SURVEY OF DIGESTION AS A WHOLE 417 



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 com- 

 plete 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 pan- 

 creatic 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 disintegration is brought 

 about. This is aided by the digestion of the gelatin-yielding con- 

 nective tissue which holds together the fibres of muscle and the cells 

 of fat, and the digestible structures in vegetable tissue which enclose 

 starch granules. Such nucleo-proteins as come into contact with the 

 gastric juice will be split up and the proteins digested to peptone. 

 The globin of the blood pigment will undergo the same change, while 

 the haematin is not much affected. 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. 353) 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 proteoses 

 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 peptone stage 

 while still in the stomach, leaving for the juices that act on it in the 

 intestine only its further hydrolysis to amino-acids, etc. But we 

 may safely assume that, in the case of a man living on an ordinary 

 mixed diet, a good deal of the food proteins passes through the 

 pylorus chemically unchanged, or having undergone only the first 

 steps of hydration. For, even a few minutes after food has been 



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