130 PHYSIOLOGICAL CHEMISTRY 



already stated, there are different enzymes which are responsible 

 for the different steps of the decomposition of starch. 



Since the action of ptyalin is inhibited by concentrations of 

 acid such as are present in gastric juice, it might be expected 

 that its activity would last only during the brief time before 

 the food enters the stomach. It has been shown, however, that 

 when a meal is taken, the food forms a bolus or mass in the 

 stomach and does not become thoroughly mixed with the acid 

 gastric juice for some time, perhaps a half hour. The ptyalin 

 thus continues to act under these conditions for some time after 

 the food is swallowed, and the decomposition of starchy ma- 

 terials may be considerable. Ptyalin is not found in the saliva 

 of all animals. It is missing, for example, in the saliva of the 

 dog, cat and some others of the carnivora. Their natural food 

 is not starchy in character. 



The saliva also contains an enzyme erepsin which has the 

 power of splitting peptids, but saliva does not digest proteins or 

 fats. Certain substances are excreted in the saliva. Potassium 

 sulphoeyanate is present in small traces, but is not known to 

 perform any definite function. If a capsule containing potas- 

 sium iodide is swallowed, iodide soon may be demonstrated in 

 the saliva. It is taken out of the blood and excreted by the 

 salivary glands. Urea, uric acid and many other substances also 

 have been found in traces in the saliva. Perhaps they merely 

 filter through from the blood. 



