The greater part of the viscera are situated in the body cavity or coelom. This is divided by the diaphragm into two parts, the thoracic cavity and the abdominal cavity. Each is lined by a serous membrane, in which the part covering the outer wall of the cavity is distinguished as the parietal layer from the part covering the viscera, which is known as the visceral layer.

The thoracic cavity is bounded by the thoracic vertebrÆ, the ribs, the sternum, and the diaphragm. The cranial opening of the cavity is filled by the trachea and oesophagus as they enter from the neck region. The thoracic cavity is lined by two thin layers of tissue, the outer one of which is the fascia endothoracica, while the inner is the pleura. The fascia endothoracica is a sheet of connective tissue which lines the entire inner surface of the thoracic cavity, descending from the dorsal median line to the heart, and passing into the fibrous layer of the pericardium. The pleura is a thin membrane covering the fascia endothoracica and corresponding to the peritoneum of the abdominal cavity. It forms two sacs, the pleurÆ, lining respectively the right and left halves of the thoracic cavity. Each of these two sacs is closed, the viscera being suspended within them by folds of the membrane, so that the cavity is everywhere separated from the viscera by a sheet of the pleura. That portion of the pleura which lines the thoracic wall is known as the parietal layer; it may be divided into that covering the ribs (costal pleura), and that covering the diaphragm. That portion which covers the viscera is the visceral layer, or, since it covers chiefly the lungs, it may be distinguished as the pulmonary pleura. The medial walls of the two pleural sacs come in contact in the median plane, forming a median vertical partition passing lengthwise of the thoracic cavity. This partition is known as the mediastinal septum. The space between the two layers which make up the mediastinal septum is known as the mediastinum, or mediastinal cavity; it contains numerous organs of the thorax. Three parts are usually distinguished in this cavity: a ventral mediastinal cavity, containing chiefly blood-vessels and the thymus gland; a middle mediastinal cavity, enclosing the heart and the anterior and posterior venÆ cavÆ; and a dorsal mediastinal cavity, containing the trachea, the oesophagus, and the aorta.

The abdominal cavity lies caudad of the diaphragm; in it are sometimes distinguished the abdominal cavity proper, extending as far caudad as the cranial edge of the pubis, and the pelvic cavity, lying caudad of this, in the region surrounded by the innominate bones and the sacrum. The two cavities are not distinctly marked off, so that it is convenient to consider the abdominal cavity as undivided. Both parts are lined by the peritoneum.

The peritoneum is a thin transparent sheet of connective tissue supporting on its surface a layer of flattened epithelial cells, the peritoneal epithelium. It forms a sac which lines the entire abdominal cavity. This sac is closed in the male; in the female, however, it communicates with the exterior through the uterine (or Fallopian) tubes and uteri. All the organs of the abdominal cavity are outside the sac. In the course of their development these organs have encroached on the peritoneal sac. Each has grown against the outer wall of the sac to a greater or less extent and has forced a part of this wall ahead of it into the cavity. In some cases the encroachment has gone so far that the organ in question lies apparently within the peritoneal cavity, suspended from the wall of the sac by a fold of that wall. The wall may thus be divided into three portions. One of these, the parietal layer, lines the wall of the body cavity. The second (the mesentery in case of the alimentary canal, or a ligament in the case of another organ) suspends the organ from the body wall. Between the layers of each mesentery or ligament blood-vessels may pass to the organs. The third portion or visceral layer covers the organ in question, forming its serous covering.

The reflections of the peritoneum to form the mesenteries and ligaments may be thus described:

Caudally the peritoneum covers the whole surface of the bladder and is reflected from its ventral wall to the linea alba as the suspensory ligament of the bladder. Farther craniad the peritoneum suspends the rectum and colon from the middorsal line, forming the mesorectum and mesocolon. The mesocolon continues craniad to the level of the caudal end of the right kidney and is broadest at its cranial end. At this end the mesocolon passes into the mesentery proper, which suspends the small intestine and is very broad and much folded. Its dorsal border is attached to the median line opposite the caudal end of the kidneys and is very short compared to its very long ventral or intestinal border. Toward the cranial end the mesentery of the jejunum passes gradually into the very much shorter duodenal mesentery. This is drawn out at the caudal end of the duodenum into a fold, the duodenorenal ligament which attaches the duodenum to the kidney.

The mesogastrium or peritoneal fold for the stomach passes from that part of the median dorsal line lying between the kidneys and the diaphragm, to the greater curvature of the stomach. It does not pass directly to the stomach, but passes first ventrad of the small intestine as far as the pelvis. Thence it turns craniad to reach the greater curvature of the stomach. The fold thus formed is called the great omentum. It forms the dorsal and ventral walls of a sac, the omental sac, the cavity of which is called the lesser peritoneal cavity. The descending limb of the fold forms the dorsal wall of the sac, and its ascending limb forms the ventral wall. Each of these walls is double like a mesentery, so that the great omentum consists of four sheets of peritoneum. Between the two sheets forming the descending limb lies the left half of the pancreas, which passes thence to the right into the duodenal mesentery. A transverse fold passes from the descending limb of the omentum along the cranial border of the pancreas to the duodenal mesentery. Farther to the right the descending limb of the omentum, which is here shorter, encloses the spleen and holds it in position parallel to the greater curvature of the stomach and about one centimeter from it. This part of the great omentum is sometimes called the gastrosplenic or gastrolienal omentum.

The great omental sac communicates with the peritoneal sac by an opening, the foramen epiploicum or foramen of Winslow. This opening lies caudad and dextrad of the caudate lobe of the liver. This lobe itself lies within the sac. Along the ventral border of the foramen epiploicum pass the common bile-duct from the liver and the portal vein to the liver.

The size of the great omental sac is increased by the lesser omentum. This is a double sheet of peritoneum which stretches horizontally from the liver to the duodenum and the lesser curvature of the stomach. It covers ventrad the caudate lobe of the liver. Its right border is at the foramen epiploicum. The part of it which stretches to the duodenum is called the duodenohepatic ligament and contains the bile-ducts and the portal vein. The part that stretches to the stomach is called the gastrohepatic ligament. The peritoneum covers the caudal and cranial surfaces of the liver and is reflected to the adjacent parts to form the ligaments of the liver.

The suspensory ligament of the liver passes from the caudal surface of the diaphragm and the median ventral line for about one or two centimeters caudad of the diaphragm, to the liver, and extends between its two halves. The ligamentum teres or round ligament is the thickened free caudal border of the suspensory ligament. It is the remains of the foetal umbilical vein. From the dorsal border of the liver the peritoneum which covers its cranial surface turns ventrad onto the caudal surface of the diaphragm, while that which covers its caudal surface turns dorsad onto the caudal surface of the diaphragm to reach the dorsal body wall. Between these two sheets a small linear part of the surface of the liver is closely applied to the diaphragm without intervening peritoneum. The two sheets which bound this area constitute the coronary ligament of the liver. This ligament is broader where it passes from the left lateral lobe to the diaphragm and is called the triangular ligament. (There is perhaps a corresponding right triangular ligament, from the cranial division of the right lateral lobe.) The caudal division of the right lateral lobe is held to the kidney of that side by the hepatorenal ligament.

II. THE ALIMENTARY CANAL. APPARATUS DIGESTORIUS.

The alimentary canal may be divided into mouth, pharynx, oesophagus, stomach, small intestine, and large intestine. With these are associated certain accessory structures,—the salivary glands, the liver, and the pancreas. The spleen, though not belonging to the digestive system, is usually described in connection with it. The respiratory organs are almost throughout in close relation with the organs of the digestive system.

1. The Mouth. Cavum oris.

—The mouth cavity extends from the lips to the pharynx. It is narrower toward the lips, broadens caudad as far back as the last teeth, then becomes narrowed to form the isthmus faucium, by which it communicates with the pharynx. The mouth cavity is divisible into the vestibule of the mouth (vestibulum oris), which comprises that part outside the jaws proper, bounded externally by the lips and cheeks, and the mouth cavity proper (cavum oris proprium), which lies within the teeth. That portion of the vestibule which is bounded by the cheeks is sometimes farther distinguished as the buccal cavity. The entire mouth cavity (except the teeth) is lined by the mucous membrane or mucosa.

The lips (labia oris) are thick folds of skin bounding the entrance to the mouth cavity. The outer surface is covered with hair; the inner surface is covered with the mucous membrane. The upper lip is marked in the median line by a deep external groove which extends upward to the septum of the nose. Along the inner surface of this groove the lip is closely united to the jaw by a thick fold, the frenulum of the upper lip. For some distance on each side of the frenulum the inner surface of the lip bears numerous large papillÆ. The lower lip is also united to the jaw by a frenulum in the median line; it is again united to the jaw just caudad of the canine tooth, in the space between the latter and the first premolar. Caudad the two lips pass into each other (forming the commissura labiorum), and unite with the cheek. The muscles of the lips have been described (page 105).

The cheeks (buccÆ) in the cat are comparatively thin and small, extending from the lips caudad to the ramus of the mandible. The outer surface is covered with hair; the inner surface is smooth and somewhat folded. The buccal cavity is rather small. On the inner surface of the cheek open the duct of the parotid (Steno’s duct), the ducts of the molar gland, and that of the infraorbital gland.

The roof of the mouth cavity is formed by the hard and soft palates. The hard palate (palatum durum) forms the cranial part of the roof; it is supported by the palatal plates of the maxillary and palatine bones. The mucosa of the hard palate is elevated to form seven or eight curved transverse ridges, which are concave caudad. Between the ridges are rows of papillÆ. In front of the most cranial ridge is a papilla in the middle line, and at each side of the papilla is the opening of a small duct (the incisive duct or Stenson’s duct), which leads dorsad through the incisive foramen to the vomeronasal organ (or organ of Jacobson), which lies on the floor of the nasal cavity. Caudad of the hard palate the roof of the mouth is formed by the soft palate or velum palatinum, described below.

The floor of the mouth cavity is formed chiefly by the tongue, which extends as far caudad as the isthmus faucium. Ventrad of the free edge of the tongue the mucosa forms a prominent median vertical fold which unites the tongue with the floor of the mouth beneath it; this fold is the frenulum linguÆ. On each side of the median line at the cranial border of the floor of the mouth is a prominent papilla, at the apex of which open the ducts of the submaxillary and sublingual glands, the former on the lateral side of the apex, the latter on the medial side.

The sides of the mouth cavity are formed by the teeth and the gums, covering the alveolar borders of the mandible, maxillaries, and premaxillaries.

The mouth cavity presents further for examination the glands, the teeth, the tongue, and the soft palate.

The Glands of the Mouth (GlandulÆ oris).

—There are five pairs of salivary glands which open into the mouth cavity.

1. The parotid gland (glandula parotis) (Fig. 65, 1; Fig. 131, 10) is flattened, rather finely lobulated, and lies ventrad of the external auditory meatus and beneath the dermal muscles. Its cranial border follows the caudal border of the masseter muscle and overlies it somewhat; its caudal border is about three centimeters caudad of the border of the masseter. Its borders are unevenly lobed. The parotid duct (ductus parotideus: frequently called Stenon’s or Steno’s duct) is formed by the union of several smaller ducts near the ventral end of the cranial border of the gland. It passes craniad imbedded in the fascia covering the masseter. At the cranial border of the masseter it turns inward and lies close against the mucous membrane of the mouth, so that from the inside of the mouth it appears as a white ridge on the mucosa. It opens on the inside of the cheek opposite the most prominent cusp of the last premolar tooth. Along the course of the parotid duct in some cases one or more small accessory parotid glands are found.

2. The submaxillary gland (glandula submaxillaris) (Fig. 65, 2, page 109, and Fig. 131, 11) is approximately kidney-shaped. Its surface is nearly smooth, the lobulations not being apparent externally. It lies ventrad of the parotid, at the caudal edge of the masseter muscle, just caudad of the angular process of the mandible. The posterior facial vein (Fig. 131, b) crosses its outer surface, and its cranioventral border is hidden by two lymphatic glands (Fig. 131, 12) lying at the sides of the anterior facial vein. The submaxillary duct (ductus submaxillaris, frequently called Wharton’s duct) leaves the inner surface of the gland and passes beneath the digastric and mylohyoid muscles and against the outer surface of the styloglossus. From the point where the styloglossus passes into the tongue the duct continues craniad close against the oral mucosa and parallel to the mandibula. It is accompanied by the duct of the sublingual, which lies at first dorsad of it and then mediad. It opens at the apex of the prominent papilla which lies at the side of the middle line at the cranial end of the floor of the mouth.

3. The sublingual gland is elongated and conical in form, with its base against the submaxillary, of which it appears to be a continuation. It stretches along the submaxillary duct for about one and one-half centimeters, lying between the masseter and digastric muscles. The sublingual duct leaves its ventral side, passes close to the submaxillary duct, at first dorsad and then mediad of it, and opens on the medial side of the apex of the same papilla with the submaxillary duct.

4. The molar gland (glandula molaris) (Fig. 65, 9) lies between the orbicularis oris and the mucosa of the lower lip. It stretches from the cranial border of the masseter to a point between the first premolar and the canine. It is flat, broad caudad, and ends in a point craniad. It has several ducts which pass straight through the cheek and open on the mucous surface of the mouth.

5. The infraorbital (or orbital) gland lies in the lateral part of the orbit on its ventral floor. It is ovoid and about one and one-half centimeters long and one-third as thick. Its ventral end rests against the mucosa of the mouth just caudad of the molar tooth. Its duct leaves the ventral end and opens into the mouth at a point about three millimeters caudad of the molar tooth.

The Teeth. Dentes.

—The adult cat has thirty teeth, fourteen in the lower jaw and sixteen in the upper jaw. There are twelve incisors, four canines, ten premolars, and four molars. The tooth formula for the cat is then

i3 — 33 — 3, c1 — 11 — 1, pm3 — 32 — 2, m1 — 11 — 1.

The teeth are implanted in the alveolar borders of the premaxillaries, maxillaries, and mandible. In each tooth can be distinguished the root, imbedded in the socket of the bone, the crown, which projects above the gums, and a narrow neck connecting the two. The root is composed of one or more separate fangs; the crown bears one or more points or cusps.

At the cranial end of each jaw are six incisor teeth (Figs. 93 and 94, a). These are imbedded in the alveolar borders of the premaxillaries and the mandible. The incisor teeth are small, with a crown bearing a sharp edge which is notched so as to form three minute cusps. The root of each has a single fang. The lateral incisors are the largest in each jaw, and those of the upper jaw are larger than those of the lower.

Caudad of the incisors, and in the upper jaw separated from them by a slight interval, are the canines (b), two in each jaw. These are long, strong, pointed teeth, deeply imbedded in the mandible and maxillaries, their large sockets causing a rounded swelling on the external surface of the bones. Each has a single fang and a single cusp. When the mouth is closed the upper canines lie laterocaudad of the lower ones.

Caudad of the canines there is in each jaw a considerable interval free from teeth: this is called the diastema. Caudad of the diastema are the premolar teeth, three pairs (c, d, e) in the upper jaw and two pairs (c, d) in the lower jaw. These teeth are compressed sideways, and those of the lower jaw fit inside of those of the upper jaw. In the upper jaw (Fig. 93) the first premolar (c) is small and usually has but a single cusp and a single fang, though occasionally there is a small supplementary cusp and fang. The second premolar is larger (d); it has a large central cusp, with a single smaller cranial cusp and two small caudal cusps, making four in all. This tooth has two fangs. The third premolar (e) is the largest tooth in the jaws; it has three large cusps in longitudinal series and a small cusp lying on the medial side of the first one in the row. Its root has three fangs. The molar tooth (f) of the upper jaw is small and lies caudomediad of the last premolar. It has two small cusps and two fangs.

In the lower jaw (Fig. 94) the two premolars (c and d) are similar, the caudal one being a little larger. Each has four cusps; a single large one, a small one craniad of this, and two small ones caudad of it. Each has two fangs. The single molar (f) is the largest tooth of the lower jaw; it has two large cusps and two fangs.

The Deciduous Teeth.—At birth the cat has no teeth. There appears later a set of twenty-six teeth: twelve incisors, four canines, and ten molars (six in the upper jaw and four in the lower). These teeth are later replaced by the permanent ones above described. The deciduous teeth of the cat are fully described by Jayne (“Mammalian Anatomy,” vol. 1. p. 319), where also an account is given of the order of appearance of the teeth.

The Tongue. Lingua

(Fig. 95).—The tongue is a muscular organ covered with mucous membrane; in life it is very mobile. It is an elongated organ, flat above, broadest in the middle, and very slightly narrowed at each end. It extends from the incisor teeth to the isthmus faucium and nearly fills the mouth cavity. The caudal third of the tongue forms the floor of the mouth cavity, so that the tongue has here no ventral surface, but is directly continuous with underlying organs. It is in this region that the extrinsic muscles of the tongue (except the genioglossus) enter it. The cranial two-thirds of the tongue is partly or entirely free from the floor of the mouth, the cranial one-third being completely free and movable. In about the middle third the ventral surface of the tongue is held to the floor of the mouth by the fold known as the frenulum linguÆ. The frenulum contains parts of the two genioglossus muscles, which enter the tongue through it. The ventral surface and lateral borders of the tongue are smooth, soft, and free from papillÆ. The dorsal surface is raised into papillÆ of various kinds, and has a slight median longitudinal furrow. The caudal part of the dorsal surface is softer, redder, and marked with papillÆ of a different kind from those of the rest of the tongue. From the caudal end a small median vertical fold, the frenulum (or plica) glossoepiglottica passes from the dorsal surface of the tongue to the cranial surface of the epiglottis.

The papillÆ of the tongue are of three kinds. 1. The very numerous filiform papillÆ (a) (papillÆ filiformes); many of them are horny and tooth-like, with points turned caudad. These are most numerous at the middle of the free end of the tongue. 2. The fungiform papillÆ (b) (papillÆ fungiformes) are found scattered over the surface of the middle of the tongue caudad of the large filiform papillÆ. They are enlarged at their free ends. There is a prominent row of very large ones (b') opposite the circumvallate papillÆ at the borders of the tongue. 3. The circumvallate papillÆ (c) (papillÆ vallatÆ) are blunt and each is surrounded by a trench which is bounded in turn by a raised wall. They are in two rows of two or three each, which converge near the base of the tongue so as to form a V with the apex directed caudad.

Muscles of the Tongue

(Fig. 96).—M. genioglossus (f) passes from the symphysis of the lower jaw into the tongue and lies beneath (dorsad of) the geniohyoid (g).

Origin from the medial surface of the mandible near the symphysis and dorsad of the origin of the geniohyoid.

Insertion.—The fibres pass dorsad, diverging in a fan-like manner and forming a flat vertical plate closely applied to the muscle of the opposite side. This plate extends along the caudal three-fourths of the tongue, i.e. as far as it is attached. The cranial fibres arch craniad to the tip of the tongue, the caudal fibres arch caudad to the root of the tongue. Dorsad the muscle is confounded with the muscle of the opposite side.

Action.—Draws the root of the tongue forward and the tip backward.

M. hyoglossus (h).—From the body of the hyoid bone to the tongue.

Origin.—(1) From the ventral surface of the body of the hyoid laterad of the geniohyoid (g), and (2) by a second head from the ceratohyal.

Insertion.—Both heads penetrate into the tongue between the styloglossus (e) and the genioglossus (f). The fibres intermingle with those of the styloglossus (e) and thus help to form the lateral parts of the tongue. They finally end in the integument on the dorsum of the tongue at the sides.

Action.—Retracts the tongue and depresses it.

M. styloglossus (e).—From the stylohyal bone to the tongue.

Origin from the mastoid process of the temporal bone, from the stylomandibular ligament (2) (which connects the border of the external auditory meatus with the angular process of the mandible) and from the proximal cartilaginous portion of the cranial cornu of the hyoid bone. The fibres pass mediad, diverging between those of the digastric and hyoglossus (h) into the lateral part of the tongue.

Insertion.—The fibres pass toward the tip of the tongue, where the mass finally ends in a point, the superficial ones gaining insertion into the integument at the sides of the tongue.

Relations.—Outer surface with the digastric (Fig. 65, b) and mylohyoid (Fig. 65, c). Inner surface with the pterygoideus internus (Fig. 96, d), the cranial cornu of the hyoid, and the tympanic bulla.

Action.—Retracts the tongue and raises it.

The intrinsic muscles of the tongue (those entirely within it) are attached to its integument at both their ends. There are three sets of fibres: a longitudinal, a transverse, and a vertical one. These are seen most readily in cross-sections.

The Soft Palate. Velum Palatinum

(Fig. 66, page 112).—The soft palate is the free curtain-like structure which forms the caudal part of the roof of the mouth. It is attached to the caudal border of the palatal plates and the ventral border of the perpendicular plates, of the palatine, and to the pterygoid processes and hamuli of the sphenoid, and extends some distance caudad of the hamuli. It thus forms a rather long and narrow curtain separating the caudal part of the nasal cavity from the mouth. Caudad it ends in a free arched border (Fig. 66, 4) which is at about the level of the epiglottis, and may lie against the cranial or the caudal surface of the latter. The narrowed passage bounded by the margin of the velum palatinum dorsad and the tongue ventrad is the isthmus faucium. From the sides of the velum a short distance from the caudal border a fold of mucosa passes ventrad to the side of the tongue; a short distance caudad of this a similar fold passes to the floor of the pharynx. These folds form the cranial and caudal pillars of the fauces. Between these folds is a shallow pocket, from the bottom of which there arises a prominent projection or swelling which is one of the two tonsils (Fig. 95, d). Each tonsil is a reddish, lobulated gland, lymphoid in the adult, nearly a centimeter in length, and about one-third as long as broad, with its long axis craniocaudad.

The velum palatinum consists of two layers of mucous membrane, oral and nasal, with intervening muscular and connective tissue. The muscles of the soft palate in the cat are as follows:

M. tensor veli palatini (Fig. 66, d, d', page 112).

Origin from the ventral surface of the body of the sphenoid between the foramen ovale and the groove for the Eustachian tube. The muscle ends in a flat tendon which passes over the hamular process (3) of the pterygoid bone.

Insertion by spreading out in the soft palate into an aponeurosis which joins the aponeurosis of the opposite muscle and lies between the mucous membrane of the mouth and that of the nose.

Action.—Stretches the palate.

M. levator veli palatini (Fig. 66, e, e').—A flat triangular muscle which lies within the tensor.

Origin from the surface of the body of the sphenoid mediad of the groove for the Eustachian tube, from the styliform process of the bulla tympani, and in part from the Eustachian tube. The muscle passes caudad, and its fibres then diverge into the velum palatinum.

Insertion into the velum palatinum, some of the fibres meeting in the middle line.

Action indicated by the name.

A number of other muscles have been described in the soft palate of the cat; they are, however, poorly developed and not easily distinguished. For an account of these, see Stowell, Proceedings of the Am. Soc. of Microscopists, 1889.

2. The Pharynx.

—At the caudal end of the mouth cavity the passage for the food and that for the air cross; at the cranial end the food-passage (mouth) is ventral, the respiratory passage (nasal cavity) dorsal. Farther caudad the food-passage (oesophagus) is dorsal, while the respiratory passage (larynx and trachea) is ventral. In the region of crossing there is therefore for a certain distance a common passageway for food and air, and this is known as the pharynx. It extends from the isthmus faucium, at the free caudal margin of the soft palate, to the beginning of the oesophagus, at the dorsal or caudal margin of the opening of the larynx. The dorsal wall of the pharynx is separated from the base of the skull and the centra of the cervical vertebrÆ only by intervening muscles (longus capitis, levator scapulÆ ventralis, and longus colli, Fig. 72, page 143). Its lateral and ventral walls are supported by the hyoid bone and the cartilages of the larynx.

Craniad the pharynx continues, usually, without break into the cavity lying dorsad of the soft palate. But at the time of swallowing the free edge of the soft palate is pushed dorsad against the dorsal wall of the pharynx, while the caudal part of the pharynx is drawn craniad, so as to form a cavity continuous with that of the mouth. In this way the cavity above the soft palate is completely separated at the time of swallowing from the rest of the pharynx. This separated portion is known as the nasopharynx: it is strictly a portion of the respiratory passage, as the food does not pass into it. The nasopharynx is continuous craniad by the choanÆ with the nasal cavity; it forms a horizontal tube between and ventrad of the perpendicular plates of the palatine bones, and has the same craniocaudal extent as the soft palate. Its dorsal wall lies against the basis cranii and the longus capitis muscles; its lateral walls against the pterygoid muscles and the perpendicular plates of the palatine bones; its ventral wall is the soft palate. At the middle of its length, at the junction of its dorsal and lateral wall, are two longitudinal slits about three millimeters long. These are the medial openings of the Eustachian tubes, by which the nasopharynx communicates with the tympanic cavity.

The pharynx proper, situated caudad of the nasopharynx, is smaller than the latter. It is bounded craniad by the epiglottis and the margin of the soft palate, and is continuous between the two, by the isthmus faucium, with the mouth cavity. Its floor is formed by the cranial end of the larynx. At its caudal end it passes dorsally into the oesophagus, while ventrally it communicates with the larynx. Its walls are muscular.

Muscles of the Pharynx

(Fig. 96).—M. glossopharyngeus (i).

Origin.—Some fibres on the ventral and lateral part of the genioglossus (f) leave that muscle near its caudal end. They form a thin band of diverging fibres which pass outside of the cranial horn of the hyoid. A similar sheet of fibres leaves the midventral part of the styloglossus (e). The two sheets unite and the united muscle crosses the hyoid, turns dorsad, and has its

Insertion into the median dorsal raphe of the pharynx.

Action.—Constrictor of the pharynx.

M. constrictor pharyngis inferior (k).—A thin sheet of muscle covering the sides of the pharynx at its caudal end.

Origin from the lateral surfaces of the thyroid and the cricoid cartilages. The fibres pass dorsad and craniad, the cranial ones covering the fibres of the middle constrictor (j).

Insertion.—The median longitudinal raphe on the dorsum of the pharynx. The caudal fibres are transverse and continuous with the circular fibres of the oesophagus. The cranial fibres may pass as far as the base of the sphenoid.

Action.—Constrictor of the pharynx.

M. constrictor pharyngis medius (j).—A thin sheet which covers the middle part of the lateral surface of the pharynx.

Origin.—The ventral two pieces of the cranial horn and the whole of the caudal horn of the hyoid. The fibres diverge, passing dorsad.

Insertion into the median dorsal raphe of the pharynx. The cranial fibres are inserted into the base of the sphenoid bone. The muscle covers part of the stylopharyngeus (l) and the superior constrictor (Fig. 66, f, page 112) and is partly covered by the glossopharyngeus (Fig. 96, i).

Action.—Constrictor of the pharynx.

M. stylopharyngeus (l).

Origin from the tip of the mastoid process of the temporal bone and from the inner surface of the cartilaginous piece between the tympanohyal and the stylohyal bones. The parallel fibres form a flat band which passes ventrocaudad over the outer surface of the constrictor superior.

Insertion.—The ventral fibres pass beneath the middle constrictor (j) at its cranial border and, continuing toward the middle line of the pharynx, gradually lose themselves among the fibres of the superior constrictor. The dorsal fibres pass onto the outer surface of the middle constrictor and are lost among its fibres.

Action.—Constrictor of the pharynx.

M. constrictor pharyngis superior or pterygopharyngeus (Fig. 66, f, page 112).—A flat, triangular sheet beneath the constrictor medius.

Origin.—The tip of the hamular process of the pterygoid bone. The muscle passes caudad, the fibres diverging, and dips beneath the cranial border of the constrictor medius.

Insertion into the median dorsal raphe of the pharynx. The dorsal fibres are inserted into the base of the sphenoid. The ventral fibres pass lengthwise of the pharynx, closely connected with those of the stylopharyngeus (Fig. 96, l), and finally reach the level of the larynx.

Action.—Constrictor of the pharynx.

3. The Œsophagus.

—The oesophagus is a straight tube, dorsoventrally flat when empty, which extends from the pharynx to the stomach. It has a uniform diameter when moderately dilated of about one centimeter. It lies dorsad of the trachea and against the longus colli muscles (Fig. 72, g') covering the centra of the cervical vertebrÆ, until it reaches the caudal end of the thyroid gland (Fig. 96, 6); then it passes to the left and lies laterodorsad of the trachea until it reaches the bifurcation of the trachea. It there returns to the median line, passes gradually distad, separated from the vertebrÆ by the aorta, and finally pierces the diaphragm about two centimeters from the dorsal body wall, and enters the stomach. Its attachment to the diaphragm is loose enough to permit of longitudinal motion. In passing through the thoracic cavity it lies in the posterior mediastinum ventrad of the aorta. Its wall consists of a muscular coat, a submucosa, and a mucosa, and its inner surface presents many longitudinal folds. It has no serous covering, its side walls being merely in contact with the halves of the mediastinal septum.

4. The Stomach. Ventriculus

(Fig. 97).—The stomach is the widest part of the alimentary canal. It is a pear-shaped sac, the long axis of which is curved nearly into a semicircle. The broad end of the sac lies to the left and dorsad; here the stomach communicates with the oesophagus (a). The narrowed end extends to the right and lies more ventrad than the other end; it passes here into the duodenum (g). That portion of the stomach which communicates with the oesophagus is known as the cardiac end (b); the opposite is the pyloric end. Owing to the curved form of the stomach above mentioned it is possible to distinguish a concave and a convex side. The concave side is directed craniad and dextrad; it is called the lesser curvature of the stomach (c). The longer convex border is directed caudad and to the left; it is called the greater curvature (d). The greater curvature extends to the left, next to the oesophagus, into a prominent convexity known as the fundus (e) of the stomach.

The stomach lies at the cranial end of the abdominal cavity, mostly to the left of the middle line. Its cardiac end is in contact by its dorsal surface with the dorsal, nearly horizontal, portion of the diaphragm. On its ventral side the cardiac end does not touch the diaphragm, so that a small part of the oesophagus passes here for a short distance into the abdominal cavity, to join the stomach. The communication of oesophagus and stomach is by a simple conical increase in size of the former. The pyloric end of the stomach extends to the right of the middle line, becoming constantly smaller; at its junction with the duodenum there is a constriction which marks the position of the pyloric valve (f). This valve is formed by a ring-like thickening of the circular muscle-fibres of the alimentary canal, forming a sphincter muscle at the junction of the stomach and duodenum and causing a projection of the mucosa into the lumen of the canal. The ventral surface of the stomach lies against the liver except when the stomach is much distended with food, when the ventral surface comes to lie against the ventral abdominal wall.

Fig. 97.—Stomach, Ventral View.

a, oesophagus; b, cardiac end of the stomach; c, lesser curvature; d, greater curvature; e, fundus; f, pyloric valve; g, part of duodenum.