to chyme, this fold relaxes every minute or two, and allows some of the chyme to escape into the intestine. [Illustration: FIG. 96.--A PORTION OF SMALL INTESTINE cut open to show the folds in its lining.] =The small intestine= is about one inch in diameter and twenty feet long, with many coils and turns in its course (Fig. 90). Its mucous lining is wrinkled into numerous _folds_ in order to increase the secreting and absorbing surface (Fig. 96). On and between the folds are thousands of little threadlike projections called _villi_ (Fig. 97). In each villus are found fine capillaries and a small lymphatic called a _lacteal_ (colored Fig. 2). The villi are so thick that they make the lining of the intestine like velvet, and enormously increase the absorbing surface. [Illustration: FIG. 97.--LINING OF SMALL INTESTINE, magnified, showing villi and mouths of intestinal glands.] =Digestion in the Small Intestine.=--This is by far the most active and important of the digestive organs. The mouth digests a small part of the starch, and the stomach digests a small part of the proteid; _the small intestine digests most of the starch, most of the proteid, and all of the fats_. The food is in the mouth a few minutes, and in the stomach two or three hours; it is in the small intestine ten or twelve hours. There are thousands of small glands called _intestinal glands_ that open between the villi (Fig. 97) and secrete the intestinal juice, which _digests cane sugar_. Besides these, there are two very large and active glands, the pancreas and liver, which empty into the intestine by ducts. =The Pancreas.=--The small intestine is the most important of the digestive organs, chiefly because it receives the secretion from the pancreas, the most important of digestive glands. The pancreas is a _long_, _flat_, _pinkish gland situated behind the stomach_ (see Fig. 90). The pancreatic juice contains _three powerful ferments_, one of which (amylopsin) digests the starches, another (trypsin) digests proteids, and the third (steapsin), with the aid of the bile, breaks up the fats into tiny globules. Fat in small globules floating in a liquid is called an _emulsion_; fresh milk is an emulsion of cream (Fig. 98). Fat is not changed to another substance by digestion, but it is emulsified, and in this condition it readily passes through the walls of the intestines and is absorbed by the lymphatics called _lacteals_ (colored Fig. 5) found in the villi. It then ascends through the _thoracic duct_ to a large vein at the left side of the neck (Fig. 100). _The digested proteid, starch, and sugar pass into the capillaries of the portal vein, and go to the liver_ on their way to the general circulation (Fig. 100). The portal circulation empties into the large ascending vein leading to the right auricle (Fig. 100). [Illustration: FIG. 98.--JUNCTION OF LARGE AND SMALL INTESTINE.]