358 FOOD AND DIGESTION 



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 with the exception of the 

 dextrins and glycogen, 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 substances. 



The salivary glands of children do not produce functionally active saliva 

 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 o . 05 per cent, of hydrochloric 

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

 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 proteins 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 protein 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 con- 

 tact 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 

 alkaline 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: 



The mouth is closed, and the food after thorough mixing with the saliva 

 is rolled into a bolus on the dorsum of the tongue. The tip of the tongue is 

 pressed upward and forward against the hard palate, thus shutting off the 

 anterior part of the mouth cavity. The mylo-hyoid muscles then suddenly 



