420 DIGESTION 



tration is practically the same as that of water. In other words, in 

 this patient the gastric contents after the test meal were neutral, and 

 it would be impossible for peptic digestion to. proceed. It must be 

 distinctly noted that the acidity of the gastric contents during the 

 digestion of test meals is not the same thing as the acidity of the pure 

 gastric juice. Observations on juice obtained from a case of gastric 

 fistula without admixture with saliva showed that in ' hunger ' juice, 

 where continuous secretion was going on, the hydrogen-ion concentra- 

 tion varied from 0-056 to o-ioo normal in several samples collected on 

 different days (Menten). 



This question of reaction has significance in two ways : in the first 

 place the reaction determines whether a given ferment shall be 

 destroyed or not by another ferment or by the alkalinity or acidity 

 of the medium. Thus pepsin can be destroyed by the alkali of the 

 pancreatic juice, enterokinase and trypsin by the hydrochloric acid 

 of the gastric juice, trypsinogen by the pepsin and hydrochloric 

 acid. Trypsin has no destructive effect on enterokinase or tryp- 

 sinogen (Mellanby). Secondly, the reaction affects the activity of 

 this or that ferment on the food substances. The optimum hydro- 

 gen-ion concentration for peptic digestion is relatively high (0-02 

 to 0-03 normal). When it is decreased to ooooi normal the rate 

 of digestion is only one-half to one-fifth as rapid. The high con- 

 centration of hydrogen-ions in the gastric contents of healthy persons 

 is clearly advantageous, and the low concentration in the gastric 

 contents in cases of hypoacidity clearly disadvantageous. Trypsin 

 acts best in a medium which contains more hydroxyl than hydrogen 

 ions. When the alkalinity is diminished it becomes less active, 

 although it is not entirely inhibited even by an acid reaction until 

 the hydrogen-ion concentration reaches about o-oooi (or IXIQ-*) 

 normal. (See footnote, p. 1139.) 



In the intestine it is possible that trypsin may perform its work 

 in a medium which is sometimes acid; and although the cause of 

 the acidity and the character of the medium are far from being the 

 same as in the gastric juice, it is obviously an advantage that the 

 chief proteolytic ferment should be able to act upon the proteins in 

 all parts of the intestine and at every stage of intestinal digestion 

 whether the reaction is acid or alkaline. The proteins of the chyme 

 are all carried by the trypsin to the stage of peptone, and the pep- 

 tone, even in perfectly normal digestion, is further split up into 

 amino- and diamino-acids by the trypsin and by the erepsin of the 

 succus entericus. 



In the lower portions of the small intestine bacteria of various 

 kinds are present and active; and it is not unlikely that even 

 throughout its whole length a certain range of action is permitted 

 to them, checked by the acidity of the chyme, though scarcely by 

 the feeble antiseptic properties of the bile. 



The lower end of the small intestine is not cut off by any bacteria- 

 proof barrier from the large intestine, in which putrefaction is con- 



