If a portion of the small intestine of any vertebrate animal be opened, washed, and floated in water, its internal surface will be seen to resemble that of velvet, and to be covered with a countless number of minute projections, or villi (Fig. 6). I have already stated that the mucous membrane of the small intestine consists of a vast number of tubular depressions or glands. It is between these glands and on their edges or surfaces that the villi of the intestine project. It will thus be seen that the area of the internal wall of the intestine is enormously increased by the alternate dippings and elevations of its surface. The part which folds in or dips, forming the tubular glands, is concerned in secretion, and the part which is elevated, forming the villi, is concerned in absorption. The construction of a villus is as follows (Fig. 6): In the centre is found an inverted tube or canal, closed at one end, the walls of which are formed of thin transparent epithelial cells; this is the blind end of a lacteal. Closely covering it is a fine network of extremely small blood vessels. The external surface of the villus is lined with a single layer of columnar epithelial cells closely set together.

Absorption of fat by the lacteals takes place in the following manner: The minute globules of fat which have been emulsified by the action of the pancreatic juice, the bile, and the intestinal juice, pass through and between the epithelial cells which form the outer lining of the villus, and also through the transparent thin wall of the lacteals. The lacteals communicate with a fine network of lymphatic vessels which ramify on the surface of the mesentery, or membrane to which the intestines are attached. Along these vessels, which are abundantly provided with valves to prevent a backward current, the oil globules, absorbed from the digested food, slowly pass, till gathered into a larger vessel called the thoracic duct. This duct passes upwards beside the vertebral column, and pours its contents into the left jugular vein in the neck.

Absorption by the capillary blood vessels of the intestine is, however, much more important than by the lacteals. It is seen from the structure of the villi that there is only a single layer of epithelial cells intervening between the digested fluid food in the intestinal canal and the extensive surface of the capillary vessels. Absorption, therefore, of all solvent and fluid matters from the intestine into the veins of the villi takes place easily and rapidly. The intestinal juice which is poured out in such abundance during digestion is also re-absorbed by the blood vessels of the villi.

Fig. 6. - Arteries and Veins of the Villi, Injected and Magnified ico Diameters.

1, 1, 1, 1, 1. Cylindrical villi receiving one single voluminous vein which occupies the centre of it, and several very small arteries all of the same calibre, arranged around the venous trunk with the divisions of which they anastomose at their ends. 2, 2, 2, 2. Flattened villi receiving two venous trunks which communicate with each other by numerous branches, and several arteries which terminate in the extremely rich network formed by these branches. 3. A larger and more flattened villus receiving three venous trunks which by their branches and anastomoses form at its summit a very close network. Around these trunks and in their interspaces very smalt and pale arterioles are seen which are connected with this network.