36. ABSORPTION.--With the elaboration of the chyle, the work of digestion is completed; but, in a certain sense, the food is yet outside of the body; that is, the blood is not yet enriched by it, and it is not in a position to nourish the tissues. The process by which the liquefied food passes out of the alimentary canal into the blood is called absorption. This is accomplished in two ways; first, by the _blood-vessels_. We have seen how the inner membrane of the stomach is underlaid by a tracery of minute and numerous vessels, and how some portions of the food are by them absorbed. The supply of blood-vessels to the intestines is even greater; particularly to the small intestines, where the work of absorption is more actively carried on. 37. The absorbing surface of the small intestines, if considered as a plane surface, amounts to not less than half a square yard. Besides, the mucous membrane is formed in folds with an immense number of thread-like prolongations, called _villi_, which indefinitely multiply its absorbing capacity. These minute processes, the villi, give the surface the appearance and smoothness of velvet; and during digestion, they dip into the canal, and, by means of their blood-vessels, absorb its fluid contents, just as the _spongioles_ which terminate the rootlets of plants, imbibe moisture from the surrounding soil. [Sidenote: 38. How is absorption effected in another way? Describe it. Name of the lacteals? Thoracic duct?] 38. Secondly, absorption is also effected by the _lacteals_, a set of vessels peculiar to the small intestines. These have their beginnings in the little villi just mentioned, side by side with the blood-vessels. These two sets of absorbents run in different courses, but their destination is the same, {97} which is the right side of the heart. The lacteals receive their name from their milky-white appearance. After a meal containing a portion of fat, they are then distended with chyle, which they are specially adapted to receive: at other times they are hardly discernible. The lacteals all unite to form one tube, the _thoracic duct_, which passes upward through the _thorax_, or chest, and empties into a large vein, situated just beneath the left collar-bone. [Illustration: FIG. 24.--THE LACTEALS. A, Small Intestine. B, Lacteals. C, Thoracic Duct. D, Absorbents. E, Blood-vessel. ] [Sidenote: 39. The absorbents? Lymph? What further of the lymph?]