The Intestine

The intestine is short—about five or six times the length of the body.

The small intestine has an average length of about thirteen feet (ca. 4 meters). It occupies most of the abdominal cavity behind the liver and stomach. The duodenum begins at the portal fissure and passes backward and upward, at first on the visceral surface of the liver, then in contact with the right flank. Near the pelvis it turns inward and sharply forward, passes along the inner border of the left kidney, and joins the jejunum to the left of the root of the mesentery. The mesoduodenum is given off from the common mesentery; it is a relatively wide fold, except at the terminal part. Here there are two short folds, one of which joins the mesocolon; the other passes on to the terminal part of the colon. The first part of the mesoduodenum contains the right branch of the pancreas. Its root is blended with the mesocolon to form a common mesentery. The remainder of the small intestine forms numerous coils, and is attached by a wide mesentery to the sublumbar region. The pancreatic and bile-ducts open into the duodenum about two or three inches (ca. 5 to 8 cm.) from the pylorus; the accessory pancreatic duct opens an inch or two (ca. 2.5 to 5 cm.) further back. The mucous membrane has very long villi. The duodenal (Brunner’s) glands occur only near the pylorus. Peyer’s patches are numerous (about twenty in young subjects), and begin in the duodenum. They are usually elliptical in outline, but the last one is band-like, reaches to the end of the ileum and is four to sixteen inches (ca. 10 to 40 cm.) long in young dogs (Ellenberger). There is an ileo-cæcal valve. The muscular coat is relatively thick.

The large intestine is two to two and a half feet (ca. 60 to 75 cm.) long. Its caliber is about the same as that of the small intestine, and it has neither longitudinal bands nor sacculations.

The cæcum is about three or four inches (ca. 8 to 10 cm.) long, and is twisted in a spiral fashion. The twist is maintained by the peritoneum, which attaches it to the ileum. It is situated usually against the middle of the upper part of the right flank, below the duodenum and the right branch of the pancreas.

The colon is attached to the sublumbar region by a mesentery, the mesocolon. It presents three parts, which correspond to the ascending, transverse, and descending colon of man. The first or right part (Colon ascendens) is very short. It begins at the junction of the ileum and cæcum, and passes forward along the inner surface of the duodenum and the right branch of the pancreas till it reaches the pyloric part of the stomach; here it turns inward and crosses the median plane, forming the transverse part (Colon transversum). The third or left part (Colon descendens) passes backward in the sublumbar region along the inner border or ventral surface of the left kidney; then inclines toward the median plane and joins the rectum. The caliber of the colon is about the same throughout. It has no bands nor sacculations.

The mucous membrane of the cæcum contains numerous solitary glands; they are found also in the first part of the colon.

The rectum is almost completely covered with peritoneum, the line of peritoneal reflection being under the second or third coccygeal vertebra. At the junction of the rectum and anus the mucous membrane has a stratified, squamous epithelium, and contains the anal glands. A small opening on either side leads into two lateral anal sacs (Sinus paranales); these are usually about the size of a hazelnut, and contain a dirty gray, fatty substance, which has a peculiar and very unpleasant odor. The skin which lines these pouches contains coil glands. Further back the skin contains large sebaceous glands and peculiar circumanal glands.

The retractor ani muscle is extensive. It arises from the shaft of the ilium, the pubis, and the symphysis pelvis, and passes upward and backward to end on the first coccygeal vertebræ and the sphincter ani externus. The retractors, together with the coccygei, form a sort of pelvic diaphragm, analogous to that of man.