The Animal Body — Digestion — Metabolism. 35 



sues for their nourishment, and thru the capillaries and the lym- 

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

 circulation. The cellular tissues of which the body is composed are 

 thus everywhere permeated by the ducts of the lymphatic system 

 and the capillaries of the blood system. The cells of the body are 

 bathed by lymph, which is the fluid that receives and temporarily 

 holds all the nutritive substances and the body waste. The mucous 

 membrane lining the small intestine has a velvety appearance, 

 caused by innumerable minute cone-like projections or tongues, 

 called villi, which project into the interior of the intestinal tube, 

 thereby coming into contact with its fluid contents. Within each 

 villus are lacteals, or drainage tubes of the lymphatic system, and 

 capillaries of the blood system. 



51. Absorption of fat. — As before told, in the small intestine a 

 part of the fat of the food is split into fatty acids and glycerin by 

 the action of steapsin. These acids and the alkalies in bile combine 

 to form soaps which aid in emulsifying the remaining fat, so that it 

 also is rapidly acted on by the steapsin and changed into fatty 

 acids and glycerin. Modern investigation supports the view that 

 the fats are absorbed as soaps and glycerin. In the intestinal wall 

 these are reconverted into neutral fats which enter the lacteals, 

 forming with the lymph a milky substance called chyle. This is 

 carried in the lymphatics and poured into a vein near the shoulder, 

 thus entering the blood circulation. 



52. Absorption of carbohydrates; formation of glycogen. — The 

 glucose and glucose-like sugars taken up from the intestinal con- 

 tents by the capillaries pass into the veins, and thence by way 

 of the portal vein into the liver. Here they are for the most part 

 withdrawn from the blood and temporarily stored in this organ 

 as glycogen, a carbohydrate which is closely related to starch and, 

 having the same percentage composition, is sometimes called ani- 

 mal starch. Normally from 1.5 to 4.0 per ct. of the weight of the 

 liver consists of glycogen. The glycogen stored in the liver is grad- 

 ually changed back into glucose, and then doled out to the system 

 as required. The property of converting glucose into glycogen is 

 not possessed by the liver alone, but by the tissues of the body gen- 

 erally, especially the muscles. When work is being done the glyco- 

 gen in the muscles is first drawn lipon to furnish glucose, and after 

 this store has been exhausted, the glycogen in the liver furnishes 

 the needed glucose. Under normal conditions some glucose is found 

 in the blood and a trace in the muscles. 



