COMPOSITION OF FOOD AND ACTION OF ENZYMES. 657 



The Chemical Changes of the Foodstuffs during Digestion. 



The physiology of digestion consists chiefly in the study of the 

 chemical changes that the food undergoes during its passage through 

 the alimentary canal. It happens that these chemical changes are 

 of a peculiar character. The peculiarity is due to the fact that the 

 changes of digestion are effected through the agency of a group of 

 bodies known as enzymes, or unorganized ferments, whose chemical 

 action is more obscure than that of the ordinary reagents with which 

 we have to deal. It will save repetition to give here certain general 

 facts that are known with reference to these bodies, reserving for 

 later treatment the details of the action of the specific enzymes 

 found in the different digestive secretions. 



ENZYMES AND THEIR ACTION. 



Historical. The term fermentation and the idea that it is 

 meant to convey has varied greatly during the course of years. The 

 word at first was applied to certain obvious and apparently spon- 

 taneous changes in organic materials which are accompanied by the 

 liberation of bubbles of gas: such, for instance, as the alcoholic 

 fermentations, in which alcohol is formed from sugar; the acid fer- 

 mentations, as in the souring of milk; and the putrefactive fer- 

 mentations, by means of which animal substances are disintegrated, 

 with the production of offensive odors. These mysterious phenom- 

 ena excited naturally the interest of investigators, and with the 

 development of chemical knowledge numerous other processes were 

 discovered which resemble the typical fermentations in that they 

 seem to be due to specific agents whose mode of action differs from 

 the usual chemical reactions, especially in the fact that the causa- 

 tive agent itself, or the ferment as it is called, is not destroyed or 

 used up in the reaction. Thus it was discovered that germinating 

 barley grains contain a something which can be extracted by water 

 and which can convert starch into sugar (Kirchhoff, 1814). Later 

 this substance was separated by precipitation with alcohol and was 

 given the name of diastase (Pay en and Persoz, 1833). Schwann 

 in 1836 demonstrated the existence of a ferment (pepsin) in gastric 

 juice capable of acting upon albuminous substances, and a number 

 of similar bodies were soon discovered: trypsin in the pancreatic 

 juice, amygdalin, invertin, ptyalin, etc. These substances were all 

 designated as ferments, and their action was compared to that of 

 the alcoholic fermentation in yeast, the process of putrefaction, etc. 

 Naturally very many theories have been proposed regarding the 

 cause of the processes of fermentation. For the historical develop- 

 ment and interrelation of these theories references must be made to 

 42. 



