THE NATURE AND THE MEANS OF DIGESTION 29 



this work of separation is accomplished in the preparation 

 of food, both industrial and domestic. In this way the 

 task of the digestive organs is lightened, and, as we are 

 often told, overeating is made easy. Some foods are 

 quite devoid of residues when successfully digested and 

 absorbed. 



The early writers, having little knowledge of chemistry, 

 were naturally led to make much of the mechanical re- 

 duction of food in the alimentary tract. Mastication sub- 

 divides the food, and it was held that the later opera- 

 tions, especially those of the stomach, were essentially 

 further grindings of a similar sort. Such mechanical proc- 

 esses as do occur continue to be of interest, but they are 

 now seen to be preliminary to actual digestion. More- 

 over, we shall see that it is easy to assume that they are 

 of a more positive nature than is really the case. The 

 human stomach is not a mill, though the gizzard of a 

 bird may be fairly described by that word. 



In the eighteenth century the emphasis passed from 

 mechanical factors to the process of dissolving the food. 

 Solution is plainly one of the features of digestion, but it 

 is a somewhat superficial one. Of course, it is natural to 

 believe that solid food must become liquid before it can 

 penetrate the intestinal wall, but mere solubility, as we 

 have already seen, is not a guarantee of fitness for the 

 use of the cells. Freely soluble foods like cane-sugar and 

 milk-sugar require to undergo digestive changes just as 

 definite as those carried out in the case of fats or coagulated 

 proteins. It is often stated that the object of digestion is 

 to produce diffusible substances. This statement is in- 

 adequate, for diffusibility like solubility does not in itself 

 determine the utility of a food. The sugars mentioned 

 above are sufficiently diffusible, and the changes which 

 they undergo before absorption serve a more fundamental 

 purpose than the mere hastening of their passage through 

 the lining of the intestine. 



In the light of modern chemical knowledge we can be 

 somewhat specific in regard to the molecular aspects of the 



