1S6 HUMAN PHYSIOLOGY 



The Nature of the Digestive Changes. Digestion is 

 often said to be a preparation of food for absorption and 

 this is correct if it is understood in the broadest sense. 

 A narrow conception is to be avoided. First of all, 

 we must not think of digestion as mere solution. Before 

 the rise of organic chemistry it was scarcely possible to 

 have any other idea concerning it and observers judged 

 its progress simply by the dissolving of food samples. 

 It is true that solid food must be dissolved but there 

 are other aspects of the process to be taken into account. 

 Foods which are in solution may yet require to undergo 

 digestion. 



Digestion is a process of refining. It effects a separa- 

 tion between the valuable and the useless. But, again, 

 this is only one feature of the change. When a food 

 already soluble is digested it is said to gain in diffusi- 

 bility. By this we mean that the power to pass through 

 ordinary membranes, like parchment, is increased. 

 This may be supposed to make absorption easier but, 

 once more, the gain in diffusibility is but one of several 

 phases in the transformation which we call digestion. 

 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. 



