68 NUTRITIONAL PHYSIOLOGY 



place than was formerly granted to it. The enzyme is 

 extremely sensitive to acid. Inasmuch as the gastric juice 

 is decidedly acid, it used to be claimed that salivary diges- 

 tion could not proceed in the stomach. But it has come 

 to be recognized that when a large mass of food is intro- 

 duced into the stomach within a short time the gastric juice 

 penetrates it rather slowly. A few minutes after the com- 

 pletion of a meal we may picture tKe stomach-contents as 

 being acidified near the surface, the acid slowly making its 

 way inward, but having a neutral or even alkaline central 

 portion. Salivary digestion will be continued in the stead- 

 ily diminishing region not yet reached by the acid, and will 

 cease only when the gastric secretion from one wall of the 

 stomach meets that from the other. Any rotation of the 

 contents would probably bring about an earlier distribution 

 of the acid and arrest of starch digestion. No such rota- 

 tion seems normally to occur. A factor which operates to 

 postpone the destruction of ptyalin is the power of the 

 proteins of the diet to engage hydrochloric acid in com- 

 bination. Since proteins are almost always present, the 

 gastric glands must secrete acid enough to satisfy their 

 capacity before there can be the excess of strictly free acid 

 which will put an end to salivary digestion. 



If the mixed food is quite acid at the outset, it is hard to 

 see how there can be any hydrolysis of starch brought 

 about by the saliva. Yet we constantly eat acid fruits 

 before our breakfast cereal and notice no ill effects. 

 Starch which escapes digestion at this stage is destined to 

 be acted upon by the pancreatic juice, and the final result 

 may be entirely satisfactory. Still it is reasonable to as- 

 sume that the greater the work done by the saliva, the 

 lighter will be the task remaining for the other secretions 

 and the greater the probability of its complete accomplish- 

 ment. The power of saliva to convert raw starch to sugar 

 is almost incomparably smaller than its capacity to digest 

 starch which has been cooked. Raw starch exists in very 

 dense grains which have to be dissolved from the surface 

 inward. Cooking, especially boiling, utterly destroys these 



