' THE ANIMAL BODY— DIGESTION— METABOLISM 31 



and peptones and splits them into amino acids. Thus thru the action 

 of the trypsin and erepsin all the protein which can be digested is split 

 into amino acids. The amino acids are absorbed thru the walls of the 

 small intestine and pass into the blood. They are then carried into the 

 general circulation, and from the blood each of the parts of the body — 

 muscles, organs, etc. — absorbs a certain amount to be used for repair or 

 in growth. 



A good picture of what takes place in protein digestion can be had 

 by likening the food proteins to a house being taken down by a builder 

 in order that he may construct another from the materials. An animal 

 eating protein compounds cannot use them just as they are, but must 

 first take them apart to a greater or less extent, and from the parts 

 reconstruct other kinds of protein suitable for its own use. In other 

 words, the proteins must have a different architecture from those in 

 the plants. The proteoses and peptones may be likened to the roof 

 and walls of the house. These walls and roof can be separated into 

 bricks and tiles, which are represented by the amino acids ; and from 

 these the animal, beginning anew, can construct new proteins of the 

 exact kind its body may require. 



Mineral matter ; water. — The mineral matter in feeding stuffs is not 

 acted upon by any enzymes, but is dissolved in its passage thru the 

 digestive tract, especially in the stomach by the acid in the gastric juice. 

 It is absorbed chiefly from the small intestine. 



Water requires no digestion and is absorbed chiefly from the small 

 intestine, but also to some extent from the stomach and large intestine. 



Distribution and use of absorbed nutrients. — We have seen that the 

 digested fats which are to nourish the body are poured into the blood 

 current by way of the lymphatics, while the glucose and the amino acids 

 enter the blood directly thru the capillaries and veins. The veins from 

 the small intestine unite and become the portal vein, which passes the 

 blood thru the liver and on into the heart. The various nutrients, 

 having been mingled with the blood, are carried thru the circulation to 

 the capillaries. 



These are so constructed that, when the blood finally reaches them, 

 the nutrients it carries pass thru their walls and into the lymph that 

 bathes the body cells. In this manner all the nutrients, having been 

 especially prepared and transported, nourish every part of the body. 



The nutrients may be oxidized, or burned, to warm the animal, or to 

 produce energy to carry on the vital processes and to perform work, as 

 shown in the following chapters. In case more nutrients are supplied 

 than are required for these purposes, the excess may be built into body 

 tissue proper, as shown in Chapter V. The glucoses may be converted 

 into fats and stored as body fat, as may also the fats derived directly 



