100 THE HUMAN MECHANISM 



To this entire process of the supply and preparation of 

 food for eating, the term "alimentation" may be conveniently 

 applied. Reflection will show that it is largely a process of 

 food refining, the principal result being a concentration of the 

 nutrients at every step. It is also a separation of the com- 

 paratively useful from the comparatively worthless (as food) ; 

 and just here, and in these points, concentration and the 

 separation of good from poor materials, we may recognize a 

 true process of digestion, but one external rather than in- 

 ternal : a refining in the field, the mill, and the kitchen 

 rather than in the stomach; in the environment rather than 

 within the organism. 



14. The ends accomplished by digestion. The processes of 

 digestion accomplish three chief results: First, they separate 

 the nutritious and therefore important part of the food from 

 the innutritious and therefore useless. This process, so con- 

 spicuous in the case of external digestion, is continued within 

 the alimentary canal. Second, digestion brings the solid part 

 of the food into solution by changing insoluble into soluble 

 substances. This is necessary, since food is received into the 

 body proper (that is, into the blood) through the lining 

 membranes of the alimentary canal, and in order that it may 

 pass through these membranes it must be dissolved. In the 

 third place, digestion transforms the food as eaten into com- 

 pounds which can be used by the cells of the body. Common 

 cane sugar, for example, is very soluble and can be absorbed 

 into the blood, but the cells of the body cannot use it. In 

 the intestine it is split into grape sugar and fruit sugar, both 

 of which can be used. Similarly, the white of egg (a protein), 

 though soluble, would be of little, if any, use if injected 

 unchanged into the blood; in the alimentary canal it is 

 transformed into available compounds. It will be helpful to 

 acquire at this time a general idea of the chemical structure 

 of two of our most important foods and of the chemical 

 changes which they undergo in the alimentary canal. 



