DIGESTIVE SECRETIONS AND THEIR CONTROL 435 



the blood through simple physical processes, filtration and dialysis 

 (Chap. XVII). The special constituents of each secretion, be- 

 ing different from anything contained in the blood, must, on the 

 other hand, be produced by chemical processes within the gland 

 itself. It is easy to show microscopically that the cells of most 

 glands during rest become filled with small granules, and that 

 when the gland is active these granules for the most part disap- 

 pear. We can picture the entire secretory process as occurring 

 in two stages : the first, a chemical stage, during which the peculiar 

 constituents of the secretion are elaborated and deposited within 

 the cells of the gland ; and a second physical stage consisting of a 

 rapid flow of water with its dissolved salts from the blood through 

 the gland into its duct, carrying with it the special materials pre- 

 viously prepared by the gland. 



Nervous Control of the Secretory Process. Considerable evi- 

 dence has accumulated indicating that gland tissue, like skeletal 

 muscle tissue, carries on its function only when stimulated to do 

 so, and that the stimulus is in many glands nervous. It has been 

 shown for the salivary glands of dogs, for example, that proper 

 stimulation of certain nerve-fibers leading to them causes them to 

 produce and store within themselves granules, while stimulation 

 of quite different nerve-fibers causes them to pour out their secre- 

 tion. The chemical part of secretion is thus controlled by one set 

 of nerves, often called trophic nerves, and the physical part by 

 another. It is interesting to note that the nerves which cause the 

 gland to pour out its secretion usually cause also vasodilatation 

 within it; an increased flow of blood through the gland therefore 

 usually accompanies the physical part of secretion. That this in- 

 creased blood-flow is not the sole cause of the outpouring of the 

 secretion, as might easily be supposed, is proved by the possibility 

 under proper conditions of stimulating a gland to pour out its fluid 

 without any accompanying vasodilatation. We must recognize, 

 then, that the physical act of secretion is the result of the action of 

 definite secretory nerves, as distinct from vasodilator nerves. Just 

 how these function to bring about the more rapid passage of water 

 and salts through the gland-cell is not clear. As we should expect, 

 continued stimulation of the secretory fibers leading to a gland, 

 without accompanying stimulation of the trophic fibers, results 

 soon in the production of a secretion which is very watery, virtu- 



