184 MOUTH DIGESTION. 



richer in alkaline-reacting salts and in amylolytic power than the 

 secretion coming without stimulation. Mixed saliva resulting 

 from stimulation with ether, alcohol, etc., contains a much larger 

 proportion of mucin than the secretion coming without stimula- 

 tion, being noticeably thick and viscid. This quality is not appar- 

 ent in the saliva resulting from mechanical stimulation. 



Duration of the Amylolytic Action of Saliva. One of the in- 

 teresting questions in regard to the amylolytic action of saliva is 

 as to its duration. As we have already seen, a very minute per- 

 centage of free acid arrests this action. The food remains in the 

 mouth but a short time, when, in portions, each of which is an 

 alimentary bolus, it passes through the esophagus into the stomach, 

 a process which is also very brief, and in the latter organ it meets 

 with a fluid, the gastric juice. 



Although the amount of hydrochloric acid in this fluid is 

 enough to arrest the action of the enzyme if it was free, still it 

 must not be forgotten that this is not the case. It is in combination 

 with the proteids of the gastric juice, and later with the proteids 

 and peptones the results of stomach digestion, so that consider- 

 able time must elapse before there is enough of the free acid 

 present to arrest the action of the ptyalin. Just how long this 

 is, it is impossible to say in all cases, for it will vary under different 

 conditions. Experiments made by Cannon (p. 201) show that 

 while the contents of the pyloric end of the stomach are strongly 

 acid, free hydrochloric acid appearing there at the end of half an 

 hour, at the end of two hours there is no free acid in the middle 

 of the food in the cardiac end, so that during all this time the 

 action of the saliva may continue. 



In discussing this subject Moore says : " The diastatic action 

 of the saliva, therefore, continues in the stomach during and after 

 a meal until (1) the alkali of the saliva has been neutralized, (2) 

 the proteid present in solution has been satisfied, and (3) a trace 

 of free hydrochloric acid remains in excess." 



Some excellent authorities are inclined to regard the chemical 

 action of the saliva as subordinate to the mechanical, inasmuch as 

 in their opinion the amount of starch converted into maltose by 

 the ptyalin is inconsiderable. This they infer from the fact that 

 a medium as acid as the gastric juice will inhibit the action of the 

 enzyme, and, since the food remains in the mouth and esophagus 

 but a short time, it follows that the conditions favorable for 

 salivary digestion must be of brief duration. It seems to us that 

 they ignore the facts, already stated, with reference to the necessity 

 of free acid to stop the action of the enzyme, and to the long time 

 before this is present in the stomach in quantity sufficient to pro- 

 duce its effect. It is undoubtedly true that the starch digestion 

 of the small intestine is very important, but certainly in the two 

 hours, approximately, that the saliva acts a considerable amount 



