Pharynx

Pharynx (Gr. 041)7 , throat), in anatomy, the cavity into which both the nose and mouth lead, which is prolonged into the oesophagus or gullet below, and from which the larynx or air tube comes off below and in front; it therefore serves as a passage both for food and air. It may be likened to an empty sack turned upside down and narrowing toward its mouth. The back and sides of the sack are formed by the three constrictor muscles of the pharynx, each of which overlaps the outer surface of the one above it, and these are lined internally by thick mucous membrane. The upturned bottom of the sack is attached firmly to the base of the skull and the internal pterygoid plates, so that this part cannot collapse, but below the anterior and posterior walls are in contact, and a transverse section of the pharynx is a mere slit.

From the front wall, on a level with the floor of the nose and roof of the mouth, a slanting shelf of muscular and glandular tissue, covered with mucous membrane, projects downward and backward into the cavity, and divides it into an upper part or naso-pharynx and a lower or oral pharynx (see fig.). This shelf is the soft palate, a.nd from the middle of its free border hangs a worm-like projection, of variable length but averaging about half an inch, the uvula.  The whole of the front wall of the naso-pharynx is wanting, and here the cavity opens into the nose through the posterior nasal apertures (see Olfactory System). On each side of the nasopharynx, and therefore above the soft palate, is the large triangular opening of the Eustachian tube through which air passes to the tympanum (see EAR). Behind this opening, and reaching up to the roof of the naso-pharynx, is a mass of lymphoid tissue, most marked in children, known as the pharyngeal tonsil. This tissue, when it hypertrophies, causes the disease known as "adenoids." From the mid-line of the roof of the pharynx a small pouch, the bursa pharyngea, best seen in childhood, projects upward, while on each side, above and behind the opening of the Eustachian tube, is a depression known as the lateral recess of the pharynx.

The oral pharynx communicates with the naso-pharynx by the pharyngeal isthmus behind the free edge of the soft palate. Above and in front it is continuous with the cavity of the mouth, and the demarcation between the two is a ridge of mucous membrane on each side running from the soft palate to the side of the tongue, and caused by the projection of the palato-glossus muscle. This is known as the anterior pillar of the fauces or anterior palatine arch. About half an inch behind this ridge is another, made by the palato-pharyngeus muscle, which gradually fades away in the side of the pharynx below. This is the posterior pillar of the fauces or posterior palatine arch, and between it and the anterior is the fossa (tonsilar sinus) in which the tonsil lies.

The Tonsil is an oval mass of lymphoid tissue covered by mucous membrane which dips in to form mucous crypts; externally its position nearly corresponds to that of the angle of the jaw. It is very vascular, deriving its blood from five neighbouring arteries. Below the level of the tonsil the anterior wall of the pharynx is formed by the posterior or pharyngeal surface of the tongue (q.v.), while below that is the epiglottis and upper opening of the larynx which is bounded laterally by the aryteno-epiglottic folds (see Respiratory System). On the lateral side of each of these folds is a pear-shaped fossa known as the sinus pyriformis. Below this the pharynx narrows rapidly until the level of the lower border of the cricoid cartilage in front and of the sixth cervical vertebra behind is reached; here it passes into the oesophagus, having reached a total length of about five inches.

The mucous membrane of the naso-pharynx, like that of the rest of the respiratory tract, is lined by ciliated columnar epithelium, but in the oral pharynx the epithelium is of the stratified squamous variety. Numerous racemose glands are present (see Epithelial Tissues), as well as patches of lymphoid tissue especially in childhood. Outside the mucous membrane and separating it from the constrictor muscles is the pharyngeal aponeurosis, which blends above with the periosteum of the base of the skull.

## Embryology

The pharynx is partly formed from the ectodermal stomatodaeal invagination (see Embryology and MouTH) and partly from the fore gut, which is the cephalic part of the entodermal mesodaeum. Up to the fifteenth day (see Mouth), the bucco-pharyngeal membrane separates these structures, and, though no vestiges of it remain, it is clear that the upper and front part of the naso-pharynx is stomatodaeal while the rest is mesodaeal. The five visceral arches with their intervening clefts or pouches surround the pharynx, and the Eustachian tube is a remnant of the first of these. The second pouch is represented in the adult by the tonsilar sinus, and until lately the lateral recess of the pharynx was looked upon as part of the same, but it has now been shown to be an independent diverticulum. The sinus pyriformis  probably represents that part of the fourth groove from which the lateral lobes of the thyroid body are derived.

The Bursa pharyngea was at one time looked upon as the place whence the pituitary body had been derived from the roof of the pharynx, but this is now disproved and its meaning is unknown. The tonsil is formed in the second branchial cleft or rather pouch, for the clefts are largely incomplete in man, about the fourth month; its lymphoid tissue, as well as that elsewhere in the pharynx, is formed from lymphocytes in the subjacent mesenchyme (see Embryology), though whether these wander in from the blood or are derived from original mesenchyme cells is still doubtful. The development of the ventral part of the pharynx is dealt with in the articles Tongue and Respiratory System.

For literature see Quain's Elements of Anatomy, vol. i. (London, 1908), and J. P. McMurrich, Development of the Human Body  (London, 1906).

## Comparative Anatomy

In the lower, water-breathing, vertebrates the pharynx is the part in which respiration occurs. The water passes in through the mouth and out through the gill slits where it comes in contact with the gills or branchiae.

The lowest subphylum of the phylum Chordata, to which the term Adelochorda is sometimes applied, contains a worm-like creature Balanoglossus, in which numerous rows of gill slits open from the pharynx, though Cephalodiscus, another member of the same subphylum, has only one pair of these.

In the subphylum Urochorda, to which the Ascidians or seasquirts belong, there are many rows of gill slits, as there are also in the Acrania, of which Amphioxus, the lancelet, is the type. In all these lower forms there are no true gills, as the blood-vessels lining the large number of slits provide a sufficient area for the exchange of gases.

In the Cyclostomata a reduction of the number of gill slits takes place, and an increased area for respiration is provided by the gill pouches lined by pleated folds of entodermal mucous membrane; these form the simplest type of true internal gills. In the larval lamprey (Ammocoetes) there are eight gill slits opening from the pharynx, but in the adult (Petromyzon) they are reduced to seven, and a septum grows forward separating the ventral or branchial part of the pharynx from the dorsal or digestive part. Both these tubes, however, communicate near the mouth.

In fishes there are usually five pairs of gill slits, though a rudimentary one in front of these is often present and is called the spiracle. Occasionally, as in Hexanchus and Heptanchus, there may be six or seven slits, and the evidence of comparative anatomy is that fishes formerly had a larger number of gill slits than at present.

In the Teleostomi, which include the bony fishes, there is an external gill cover or operculum.

In the Dipnoi or mud fish the work of the gills is shared by that of the lungs, and in the African form, Protopterus, external' gills, developed from the ectodermal parts of the gill slits, first appear. In the tailed Amphibians (Urodela) the first and fifth gill clefts are never perforated and are therefore in the same condition as all the gill clefts of the human embryo, while in the gilled salamanders (Necturus and Proteus) only two gill clefts remain patent. The gills in all the Amphibia are external and of ectodermal origin, but in the Anura (frogs and toads) these are succeeded before the metamorphosis from the tadpole stage by internal gills, which, unlike those of fish, are said to be derived from the ectoderm.

In the embryos of the Sauropsida (reptiles and birds) five gill clefts are evident, though the posterior two are seldom at any time perforated, while in the Mammalia the rudiments of the fifth cleft are no longer found in the embryo, and in man, at all events, none of them are normally perforated except that part of the first which forms the Eustachian tube. It will thus be seen that in the process of phylogeny there is a gradual suppression of the gill clefts beginning at the more posterior ones.

The soft palate is first found in crocodiles as a membranous structure, and it becomes muscular in mammals. The bursa pharyngea and pharyngeal tonsil are found in several of the lower mammals. In the sheep the latter is particularly large.

For literature and further details, see R. Wiedersheim's Comparative Anatomy of Vertebrates, translated by W. N. Parker (London, 1907); also Parker and Haswell's Zoology (London, 1897). (F. G. P.)