94 ADVANCED PHYSIOLOGY 



to be proteids. They cannot be absorbed as proteid even though 

 our own muscles and blood are largely of proteid material. 

 The change which proteids must go through is a chemical 

 'one, and the first step toward it is brought about by the 

 hydrochloric acid and the pepsin in the gastric juice. This 

 pepsin is one of the ferments, or enzymes, to which attention 

 was directed in an earlier chapter, and under its influence the 

 proteids, which are very complex bodies, are broken into simpler 

 ones. These new bodies, peptones, proteoses, and allied sub- 

 stances, are no longer real proteids but are far simpler chem- 

 ically, and may readily pass through the wall of the stomach. 

 It is probable, however, that a large part of the proteid eaten 

 is carried on into the intestine and still further modified 

 before absorption. 



Milk contains much proteid in the form of casein. This 

 casein, however, is not readily acted on by pepsin until the 

 milk is curdled. Rennin, the second enzyme in gastric juice, 

 produces the curdling action on milk. It is from a similar 

 curd that cheese is made, cheese-makers adding to their 

 milk rennet which is usually obtained by a process of extrac- 

 tion and refinement from the stomachs of calves. Calves' 

 diet consists mainly of milk and their gastric glands contain 

 great quantities of rennin. 



From these facts it is evident that milk taken into the 

 stomach is speedily soured by the acid in the gastric juice 

 and curdled by the rennin. If a baby vomits, and its milk 

 is found to be curdled and sour it may indicate that the child 

 has eaten more than it can comfortably hold in its stomach, 

 but the changes in its food only show that digestion is going 

 on properly. 



On sugars and starches gastric enzymes have practically no 

 effect. Indeed, the digestion of starches which was begun 

 by the saliva in the mouth stops after the food has become 

 mixed with the gastric juice, because the salivary enzymes will 

 not act in an acid solution, and there is considerable acid in 



