350 HUMAN PHYSIOLOGY 



lactase, which splits inilk sugar. (3) Erepsin, an enzyme 

 which seems to act more powerfully than trypsin in splitting 

 peptone and many other proteids into their component 

 non-proteid crystalline constituents such as the di-amido 

 acids and non-amido acids, e.g. leucin and tyrosin. The 

 object of this is not easy to understand, but it may be that 

 the nitrogen of the proteid is largely treated as a waste 

 product. Vernon has shown that a similar enzyme is widely 

 distributed in the tissues, being specially abundant in the 

 kidney. (4) Enterokinase a zymin which, acting on tryp- 

 sinogen, converts it into active trypsin (p. 348). 



Nervous Mechanism of Secretion. So far very little is 

 known on this point. It has been found that, when the in- 

 testine is ligatured in three places so as to form two closed 

 sacs, if the nerves to one of these be divided, it becomes filled 

 with a clear fluid closely resembling lymph. The dilatation 

 of the blood-vessels may account for this without secretion 

 being implicated. 



Bacterial Action in the Alimentary Canal. 



With the food, water, and saliva, numerous micro- 

 organisms of very diverse character are swallowed. It has 

 been suggested that the leucocytes formed in the lymphoid 

 tissue at the back of the mouth 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 these, by splitting sugars, form lactic and 

 sometimes acetic acid. But when sufficient gastric juice is 

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

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

 killed. Others 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 pro- 

 duced the putrefactive organisms begin to flourish and to 



