as the _ileum_. The ileum joins that
part of the large intestine known as the caecum, and at their place of
union is a marked constriction which prevents material from passing from
the large into the small intestine (Fig. 73). This is known as the
_ileo-caecal valve_.

_The mucous membrane_ of the small intestine is richly supplied with blood
vessels and contains glands that secrete a digestive fluid known as the
_intestinal juice_. The membrane is thrown into many transverse, or
circular, folds which increase its surface and also prevent materials from
passing too rapidly through the intestine. One important respect in which
the small intestine differs from all other portions of the food canal is
that its surface is covered with great numbers of minute elevations known
as the villi. The purpose of these is to aid in the absorption of the
nutrients as they become dissolved (Chapter XI).

_The muscular coat_ of the small intestine is made up of two distinct
layers--the inner layer consisting of circular fibers and the outer of
longitudinal fibers. These muscles keep the food materials mixed with the
juices of the small intestine, but their main purpose is to force the
materials undergoing digestion through this long and much-coiled tube.

The outer, or _serous_, coat of the small intestine, like that of the
stomach, is an extension from the general lining of the abdominal cavity,
or peritoneum. In fact, the intestine lies in a fold of the peritoneum,
somewhat as an arm in a sling, while the peritoneum, by connecting with
the back wall of the abdominal cavity, holds this great coil of digestive
tubing in place (Fig. 64). The portion of the peritoneum which attaches
the intestine to the wall of the abdomen is called the _mesentery_.

Most of the liquid acting on the food in the small intestine is supplied
by two large glands, the liver and the pancreas, that connect with it by
ducts.

[Fig. 71]

Fig. 71--*Abdominal cavity* with organs of digestion in position.

*The Liver* is situated immediately below the diaphragm, on the right side
(Figs. 71 and 72), and is the largest gland in the body. It weighs about
four pounds and is separated into two main divisions, or lobes. It is
complex in structure and differs from the other glands in several
particulars. It receives blood from two distinct sources--the portal vein
and the hepatic artery. _The portal vein_ collects the blood from