The small intestine commences at the pylorus, which is the strong muscular ring that separates the stomach from the intestine, and it terminates at the point where the alimentary canal suddenly enlarges to form the large intestine. Its length is more than 70 feet, and its width, when undistended with food or gases, about 1 1/2 inch. It is suspended from the vertebral column by a double fold of membrane (peritoneum), which is here named the mesentery. The blood-vessels, lacteals, and nerves of the intestine reach it by running between these two layers of the mesentery. In structure the small intestine resembles the stomach in having an outer serous covering of peritoneum, a middle layer of longitudinal and circular muscular fibres, and an internal mucous layer. When closely examined the mucous membrane of the small intestine exhibits a velvety or pile-like arrangement, which is most conspicuous when the membrane is floating in water. This appearance is due to the presence of an immense number of small projections about 1/50 of an inch long, named villi (1, fig. 84), between which are the openings of minute glands (Lieberkuhnian follicles), named after Lieberkuhn, a Dutch anatomist, who first described them about 1745.
Fig. 84. - Section through the Small Intestine.
1 Villi. - Lacteal vessels. 3 Muscular coat. 4 Serous coat.
Fig. 85. - Large Intestine. a, Caecum; b, c, muscular bands; d, ileum, or terminal portion of small intestine; e, e',f, f ', large colon; g, pelvic flexure; h, single colon.
The villi number many millions, and are interesting as containing the lacteals, a set of vessels by which the food is absorbed and conveyed into the system. It is sufficient in this place to say that they present a central tube, which commences with a free extremity above, and ends in a network at the base of the villus in the wall of the intestine. These lacteal vessels are each invested by a mantle of smooth muscular tissue, and this again by a plexus of capillary blood - vessels. The whole is enclosed by a transparent membrane covered with cells that project into the cavity of the intestine. The Lieber-kuhnian follicles are simple tubes very closely set and lined by columnar epithelium. The secretion they pour into the intestine has high digestive powers for all kinds of aliment.
The large intestine consists of three parts, the caecum, the colon, and the rectum. The small intestine opens into the first part of the large intestine by an orifice that is guarded by a fold of mucous membrane named the ileo-caecal valve, which prevents the return into the small intestine of the food which has entered the caecum. True villi stop abruptly at this spot, and are not found in the large bowel.
The caecum is a greatly enlarged portion of the intestine. It occupies the right side of the abdomen, and has a capacity of 7 or 8 gallons. The surface is marked by four longitudinal bands of muscle, which being shorter than the other coats, pucker them into great bulging pouches. The upper extremity is curved and presents two openings, one by which the small intestine terminates in it; and the other very narrow, which communicates with the colon, or second division of the large intestine.
Fig. 86. - Large Intestine.
A, Ileum, or terminal portion of small intestine. B, Caecum. C, E, Large colon. D, Pelvic flexure. F, Single colon. G, Rectum. H, Anus. The arrows indicate the course taken by the food in the process of digestion.
The colon is divisible into two parts - the large or double colon, and the small or floating colon. The length of the large colon is about 12 feet, and its capacity about 18 gallons. The length of the small colon is about 10 feet. The caecum and both the large and small colon have the same general arrangement of the coats as the other parts of the intestine.
The rectum is a short, straight tube extending from the last and shortest division of the colon. It takes a straight course backward, and terminates in the anus or external orifice. The muscular layer of this segment of the intestine is very strongly developed. The orifice is surrounded by circular fibres, which form a tolerably well - defined ring to which the name of sphincter ani has been given. This ring keeps the aperture closed. There is also another muscle connected with it, which pulls the posterior part of the bowel forward after de-ftecation, and is known as the retractor ani.