FOOD AND DIGESTION. 34^ 



The salivary glands of children do not become functionally active till 

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

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

 soluble and capable of absorption. The salivas of the dog, cat, bear, and 

 pig are almost inactive, whereas that of monkeys, rabbits, mice, squirrels 

 and guinea-pigs, are strongly diastasic. 



Salivary Digestion in the Stomach. Under proper conditions salivary 

 digestion may continue for some time after the food has entered the stom- 

 ach. In laboratory experiments it is found that while the addition of 

 even .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 loose 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 secreted combines with the proteid food-stuffs and 

 so does not affect the ptyalin. It usually requires at least 15 to 20 min- 

 utes before the acid is secreted in sufficient quantity to be in exces^ as 

 free acid, of the amount which can combine with the proteids, and during 

 this time salivary digestion may continue. Of course the action of pty- 

 alin on food taken later in a meal is promptly stopped when it reaches the 

 stomach because of the presence of free acid. 



The Nervous Mechanism of the Secretion of Saliva. 



The secretion of saliva is under the control of the nervous system. It 

 is a reflex action. Under ordinary conditions it is excited by the stimu- 

 lation of the peripheral branches of two nerves, viz., the gustatory or 

 lingual branch of the inferior maxillary division of the fifth nerve, and 

 the glosso-pharyngeal part of the eighth pair of nerves, which are distrib- 

 uted to the mucous membrane of the tongue and pharynx conjointly. 

 The stimulation occurs on the introduction of sapid substances into the 

 mouth, and the secretion is brought about in the following way: From 

 the terminations of the above-mentioned sensory nerves distributed in the 

 mucous membrane an impression is conveyed upward (afferent) to the 

 special nerve centre situated in the medulla-oblongata which controls 

 the process, and by it is reflected to certain nerves supplied to the 

 salivary glands, which will be presently indicated. In other words, 

 the centre, stimulated to action by the sensory impressions carried 

 to it, sends out impulses along efferent or secretory nerves supplied 

 to the salivary glands, which cause the saliva to be secreted by and dis- 



