The nutritive materials must be distributed to the textures and organs in order that the food stuffs, when altered by. the various processes described under digestion, may be of any use to the economy. For this purpose they must pass through the lining membrane of the alimentary canal and gain access to the blood, which is the common mode of intercommunication between the various parts of the body.

The nutrient part of the food has then to be absorbed out of the alimentary canal by the surrounding tissues, and mixed with the general circulating fluids, lymph and blood.

But the blood is separated from the intestinal contents by barriers, which, as far at least as the blood is concerned, are impassable, although it exerts considerable pressure, and thereby tends to escape from the blood vessels.

The question then arises, How does the elaborated chyme make its way through this barrier, which is sufficient to prevent the flow of blood into the intestinal tract?

The general answer is easily given, viz.: the blood cannot pass through an animal membrane. But this is not a satisfactory solution of the question, for under certain abnormal circumstances, the blood does pass through the wall of the vessels, and normally the plasma escapes from the capillaries into the tissues, in order to nourish them. We must further remember, in considering this point, that the wall of the vessels and the membrane lining the intestine are both made up of living cells which are endowed with a capability, coincident with their lives, of controlling any passage through or between them. Some of these cells, which we might call secreting agents, do allow, or rather cause, a passage of fluid from the blood to the intestinal cavity, and, as we shall presently see, others of them induce a passage of the nutritious materials from the intestinal canal into the surrounding tissues.

In order clearly to understand the method by which absorption is accomplished, it is necessary to have some idea of the absorbent system generally; it may be well, therefore, at this place to give a brief account of the construction of the special apparatus which carries on this function. Although the absorbent vessels form one continuous system, they may be conveniently divided into two departments, namely, interstitial and surface absorption. A certain modification of the latter, called the lacteal system, occurs in the alimentary canal, and is described under intestinal absorption.

Diagram showing the Course of the Main Trunks ol the Absorbent System.

Fig. 80. Diagram showing the Course of the Main Trunks ol the Absorbent System. The lymphatics of lower extremities (D) meet the lacteals of intestines (LAC) at the receptacu-lum chyli (R. C ), where the thoracic duct begins. The superficial vessels are shown in the diagram on the right arm and leg (S), and the deeper ones on the arm to the left (D). The glands are here and there shown in groups. The small right duct opens into the veins on the right side. The thoracic duct opens into the union of the great veins of the left side of the neck (T).