324 SECRETION. 



CHAPTER XYI. 



SECRETION. 



WE have already seen, in a previous chapter, how the elements of 

 the blood are absorbed by the tissues during the capillary circula- 

 tion, and assimilated by them or converted into their own substance. 

 In this process, the inorganic or saline matters are mostly taken up 

 unchanged, and are merely appropriated by the surrounding parts in 

 particular quantities ; while the organic substances are transformed 

 into new compounds, characteristic of the different tissues by which 

 they are assimilated. In this way the various tissues of the body, 

 though they have a different chemical composition from the blood, 

 are nevertheless supplied by it with appropriate ingredients, and 

 their nutrition constantly maintained. 



Beside this process, which is known by the name of "assimila- 

 tion," there is another somewhat similar to it, which takes place in 

 the different glandular organs, known as the process of secretion. It 

 is the object of this function to supply certain fluids, differing in 

 chemical constitution from the blood, which are required to assist 

 in various physical and chemical actions going on in the body. 

 These secreted fluids, or "secretions," as they are called, vary in 

 consistency, density, color, quantity, and reaction. Some of them 

 are thin and watery, like the tears and the perspiration ; others are 

 viscid and glutinous, like mucus and the pancreatic fluid. They 

 are alkaline like the saliva, acid like the gastric juice, or neutral 

 like the bile. Each secretion contains water and the inorganic salts 

 of the blood, in varying proportions; and is furthermore distin- 

 guished by the presence of some peculiar animal substance which 

 does not exist in the blood, but which is produced by the secreting 

 action of the glandular organ. As the blood circulates through the 

 capillaries of the gland, its watery and saline constituents transude 

 in certain quantities, and are discharged into the excretory duct. 

 At the same time, the glandular cells, which have themselves been 

 nourished by the blood, produce a new substance by the catalytic 



