DIGESTION 111 



dent unit. By its own activity it takes up materials from the 

 lymph, out of which it manufactures its own special products. 

 It stores its products until they are wanted. Then by its own 

 activity it extrudes them into the lumen of the gland-tube. It 

 has, indeed, been shown that, when the nerve going to a salivary 

 gland is stimulated, the gland shrinks, notwithstanding the 

 great dilation of its bloodvessels. Under the influence of the 

 stimulation the granules in the gland-cells imbibe water, swell 

 up, and escape from the cells. The cells discharge their 

 accumulated stores, in the first instance, more rapidly than they 

 take up materials (even fluid) from the blood. For its know- 

 ledge (if the term may pass) of what is wanted the gland-cell is 

 dependent upon messages which reach it through the nervous 

 system. These messages take origin in the endings of the 

 sensory nerves of the mouth, pass up to the brain, and are 

 reflected down the nerves to the gland. So accurate is the 

 information conveyed to the glands, that when a horse transfers 

 the work of mastication from one side of its mouth to the other, 

 as it is in the habit of doing about every quarter of an hour, the 

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

 increased; on the other side it is diminished. Two or three times 

 as much saliva is poured out on the one side as on the other. 



Not only is the amount of saliva poured out in response to 

 stimulation proportional to the needs of mastication, but the 

 kind of saliva is adapted to the nature of the food. In a dog 

 and this is an observation which can be made only on an 

 animal which lives on a mixed diet it is possible to determine 

 the amount of the two kinds of saliva secreted and the relation 

 of flow to food. When meat is given to the animal, the 

 submaxillary gland yields its secretion ; when it is fed on 

 biscuit, abundance of the watery parotid saliva is poured 

 forth. A mouthful of sand also causes the parotid saliva to 

 flow, in order that the sand may be washed out of the mouth. 



More remarkable than the response to direct stimulation is the 

 effect produced by the sight and smell of food. When meat 

 is shown to a dog, submaxillary saliva begins to flow ; when 

 it is offered bread, parotid saliva is secreted. And the activity 

 of the glands is not merely a nervous reflex independent of the 

 animal's mind. The moment the dog realizes that it is being 

 played with that there is no intention of giving it the coveted 



