ELEMENTARY PHYSIOLOGICAL CHEMISTRY 219 



the mouth, and after about two minutes transfer this to a test tube. 

 Ascertain if reduction of cupric salts occurs. Repeat this experiment 

 with a grain of unboiled starch and note the difference in both cases 

 (see Carbohydrates, p. 167). 



The ptyalin will only act in neutral or alkaline reaction, but not in the 

 presence of free acid. 



EXPERIMENT VIII. If experiment VI. be repeated with the addition 

 of a few drops of 0*2 per cent, hydrochloric acid, so that the fluid reacts 

 acid to litmus, it will be noticed that no dextrine is produced. If the 

 mixture be heated for a considerable time a trace of a reducing sugar 

 may appear because of the hydrolysing action of the acid. 



From the results of this last experiment we see, therefore, that it 

 will be impossible for the action of the ptyalin to proceed in the acid 

 gastric contents. If the stomach be empty at the beginning of the 

 meal the process goes on for about half an hour, as the first portion of 

 acid which is secreted is bound to proteid, so that it does not exercise 

 its inhibiting influence on the ptyalin which has been swallowed. 



Only a small percentage of the starch taken with the food, however, 

 is changed by the time it leaves the stomach, and it would appear that 

 ptyalin, the only ferment in saliva, is of very little importance 

 for the digestion of starchy foods, the main seat of this being in the 

 duodenum by means of the amylopsin of the pancreatic juice. 



The actual function of the saliva is undoubtedly a mechanical one, 

 acting as a solvent for certain foods and assisting in the mastication and 

 swallowing of others. A body must be in solution before it can be 

 tasted, so that the saliva assists in the appreciation of taste. It is also 

 necessary for articulation and for preserving the sensitiveness of the 

 nerve endings of taste and common sensation. This explains why a 

 fever patient cannot taste things so well as during health. It is 

 interesting to note that in some animals the saliva contains little or no 

 amylolytic ferment. 



CHAPTER XII. 

 DIGESTION IN THE STOMACH. 



THE food, after being masticated in the mouth, is passed down the 

 oesophagus into the stomach, where it is collected, and remains for two 

 or three hours, meanwhile being acted on by the gastric juice. In 

 order that each particle of food may be efficiently digested, there is a 



