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. 98.—Junction of Small and Large Intestine.