742 PHYSIOLOGY 



The process by which the huge starch molecule is converted into 

 dextrins and maltose is a very complicated one, and a number of 

 intermediate compounds of dextrins and maltose can exist between 

 those whose presence is revealed by their varying reaction to iodine. 



Ptyalin is most active in a neutral medium, so that the addition 

 of minute traces of acid to the saliva increases its diastatic power. 

 In the presence of free mineral acid ptyalin is rapidly destroyed, 

 003 per cent, 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 salivary 

 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, however, formed in man by three 

 pairs of glands, viz. the sub-lingual and the sub- maxillary glands, 

 situated in the floor of the mouth below the jaw, and the parotid 

 gland, lying in the cheek over the ramus of the superior 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 



