SALIVARY DIGESTION IN THE STOMACH 313 



very faintly alkaline medium and is inhibited by strong alkalies and especially 

 by acids even as weak as the acidity of the gastric juice. This last is of 

 particular importance since it raises the question as to how long the ptyalin 

 may act. 



The action of saliva on starch is not limited to the brief interval during 

 which food remains in the mouth, as is now well known, but may continue 

 for a time in the stomach. 



Ptyalin is strictly an amylolytic ferment. 



Starch appears to be the only principle of food upon which the saliva acts 

 chemically. The secretion has no apparent influence on gum, cellulose, or 

 on fat, and is equally destitute of power over albuminous and gelatinous sub- 

 stances. 



The salivary glands of children do not become functionally active till the 

 age of 4 to 6 months, and hence the bad effects of feeding them before this 

 age on starchy food, corn-flour, etc., which they are unable to render soluble 

 and capable of absorption. 



Salivary Digestion in the Stomach. Laboratory experiments 

 have demonstrated that while the addition of even 0.05 per cent of hydrochloric 

 acid will inhibit the action of ptyalin on a solution of starch, if any proteids 

 be present in the solution much more acid must be added before the action 

 of the ptyalin is stopped. The explanation of the latter fact is that the acid 

 unites with the proteids in some chemical combination forming "combined 

 acid," which has little effect, comparatively, on ptyalin. This "combined 

 acid" gives a red color with litmus, but is distinguished from free acid by 

 giving a brownish instead of a bluish color with Congo red. When food enters 

 an empty stomach, as happens at the beginning of a meal the acid first com- 

 bines with the proteid food stuffs and so does not at once affect the ptyalin. 



A still more important fact in its bearing on this subject was recently 

 discovered by Cannon, who showed experimentally that starchy foods mixed 

 with weak alkali remain alkaline in the stomach for as much as an hour and 

 a half. Such foods when swallowed into the stomach are packed away in 

 that organ in a mass. The secretion of the acid gastric juice comes in contact 

 only with the outer surface of the mass, which is not materially disturbed by 

 the stomach peristalses. The center of the mass may, therefore, remain alka- 

 line until the outer layers are completely eroded away, and the ptyalin may 

 continue to act on starch during the whole time. 



DEGLUTITION. 



When properly masticated, the food is transmitted in successive portions 

 to the stomach by the act of deglutition or swallowing. The following account 

 of deglutition is based upon the researches of Kronecker and Meltzer, whose 

 experiments seem to modify in some details the earlier theory of Magendie: 



