708 PHYSIOLOGY 



hydrochloric being sufficient for this purpose. It acts most rapidly at the 

 body temperature. At C. its action is still just perceptible. If heated to 

 60 C. it is destroyed. 



We have seen that boiled starch solution is changed by saliva when kept 

 only a few seconds in the cavity of the mouth. When the starch is in the 

 solid condition, as in biscuits and- most farinaceous foods, its stay in the 

 mouth during the normal process of mastication is not long enough to allow 

 of any considerable hydrolysis occurring. When a meal is taken, the food 

 which is swallowed forms a mass lying in the fundus of the stomach. This 

 mass is penetrated only with difficulty by the acid gastric juice secreted 

 by the mucous membrane of the stomach within five minutes of the taking 

 of food. Even half an hour after a meal the interior of the mass of food in the 

 stomach may be still found to be neutral or slightly alkaline. The food 

 therefore, thoroughly moistened by and mixed with saliva, remains in the 

 stomach for thirty to forty minutes before the sah' vary ferment is destroyed 

 by the penetration of the acid gastric juice. During this time the ptyalin 

 continues to exert its effect, so that we may say that the chief part of the 

 salivary digestion occurs actually in the stomach, and results in an almost 

 complete alteration of the starch into dextrins and maltose. Unboiled 

 starch is attacked with extreme slowness by the diastatic ferments either of 

 the saliva or the pancreatic juice, so that, if taken by man, large quantities 

 are unutilised and reappear in the faeces. Thirty to forty minutes after a meal 

 the food becomes thoroughly soaked with the acid gastric juice, and salivary 

 digestion gives place to gastric digestion. 



THE SECRETION OF SALIVA 



The mucous membrane of the mouth, especially on the under surface of 

 the tongue, presents a number of small glands which contribute by their 

 secretions to the moistening of the mouth. The greater part of the saliva 

 is formed in man by three pairs of glands, viz. the sublingual and the 

 submaxillary glands, situated in the floor of the mouth below the jaw, and 

 the parotid gland, lying in the cheek over the ramus of the inferior maxilla. 



The arrangement of these glands, especially of those in the floor of the mouth, 

 varies somewhat in different animals. In the dog and cat the sublingual gland is 

 wanting, its place being taken by a gland situated somewhat further back and known as 

 the retrolingual gland. In the pig both retrolingual and sublingual glands are present 

 in addition to the submaxillary, and one may sometimes find traces of the retro- 

 lingual gland in man. Many animals, e. g. the dog, also possess a gland situated in t he 

 orbit, which pours its secretion into the mouth the orbital gland. These glands can 

 be divided, according to their structure and the nature of the secretion which they form, 

 into several classes. Among the salivary glands of the mucous membrane wo may 

 distinguish two types, the mucous gland and the serous glaiul. In specimens hardened 

 and stained by the ordinary methods, the mucous ^land is distinguished by the fuel 

 that its short duct opens into wide alveoli, the lining cells of which are distended 

 with mucin and therefore present a clear unstained space in the section. In the 

 other type, the serous gland, the duct lined with columnar cells branches into a 

 series of acini which pn-M-nt a well -111,11 ked lumen and arc lined with small granular 

 cells with a very distinct and well-staining nucleus. The same general distinction 

 can be made out in the large salivary glands. The parotid gland in man and in 



