35 o A MANUAL OF PHYSIOLOGY 



forcing of the fluid through the cardiac orifice of the stomach 

 by the contraction of the oesophagus. 



Chemical digestion in man begins already in the mouth, a 

 part of the starch being there converted into dextrins and 

 sugar (maltose), as has been shown by examining a mass of 

 food containing starch just as it is ready for swallowing 

 (p. 376). This process is no doubt continued during the 

 passage of the food along the oesophagus. 



The first morsels of a meal which reach the stomach find 

 it free from gastric juice, or nearly so. They are alkaline 

 from the admixture of saliva ; and the juice which is now 

 beginning to be secreted, in response to the presence of the 

 food, and to reflex excitement starting in the mouth, is foi 

 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 in the stomach, although 

 some is combined with proteids, and during this period the 

 ptyalin of the swallowed saliva will be able to act even 

 better than in the mouth, being favoured by the weakly acid 

 reaction due to the lactic acid(and the traces of butyric and 

 acetic acids) produced during the first part of the digestive 

 period by the action of the Bacillus acidi lactici on the carbo- 

 hydrates of the food. 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 pan- 

 creatic 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 proteids 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 



