86 NUTRITIONAL PHYSIOLOGY 



since, regarded as a solution of proteins, it is much more 

 dilute than the milk of the cow. 



Peptic Digestion. The chief enzyme of the gastric juice 

 is the one commonly called pepsin. Its relations with the 

 acid of the stomach are so close that many writers urge 

 that we should speak rather of "pepsin-hydrochloric acid/* 

 the term suggesting the existence of a compound of the 

 two which is responsible for the action on the food. The 

 power to digest proteins is manifested only with an acid 

 reaction, and is permanently lost when the mixture is made 

 distinctly alkaline. The conditions which permit peptic 

 digestion to take place are, therefore, precisely those which 

 exclude the action of the saliva. 



When protein in solid form, such as boiled white of 

 egg, is subjected to the influence of gastric juice the pieces 

 swell and become softened. Later they are dissolved. 

 When the trial is made with protein which is originally in 

 solution, such as unboiled white of egg, there is no visible 

 evidence of change. But there are physical and chemical 

 tests which can be employed to show that digestion is as 

 definite a change in this case as in the other. An early 

 manifestation of this fact is the loss of the property of 

 coagulation on heating. Later there are indications that 

 the molecules are undergoing cleavage. At each successive 

 stage there is a gain in the power of diffusion, a reduction 

 of viscosity, and a diminution in the number of precipitants 

 which can be employed to throw the protein out of solution. 



Physiologic chemists have studied minutely the charac- 

 teristics of the hydrolytic products during the advance of 

 peptic digestion. They have attempted to identify nu- 

 merous compounds, each of which has a transient existence 

 and is then itself hydrolyzed. For our present purposes it 

 would be unprofitable to dwell upon such questions of detail. 

 Certain of the earlier cleavage products are included under 

 the general name of proteases or albwnoses; others, arising 

 later and of a simpler character, are called peptones. 

 Roughly speaking, there is a parallel between salivary and 

 peptic digestion. In either case, molecules of great size 



