390 A MANUAL OF PHYSIOLOGY 



reflexly through the presence of the food and the water of the 

 saliva in the stomach, is for a time neutralized, and amylolytic 

 digestion still permitted to go on. For 20 to 40 minutes after 

 digestion has begun there is no free hydrochloric acid jn the 

 stomach, although some is combined with proteins, and during 

 this period the ptyalin of the swallowed saliva will be able to 

 act even better than in the mouth, being favoured by a weakly 

 acid reaction. Indeed, for a time, as the meal goes on, the 

 successive portions of food which arrive in the stomach will 

 find the conditions more and more favourable for amylolytic 

 digestion. But as the acidity continues to increase, the activity 

 of the ptyalin will first be lessened, and ultimately abolished ; 

 and, upon the whole, a considerable proportion of the starches 

 must usually escape complete conversion into sugar until they 

 are acted upon by the pancreatic juice. This is particularly 

 the case with unboiled starch, as contained in vegetables which 

 are eaten raw ; and, indeed, we know that sometimes a certain 

 amount of starch may escape even pancreatic digestion, and 

 appear in the faeces. Meanwhile, pepsin and hydrochloric acid 

 are being poured forth ; the latter is entering into combination 

 with the proteins of the food ; and before the end of an ordinary 

 meal peptic digestion is in full swing. The movements of the 

 pyloric end of the stomach increase, and eddies are set up in its 

 contents, which carry the morsels of food with them, and throw 

 them against its walls. In this way not only are the contents 

 thoroughly mixed, and fresh portions of food constantly brought 

 into contact with the gastric juice secreted mainly in the more 

 passive cardiac end, but a certain amount of mechanical disinte- 

 gration is brought about. This is aided by the digestion of 

 the gelatin-yielding connective tissue which holds together the 

 fibres of muscle and the cells of fat, and the digestible structures 

 in vegetable tissue which enclose starch granules. If milk has 

 formed a portion of the meal, the caseinogen will have been 

 curdled soon after its entrance into the stomach, by the action 

 of the rennet ferment alone (see p. 326) when the milk has 

 been taken at the beginning of digestion before the gastric 

 contents have become distinctly acid, by the acid and ferment 

 together when it has been taken later. The caseinogen and other 

 proteins of milk, like the myosinogen and other proteins of 

 meat, and the globulins, albumins, and other proteins of bread and 

 of vegetable food in general, are acted upon by the pepsin and 

 hydrochloric acid, yielding ultimately peptones ; while variable 

 quantities of these proteins and of the acid-albumin and pro- 

 teoses derived from them may escape this final change, and pass 

 on as such into the duodenum. In the dog, indeed, a very large 

 proportion of a meal of flesh has been found to be digested to the 



