DIGESTION 189 



The gastric juice of the dog withstands putrefaction for a long 

 time ; it may be kept for months without undergoing any im- 

 portant change. Not so with herbivora ; the mixed gastric 

 fluids of the horse rapidly putrefy. The antiseptic properties 

 of the dog's juice are attributed to its hydrochloric acid ; if this 

 is so, it is additional evidence against the acid of herbivora being 

 hydrochloric. There appears to be no reason why lactic acid 

 should not be formed by the marginal cells of the fundus glands, 

 but a source of lactic supply in herbivora is the carbohydrate 

 of their food. 



Pepsin is of a protein nature, though very little is known of it 

 chemically. It exhibits its action best at a temperature of the 

 interior of the body (37 to 40 C.) ; a low temperature retards 

 its activity, while it is destroyed at a high one. The ordinary 

 commercial product is very impure ; it is an extract of the 

 mucous membrane of the stomach, to which starch or milk 

 sugar has been added. In physiological work a glycerin 

 extract of the mucous membrane of the stomach suffices, to 

 which is added some dilute HC1. Glycerin has the power of 

 extracting the ferments both from the stomach and other portions 

 of the digestive tract, such as the pancreas. The pepsin is 

 formed, as we have seen, in the chief cells of the gastric glands 

 as a zymogen or propepsin, which becomes pepsin after secre- 

 tion. The action of pepsin is almost wholly, if not entirely, 

 confined to the protein constituents of food. It converts the 

 insoluble proteins into soluble ones, not by direct transforma- 

 tion, but by several stages. The products intermediate between 

 protein and peptone have received certain names suggestive of 

 differences in their chemical nature, but as to all of this a good 

 deal of doubt and speculation exists. 



In the following table the various stages of conversion are 

 indicated in the order in which they are found to occur as deter- 

 mined by small differences in the chemical tests, such as solu- 

 bility or colour reaction, yielded by the peptonised product. 

 The table is the one drawn up by Ktihne : 



1. The protein as consumed, or native albumin. 



2. Acid albumin or syntonin. 



3. Primary proteoses. 



4. Secondary proteoses. 



5. Peptones. 



The protein, having reached the stage of peptones, is now 

 capable of being absorbed, but the conversion from protein to 

 peptone is a most complex one, during which the large protein 

 molecule is converted into simpler products of an infinitely 

 smaller molecular weight, while so great is the complexity that 



