I go A MANUAL OF VETERINARY PHYSIOLOGY 



the resulting product, peptone, is in all probability a group of 

 compounds, rather than a single one, which only resemble each 

 other in their solubility and their definite reaction to certain 

 chemical tests. There is good reason for thinking that peptone 

 may not even represent the final end-product in the stomach, 

 but that still simpler bodies, to which the name pept<j>ids has 

 been given, may be formed. 



Rennin is the second enzyme found in the gastric juice ; it is 

 produced in the chief cells of the gastric glands as a prorennin, 

 and is subsequently, on secretion, changed into rennin under the 

 influence of the acid present. The enzyme can be readily ob- 

 tained from the true stomach of the calf,' but not from the pyloric 

 end. Commercially, rennin is used in the manufacture of cheese, 

 a good extract causing very rapid clotting of milk. This process 

 closely resembles the clotting of blood, and, like it, is followed by 

 a contraction of the clot, resulting in a solid substance known as 

 curds, and a yellowish fluid, whey. The above process further 

 resembles blood-clotting in requiring the presence of a calcium 

 salt. The conversion of the casein of milk into curd is believed 

 to involve two distinct processes : first, the formation of a sub- 

 stance known as paracasein by the action of rennin ; and, 

 secondly, the action of lime salts on paracasein, by which means 

 the curd is produced. It is even considered that the lime salts 

 are of more importance than the rennin, as no clotting occurs 

 in milk deprived of its calcium salts. The curdling of milk 

 brought about by the lactic acid organism may be familiarly 

 recognised when milk ' turns sour '; it is not the same as clotting, 

 but represents the precipitation of casein by an acid. 



There would appear to be no necessity for animals to possess 

 this ferment after weaning, as milk does not form an article of 

 diet for animals, if we except the chemically altered milk given 

 to the pig. Once the curd is formed in digestion, the process is 

 carried on by pepsin, rennin taking no further part. 



Rennin has been described as existing in the pancreas, testis, 

 and in vegetable tissues, but little is definitely known. It is 

 stated that an antirennin may be formed in the blood by im- 

 munising an animal by injections of rennin, and that the substance 

 produced prevents milk clotting. 



Other ferment actions of the gastric juice have been described, 

 such as fat- and starch-splitting. A fat-splitting ferment, allied 

 to pancreatic lipase, may, it is said, be extracted from the gastric 

 mucous membrane of young animals, but of the existence of 

 this ferment there is very little evidence. Protein digestion is 

 the essential duty of the stomach, while in all vegetable feeders 

 maceration of the vegetable fibres is begun in the stomach as 

 a preliminary measure. A stomach is not essential to life in all 



