184 HUMAN PHYSIOLOGY 



With the advance of chemistry it was found that the 

 digestive changes are always cleavages, large and com- 

 plex molecules giving rise to new ones smaller and 

 more numerous. This reduction in the size of molecules 

 naturally favors diffusion. 



Most significant of all, digestion obliterates many of 

 the characters which differentiate foods and gives us at 

 last much the same set of products whatever the meal 

 may have been. Day by day we make different choices 

 but we do not greatly alter the nature of the contribu- 

 tion made by the intestine to the blood. A com- 

 paratively small number of individual substances result 

 from the serial cleavages which have occurred. So 

 we may say that digestion standardizes our food; it 

 prepares for the body a few acceptable compounds from 

 the many strange and foreign ones which were taken 

 into the stomach. 



The importance of this standardization can be made 

 clear by an illustration. Take cane sugar as an example. 

 Here is a food which contains no waste matter and needs 

 no further refining. It is soluble and diffusible. Yet 

 it is not fit to be introduced into the circulation and if 

 the experiment is tried it will be excreted through the 

 kidneys, in other words, treated as useless material. 

 The trouble is that it is not a native compound. A 

 single change quickly accomplished in the intestine 

 transforms it into two other kinds of sugar which the 

 body can utilize. 



Digestive Secretions. The process of digestion is 

 carried on under the influence of the several digestive 

 juices. When one of these is found to have power to 

 advance the digestion of a certain kind of food we 

 naturally assume that an agent exists to bring about 

 the observed effect and we call the supposed agent an 

 enzyme. Enzymes are not known in an isolated or 

 pure condition; their existence is inferred from the be- 

 havior of mixtures of a very heterogeneous sort. We 

 say that saliva contains an enzyme capable of digesting 



