Fusion may be either collateral or longitudinal. The former is seen in the case of the flexor carpi ulnaris. 

In many mammals (e.g. the dog), there are two muscles inserted separately into the pisiform bone, one rising from the internal condyle of the humerus, the other from the olecranon process, but in many others (e.g. man) the two muscles have fused. Longitudinal fusion is seen in the digastric, where the anterior belly is part of the first (mandibular) branchial arch and the posterior of the second or hyoid arch; in this case, as one would expect, the anterior belly is supplied by the fifth nerve and the posterior by the seventh. Partial suppression of a muscle is seen in the rhomboid sheet; in the lower mammals this rises from the head, neck and anterior (cephalic) thoracic spines, but in man the head and most of the neck part is completely suppressed. Complete suppression of a muscle is exemplified in the omo-trachelian, a muscle which runs from the cervical vertebrae to the acromian process and fixes the scapula for the strong action of the triceps in pronograde mammals; in man this strong action of the triceps is no longer needed for progression, and the fixing muscle has disappeared. Shifting of origin is seen in the short head of the biceps femoris. This in many lower mammals (e.g. rabbit) is a muscle running from the tail to the lower leg; in many others (e.g. monkeys and man) the origin has slipped down to the femur, and in the great anteater it is evident that the agitator caudae has been used as a muscle slide, because the short head of the biceps or tenuissimus has once been found rising from the surface of this muscle. Shifting of an insertion is not nearly as common as shifting of an origin; it is seen, however, in the peroneus tertius of man, in which part of the extensor longus digitorum has acquired a new attachment to the base of the fifth metatarsal bone. The new formation of a muscle is seen in the stylo-hyoideus alter, an occasional human muscle; in this the stylo-hyoid ligament has been converted into a muscle. The transference of part of one muscle to another is well shown by the human adductor magnus; here the fibres which pass from the tuber ischii to the condyle of the femur have a nerve supply from the great sciatic instead of the obturator, and in most lower mammals are a separate part of the hamstrings known as the presemimembranosus.

For further details see Bronn's Classen and Ordnungen des Thierreichs; " The Muscles of Mammals," by F. G. Parsons, Jour. Anat. and Phys. xxxii. 428; also accounts of the musculature of mammals, by Windle and Parsons, in Proc. Zool. Soc. (1894, seq.); Humphry, Observations in Myology (1874). (F. G. -P.)