DIGESTION 329 



Mechanical stimulation unJoubtedly causes a secretion, 

 probably through a reflex in the nerve plexuses in the wall 

 of the gut. 



As regards the action of extrinsic nerves very little is 

 known. It has been found that, if the intestine be ligatured 

 in three places so as to form two closed sacs, and the nerves 

 to one of these be divided, that part becomes filled with a 

 clear fluid closely resembling lymph. The dilatation of the 

 blood-vessels may, however, account for this, without 

 secretion being implicated. 



D. Bacterial Action in the Alimentary Canal. 



Numerous micro-organisms of very diverse character are 

 swallowed with the food and saliva. It has been suggested 

 that the leucocytes, formed in the lymphoid tissue of the 

 tonsils and pharynx, attack and destroy such organisms, but 

 so far, definite proof of this is not forthcoming. 



When the food is swallowed, the micro-organisms 

 multiply for some time in the warm, moist stomach, and 

 certain of them form lactic and sometimes acetic acid by 

 splitting sugars. But, when sufficient gastric juice is poured 

 out for the hydrochloric acid to exist free, the growth of 

 micro-organisms is inhibited, and some of them, at least, are 

 killed. 



Those which are not killed pass on into the intestine, 

 and, as the acid in the chyme becomes neutralised, the acid- 

 forming organisms begin to grow, and, by splitting the 

 sugars, form lactic or acetic acid and render the contents of 

 the small intestine slightly acid. Towards the end of the 

 small intestine, and more especially in the large intestine, 

 the alkaline secretions have neutralised these acids, and, in 

 the alkaline material so produced, the putrefactive organisms 

 begin to flourish and to attack any protein which is not 

 hydrolysed by the digestive enzymes — splitting it up and 

 forming among other substances a series of aromatic bodies, 

 of which the chief are indol, skatol, and phenol. 



Aromatic Bodies. — This splitting probably occurs through 



