Physiology and Hygiene for Secondary Schools

by Francis M. Walters

1. An outer layer of epithelial cells, resting upon a connective tissue support.

[pg 174]2. A small lymph tube, called a lacteal, which occupies the center of the villus and connects at the base with other lymph tubes, also called lacteals (B, Fig. 76).

3. A network of capillaries.

The villi are structures especially adapted to the work of absorption, and they are found only in the small intestine. The mucous membrane in all parts of the canal, however, is capable of taking up some of the digested materials.

Fig. 76—The villi. A. Diagram of a small section of mucous membrane of small intestine. 1. Villi. 2. Small glands, called crypts.
B. Diagram showing structure of villi. 1. Small artery. 2. Lacteal. 3. Villus showing termination of the lacteal. 4. Villus showing capillaries. 5. Villus showing both the lacteal and the capillaries. 6. Small vein. 7. Layer of epithelial cells.

Work of Capillaries and Lacteals.—The capillaries and lacteals act as receivers of material as it passes through the layer of epithelial cells covering the mucous membrane. The lacteals take up the digested fats,[66] and the capillaries receive all the other kinds of nutrients. These vessels do not, of course, retain the absorbed materials, but pass them on. Their final destination is the general circulation, which they reach by two well-defined channels, or routes.

Routes to the Circulation.—The two routes from the[pg 175] place of absorption to the general circulation are as follows:

1. Route taken by the Fat.—The fat is conveyed by the lacteals from the villi to the receptacle of the chyle. At this place it mingles with the lymph from the lower parts of the body, and with it passes through the thoracic duct to the left subclavian vein. Here it enters the general circulation. Thus, to reach the general circulation, the fat has to pass through the villi, the lacteals, the receptacle of the chyle, and the thoracic duct (Fig. 77). Its passage through these places, like the movements in all lymph vessels, is slow, and it is only gradually admitted to the blood stream.