HORMONES 41 



endoenzymes. They are best known in connection with de- 

 structive metabolism, the modern conception being that endo- 

 enzymes are the causes of a series of progressive chemical decom- 

 positions. Each chemical process is presided over by a specific 

 endoenzyme which acts only on certain chemical substances and 

 gives rise to other chemical substances to be acted on in turn by 

 other enzymes. "The processes which years ago were consid- 

 ered as due to the peculiar vital properties of the tissue cells, 

 and which were supposed to be entirely dependent upon their 

 morphological and functional integrity, are now seen to be due 

 primarily to a great variety of enzymes, manufactured indeed 

 by the living cells, but capable of manifesting their activity 

 even when free from the influence of the living protoplasm. 

 The varied processes of tissue katabolism are the result of or- 

 derly and progressive chemical changes, in which cleavage, 

 hydrolysis, reduction, oxidation, deamidization, etc., alternate 

 with each other under the influence of specific enzymes, where 

 chemical constitution and the structural make-up of the various 

 molecules are determining factors in the changes produced/' 

 (Chittenden, The Nutrition of Man, pp. 75, 76.) 



Hormones. A second group of enigmatical chemical sub- 

 stances produced by living organisms includes the hormones. 

 These are extremely difficult to study, and facts regarding them 

 have come to light only recently. A good example is the hor- 

 mone secretin in man, which causes the pancreas cells to secrete 

 the digestive ferments of the pancreatic juice. This secretin 

 is formed after contact of the acidified contents of the stomach 

 with the mucous membrane of the small intestine. The acid 

 food stuffs do not stimulate nerves which start secretion in the 

 pancreas, but they act apparently upon a substance formed in 

 the cells of the mucous membrane, transforming this substance 

 into a heat-resisting hormone secretin which reaches and stimu- 

 lates the appropriate nerves through the blood and these, in 

 turn, stimulate the pancreas cells to secrete. Other hormones 

 are responsible for many of the phenomena of growth and 

 differentiation, and possibly they play an important part in 

 early development of the individual. Again, there is strong 



