General Pliysiology of Animals. 33; 



course, to render the food-particles soluble. For that purpose 

 various secretions are poured into the alimentary canal from 

 glands in its vicinity. The most important of these are 

 saliva, gastric juice, intestinal juice, pancreatic juice, and 

 bile. It may be convenient if we summarise the functions 

 of these fluids and the action they severally have on the 

 contents of the alimentary canal. 



Saliva. Saliva, though absent from the Amphibia, is 

 so important a fluid that a notice of it cannot be omitted 

 in a general sketch of animal physiology. It is a colourless, 

 watery, but slightly viscid fluid, produced in abundance in 

 certain glands situated in or near the buccal cavity. The 

 essential constituent of saliva is a ferment, ptya^in, which 

 has the power of transforming starch into sugar. Starch, 

 being one of the substances already described as colloids, 

 is incapable of being absorbed through an animal mem- 

 brane, while sugar, being a crystalloid, may be so absorbed. 

 Ptyalin, by changing starchy substances in the food into 

 sugar, thus renders them soluble, while the water of the 

 saliva dissolves the sugar so formed, which thus is rendered 

 capable of subsequent absorption and assimilation. When 

 food is introduced into the mouth, and the masticatory 

 movements take place, the salivary glands are reflexly (p. 317) 

 stimulated to secrete a copious supply of saliva, which in 

 the process of mastication becomes thoroughly mixed with 

 the food. All the digestive glands act in the same way, i.e. 

 reftexly, the medium being a sensory or afferent nerve dis- 

 tributed to the mucous membrane of the alimentary canal, 

 and an efferent or secretory nerve going to the secretory 

 cells of the glands, the stimulation of which brings about 

 glandular activity and a flow of the secretion. 



Gastric juice. The digestive fluid formed by the glands 

 in the mucous membrane of the stomach-wall also con- 

 tains one essential ferment, namely, pepsin, which in the 

 presence of a small amount of hydrochloric acid (0-2 per 

 cent.) transforms insoluble proteids into soluble substances 



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