186 PHYSIOLOGY OF ALIMENTATION. 



salivary glands are turned outward, sufficient saliva is still 

 secreted to keep the mouth moist, and according to COLIN 

 this may amount, even while no food is being chewed, to 

 from 100 to 150 c.c. an hour in the horse. 



4. On the Nature of Salivary Secretion. The impression 

 might readily be obtained from the preceding paragraphs 

 that the secretion of saliva is primarily a nervous act. This 

 is, of course, not the case, and in the last analysis the secre- 

 tion of saliva must be regarded as a function of the cells 

 themselves constituting the salivary glands, only their 

 activity is markedly influenced through impulses which 

 pass to them over the nerves connected with them. We 

 have already mentioned the fact that the salivary glands 

 will secrete (paralytic secretion) when all the nerves sup- 

 plying them have been cut. It is much more reasonable 

 to suppose that this secretion is an expression of activity 

 on the part of the glands themselves than one induced 

 through stimuli passing into them from the cut nerves. 

 In a similar way modern experiments with atropin, pilo- 

 carpin, and other poisons seem to indicate that their action 

 is at least not limited to their effect upon the nerves, if per- 

 haps they do not act solely upon the secreting epithelium itself. 



We know as yet nothing definite regarding the nature 

 of the process of salivary secretion, any more than we know 

 regarding the forces at work in any process of secretion. 

 It was once believed that the saliva represented nothing 

 but a filtrate from the blood which was squeezed through 

 the gland-cells under the influence of the blood-pressure. 

 This idea must be given up entirely, for the chemical com- 

 position of the saliva differs from that of blood-plasma 

 or lymph, not only quantitatively but also qualitatively. 

 Certain salts are present in greater, others in less amount 

 than in the blood, and while some chemical constituents 

 found in the blood do not appear at all in the saliva, the 

 reverse is also true. We must therefore conclude that the 

 gland cells themselves have the power of forming new chem- 



