THE ANIMAL BODY— DIGESTION— METABOLISM 29 



nect them with the veins. If one extends his arms in front of him with 

 his finger tips touching, his body will represent the heart, while one 

 arm will represent an artery carrying blood from the heart, and the 

 other a vein conveying blood to the heart. The touching fingers will 

 correspond to the capillaries connecting the arteries* with the veins, and 

 the space all about the fingers will represent the surrounding body 

 tissues. In general, neither the veins nor the arteries allow any sub- 

 stance within them to escape thru- their walls proper. It is thru- the 

 capillaries that the nutritive matter carried by the blood finds its way 

 into the body tissues for their nourishment, and thru the* capillaries and 

 the lymphatics, in turn, the waste of the body drains back into the blood 

 circulation. 



The tissues of the body are thus everywhere permeated by the ducts 

 of the lymphatic system and the capillaries of the blood system. The 

 cells are bathed by lymph, which is the fluid that receives and tem- 

 porarily holds all the nutritive substances and the body wastes. 



The digested nutrients are absorbed into the circulation chiefly thru 

 the walls of the small intestine. The mucous membrane lining it has a 

 velvety appearance, caused by innumerable minute, cone-like projec- 

 tions, or tongues, called villi, which project into the interior of the intes- 

 tinal tube, thereby coming into intimate contact with its fluid contents. 

 Within each villus are lacteals, or drainage tubes of the lymphatic 

 system, and capillaries of the blood system. 



Digestion and absorption of fat. — In discussing the manner in which 

 the various nutrients are absorbed, let us at the same time review their 

 digestion, tracing just-what becomes of each after entering the mouth. 



The fats of food undergo no appreciable digestion until they reach 

 the small intestine. Here thru the aid of the bile they are split by the 

 lipase of the pancreatic juice into fatty acids and glycerin. The 

 alkalies in the bile then unite with these fatty acids to form soaps. 

 It is believed that practically all of the fats are absorbed in the form 

 of soaps and glycerin ; these are then reunited into fats in the intestinal 

 wall. Some of the fatty acids and glycerin formed by the action of 

 the lipase may perhaps be absorbed directly, without the fatty acids 

 being first changed to soaps. In the villi of the intestinal wall the 

 fats enter the lacteals, forming, with the lymph a milky substance called 

 chyle. This is carried by the lymphatics and poured into a vein near 

 the shoulder, thus entering the blood circulation. 



Digestion and absorption of carbohydrates. — Carbohydrate diges- 

 tion begins in the mouth, where the ptyalin in the saliva changes starch 

 into malt sugar. This action continues in the first part of the stomach, 

 but ceases when the food becomes acid in the rear end of that 

 organ. Simple, glucose-like sugars may be absorbed directly from the 



