[Illustration:  Fig. 55.—­A Small Portion of the Mucous Membrane of the Small Intestine. (Villi are seen surrounded with the openings of the tubular glands.) [Magnified 20 diameters.]]

143.  The Small Intestine.  At the pyloric end of the stomach the alimentary canal becomes again a slender tube called the small intestine.  This is about twenty feet long and one inch in diameter, and is divided, for the convenience of description, into three parts.

The first 12 inches is called the duodenum.  Into this portion opens the bile duct from the liver with the duct from the pancreas, these having been first united and then entering the intestine as a common duct.

The next portion of the intestine is called the jejunum, because it is usually empty after death.

The remaining portion is named the ileum, because of the many folds into which it is thrown.  It is the longest part of the small intestine, and terminates in the right iliac region, opening into the large intestine.  This opening is guarded by the folds of the membrane forming the ileo-caecal valve, which permits the passage of material from the small to the large intestine, but prevents its backward movement.

144.  The Coats of the Small Intestine.  Like the stomach, the small intestine has four coats, the serous, muscular, sub-mucous, and mucous.  The serous is the peritoneum.[22] The muscular consists of an outer layer of longitudinal, and an inner layer of circular fibers, by contraction of which the food is forced along the bowel.  The sub-mucous coat is made up of a loose layer of tissue in which the blood-vessels and nerves are distributed.  The inner, or mucous, surface has a fine, velvety feeling, due to a countless number of tiny, thread-like projections, called villi.  They stand up somewhat like the “pile” of velvet.  It is through these villi that the digested food passes into the blood.

[Illustration:  Fig. 56.—­Sectional View of Intestinal Villi. (Black dots represent the glandular openings.)]

The inner coat of a large part of the small intestine is thrown into numerous transverse folds called valvulae conniventes.  These seem to serve two purposes, to increase the extent of the surface of the bowels and to delay mechanically the progress of the intestinal contents.  Buried in the mucous layer throughout the length, both of the small and large intestines, are other glands which secrete intestinal fluids.  Thus, in the lower part of the ileum there are numerous glands in oval patches known as Peyer’s patches.  These are very prone to become inflamed and to ulcerate during the course of typhoid fever.

145.  The Large Intestine.  The large intestine begins in the right iliac region and is about five or six feet long.  It is much larger than the small intestine, joining it obliquely at short distance from its end.  A blind pouch, or dilated pocket is thus formed at the place of junction, called the caecum.  A valvular arrangement called the ileo-caecal valve, which is provided with a button-hole slit, forms a kind of movable partition between this part of the large intestine and the small intestine.