DIGESTION 



165 



and swallowing, stimulating the nerves of taste, and in ruminants 

 assisting in rumination. According to the writer's observations 

 on the horse, saliva has no chemical action on the raw starch of 

 its food, and this is not surprising when we remember that the 

 starch grains are enclosed in an envelope of cellulose, a sub- 

 stance on which saliva has no action. So intimately, however, 

 is salivary secretion associated with starch conversion, that it is 

 not possible to pass over without further notice the action pro- 



Fig. 58. 



-Apparatus employed by Colin in Experiments on the Secretion 

 of Parotid and Submaxillary Saliva. 



duced on starch in man, and, according to some observers, in 

 horses and cattle, by the presence of ptyalin in the saliva. 



The starch found in plants exists in the form of granules 

 possessing a shape peculiar to the species ; these granules are 

 enveloped in a tough envelope of cellulose ; before the true 

 starch, the granulose contained in the cellulose envelope, can be 

 reached the cellulose must be traversed. For this reason some 

 animals, like man, cannot digest raw starch, but by cooking, 

 the starch (granulose) is liberated and free to be acted upon ; 



