From the duodenum the food passes into the small intestine. The small intestine measures in the adult male seven and a half yards long. Throughout the whole of this length its internal surface or mucous membrane is closely set with small tubular glands, called the crypts of Lieberkuhn. These glands, which are present in countless millions, secrete and pour out into the intestine a watery alkaline fluid. The intestinal juice has, though in a much smaller degree, the same properties as the more active juices of the stomach and pancreas. Thus the processes of digestion are continued in a lesser degree throughout the whole tract of the intestinal canal. The albumen which has escaped change into albumose in the stomach, and the starch which has not been converted into glucose by the action of the pancreatic juice in the duodenum, slowly undergo those necessary changes in the intestines.

Fig. 4. - Tubular Glands of the Small Intestine opening on the surface of the mucous membrane between the villi. Magnified 40 Diameters.

1, 1. A vertical section of the mucous membrane of the small intestine. 2, 2. Tubular glands, their bases resting on the muscular coat, and their mouths opening on the free surface of the mucous membrane. 3, 3. Mouths of these glands. 4, 4, 4, 4. Villi covering the free surface of the mucous membrane: they are here mostly conical, are very large and placed close together.

The presence of food in the intestine acts as a stimulant to its muscular walls, and slow contraction of the involuntary muscular fibres of these walls takes place, by means of which a vermiform movement of the intestine is set up, which slowly passes the chyle on towards the large intestine.