Part 58

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 œsophagus 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 œsophagus (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 œsophagus 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 œsophagus, 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 œsophagus passes here for a short distance into the abdominal cavity, to join the stomach. The communication of œsophagus 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.

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

The stomach is supported by the =great omentum= and the =gastrohepatic= ligament. It is connected with the duodenum by the =gastroduodenal= ligament; with the spleen by the =gastrolienal= ligament.

The inner surface of the stomach presents longitudinal folds at its pyloric end and along the greater curvature as far as the fundus. The prominence of these depends on the degree of distension. Its walls are composed of an external peritoneal layer, an internal mucous layer, and an intervening muscular layer. This may be seen with the unaided eye in sections of the wall.

5. =The Small Intestine. Intestinum tenue.=--The small intestine lies in numerous coils which take up the greater part of the space in the abdominal cavity. It has a length about three times that of the body of the cat. It is usually considered as divided into three parts, =duodenum=, =jejunum=, and =ileum=; these divisions are, however, not clearly marked off. The small intestine is suspended by the =mesentery= already described.

The =duodenum= is that part of the small intestine which follows the stomach. At the pylorus (Fig. 97, f) the alimentary canal makes a rather sharp turn so that the first part of the duodenum forms an angle with the pyloric portion of the stomach, and extends caudad and slightly toward the right, soon becoming directed almost entirely caudad and lying along the right side. About eight or ten centimeters caudad of the pylorus it makes a U-shaped bend, extending thus craniosinistrad for four or five centimeters. Here it passes without definite limit into the jejunum, the duodenum being considered to end at the next turn caudad. The entire duodenum is about fourteen to sixteen centimeters in length. Between the two limbs of the U-shaped bend formed by the duodenum, the duodenal half of the pancreas is enclosed (Fig. 102, a).

The walls of the duodenum are composed of the serous (peritoneal) investment, a muscular coat which is made up of an outer thin, longitudinal layer of fibres and an inner thick, circular layer, a submucous coat, and inside this a mucous coat. The mucosa is thrown up into numerous delicate finger-like villi which give to it a velvety appearance. On the dorsal wall of the duodenum, about three centimeters distad of the pylorus, the mucosa presents a slight papilla, at the apex of which is seen the oval opening of the =ampulla of Vater=. This is an ovoid space in the wall of the duodenum. The space is encroached upon by numerous folds of the walls. The common bile-duct and the pancreatic duct open into it, the former extending from the bottom of the ampulla nearly to its mouth, and the latter extending from the bottom about half-way to the mouth. Two centimeters caudoventrad of the opening of the ampulla of Vater is the opening of the accessory pancreatic duct. It can usually be demonstrated only by passing a bristle into the duodenum through an opening in the duct.

The =jejunum= is the part of the small intestine following the duodenum. It is not separated from the part of the small intestine following it by any sharp line. In man it constitutes two-fifths of the small intestine exclusive of the duodenum, and is characterized by its emptiness after death and by the absence from it of Peyer’s agminated glands (Peyer’s patches).

The =ileum= is the portion of the small intestine between the jejunum and colon. It lies suspended by its mesentery in numerous folds in the caudal part of the abdominal cavity, separated from the ventral abdominal wall only by the great omentum. It is of nearly uniform diameter, but its caudal portion is thinner-walled than its cranial portion. Its walls have a microscopic structure like that of the duodenum and jejunum. On its inner surface and on the inner surface of the jejunum are seen close-set villi, but these become rather sparser toward the caudal end of the ileum and disappear about one centimeter from the opening into the colon. Among the villi of the caudal end of the ileum are numerous rounded elongations free from villi. These are the solitary follicles or solitary glands (lymphatic) of the intestine. These glands when aggregated together form the agminated glands or patches of Peyer. The ileum passes at the caudal end into the colon, the opening being guarded by the =ileocolic valve= (Fig. 99). This is formed by a marked projection of the mucosa (f) and transverse muscle layer (e) of the ileum into the colon. Its surface is free from villi.

6. =The Large Intestine. Intestinum crassum=.--The large intestine is divided into =colon= and =rectum=. The =colon= or first part of the large intestine lies against the dorsal body wall and is separated from the ventral body wall by the folds of the ileum. It has a diameter about three times that of the ileum. The opening of the ileum into it is on its side between one and two centimeters from its cranial end (Fig. 98). The blind pouch thus formed by the cranial end of the colon is the =cæcum= (Fig. 98, c; Fig. 99, a). The cæcum ends in a slight conical projection which may be considered as the rudiment of a vermiform appendix. The colon lies at first on the right side and passes at first craniad; then transversely to the left, then caudad, lying nearly in the middle line and next to the dorsal abdominal wall. The colon may thus be distinguished according to its direction into =ascending, transverse,= and =descending= colon. At its caudal end the colon passes without sharp limit into the rectum.

FIG. 99.--SECTION OF THE ILEOCOLIC VALVE.

Fig. 98.--a, ileum; =b=, ascending colon; c, cæcum; d, position of ileocolic valve.

Fig. 99.--a, cæcum; b, colon; c, ileum; d, longitudinal muscle layer; e, transverse muscle layer; f, mucosa; g, ileocolic valve (opened, as when material is passing into the colon).]

At the bottom of the cæcum on its inner surface is seen a collection of solitary glands forming one of the agminated glands of Peyer, or Peyer’s patches. The mucous membrane is without villi. It presents a few considerable elevations, probably solitary glands.

The =rectum= is the terminal portion of the large intestine lying in the median line close to the dorsal body wall, from which it is suspended by the short mesorectum. Its structure is like that of the colon. It opens externally at the anus. The entire large intestine has a length about one-half that of the animal. At each side of the anus are two large secreting sacs, the =anal sacs= or =glands=, each about a centimeter in diameter. These open into the anus one or two millimeters from its caudal boundary.

=Muscles of the Rectum and Anus.=--Owing to the close interrelation of the muscles of the rectum and anus with those of the urogenital organs, all these muscles will be described together at the end of the description of the urogenital system.

7. =The Liver=, =Pancreas=, and =Spleen=.--=The Liver.= =Hepar.=--The liver (Figs. 100 and 101) is a large red-brown organ occupying the cranial part of the abdominal cavity. It is closely applied to the caudal surface of the diaphragm and extends thence ventrad of the stomach so as to conceal all but its pyloric end. Owing to the position of the stomach the larger mass of the liver is on the right side and it extends somewhat further caudad on this side.