90 PRINCIPLES OF VETERINARY SCIENCE 



supply the gland. These facts are conclusive evidence that secre- 

 tion is an active or vital process carried on by the gland-cells, 

 under the influence of nerves, and is not a simple filtration process 

 in which fluid passes through a filter bed from a higher to a lower 

 pressure. The gland-cells actually determine the amount of 

 salts which shall accompany the water that is secreted. 



Another means of proving that the secretory product is manu- 

 factured by the gland is to compare the constituents of the secre- 

 tion with those of the blood which supplies all nutrient and other 

 substances to the gland. Saliva again may be taken as an ex- 

 ample. It contains mucus and an enzyme. Neither of these 

 substances is found even in the half-formed state in the blood. 

 The inevitable conclusion is that the gland-cells manufacture the 

 peculiar substances which they secrete. Each gland may accord- 

 ingly be looked upon as an independent unit. It takes up ma- 

 terials from the lymph and from these manufactures products 

 peculiar to itself. The stimulus which causes a gland to start 

 secreting is received from the central nervous system. In the 

 case of a salivary gland, the message originates from direct 

 stimulation of the sensory nerves in the mouth, or reflexly from 

 the sight or smell of appetizing food. So accurate is the informa- 

 tion carried to the glands in the mouth, that when a horse trans- 

 fers the work of mastication from one side of its mouth to the 

 other, as it is in the habit of doing every fifteen or twenty minutes, 

 the flow of saliva from the parotid gland on the masticating side 

 is increased, while on the other side it is diminished. Often two 

 or three times as much saliva is poured out on one side as on the 

 other. A remarkable control of secretion is seen in the ox during 

 rumination when the submaxillary glands do not secrete, although 

 the other salivary glands are very active. 



THE PHYSIOLOGY OF DIGESTION 



Digestion is the process of separating the available from the 

 unavailable materials of the food. It consists in dissolving, 

 breaking down, and chemically changing the food so that it 

 can be absorbed and used by the body. Gross digestion is 

 carried on in the digestive canal, but the special preparation of 

 the food for the body cells is carried on in the protoplasm of the 

 cells themselves. 



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