DIGESTION AND ABSORPTION l6l 



converted into conditions necessary for absorption and metabo- 

 lism, (b) the region of absorption, where the digested food is 

 taken up and conveyed to the cells, (c) the region of metabolism, 

 where the digested and absorbed food is built up into body 

 substance, or is oxidized, (d) the region of excretion, where the 

 products of metabolism are removed from the body. It must 

 be noted that these are not regions defined anatomically by 

 location in certain organs but by the process which takes place. 



We shall now consider each one of these processes in the order 

 in which they take place in the animal body. In the following 

 discussion of the physiological processes of animals the facts 

 refer primarily to human beings. In general they apply also 

 to domestic animals, but in cases where they are known to be 

 different some qualifying statement will be made. 



Mastication or Chewing. — The mastication of the food in 

 the mouth, or in other regions where this is accomplished in 

 some animals, as the gizzard of fowls, is not, strictly speaking, 

 part of the process of digestion. In most cases it must precede 

 digestion and is part of the physical operation by which the 

 food is obtained and introduced into the animal body in a proper 

 condition for digestion. It results in the finer division of the 

 food substance so that it will be in such a physical condition 

 that digestive juices may act upon it. It will naturally vary as 

 the character of the food substances varies. In the case of 

 liquid foods or foods already in solution no mastication is neces- 

 sary. We must caution against assuming from what has 

 previously been said that all soluble substances are on that 

 account already digested and do not need digestive action. A 

 soluble substance, as we shall soon explain, may or may not be 

 subject to digestion. For each food constituent there is a final 

 form which it must assume before it can be absorbed and 

 metabolized. The food material in a few cases may be already 

 in this form, but it may also be readily soluble and still require 

 digestion. The solid foods require more or less thorough 

 mastication in order to make them subject, under the best 

 conditions, to digestive action. Plainly, the tougher and 



M 



