DEPURATION 479 



being injurious to its welfare, and therefore to be discarded. The term 

 secx'etiou is sometimes confined to the former, while the latter action receives 

 the distinguishing term excretion : but as in many cases the fluid which is 

 removed as being injurious to the system is also used for beneficial purposes, 

 the distinction is not capable of being strictly maintained. The nature of 

 the process is essentially the same in all cases, being carried out by the 

 development of simple cells, each possessing its own independent vitality. 

 These cells select certain ingredients from the blood, and then set them 

 free by the rupture of their walls ; and being situated on the free 

 surface of the lining membrane of the gland, which is continuous with 

 the mucous membrane or skin, the secreted fluid gradually reaches the 

 one or the other. It is impossible, at present, to ascertain the precise 

 means by which each gland is made up of cells having special powers of 

 selection ; but that the fact is so is capable of demonstration. Thus, the 

 cells of the liver select the elements of bile ; those of the salivary glands 

 saliva ; and so on. But, as we shall hereafter find, there are minute points 

 of diflference in the arrangement of these cells in the different glands. It 

 is now ascertained that the elements of the various secretions exist in the 

 blood ; and therefore the otHce of the glands is confined to the selection and 

 separation of their products, and they have little or nothing to do with 

 their conversion. 



DEPURATION, AND ITS OFFICE IN THE 

 ANIMAL ECONOMY 



The whole op the various secretions which go on in the body are 

 necessary for the due preservation of its health ; but the most important of 

 the class alluded to above as excretions, must be removed from the blood, or 

 death will speedily ensue. Thus, if saliva and gastric juice, as well as the 

 other secretions aiding digestion, are not mixed with the food, the nutrition 

 of the body will be imperfectly carried on, and its health will suffer. But 

 if the elements of bile and urine are retained in the blood, not only is the 

 system upset, but absolute death is pi-oduced in severe cases. Hence it 

 follows, that attention to the state of the organs of depuration, or excretion, 

 is of more importance even than to those of secretion, using these terms in 

 the sense explained in the last paragraph. The chief organs of depuration 

 are the lungs, which remove carbon from the blood ; the liver, which 

 secretes the bile ; the kidneys, which get rid of the urea ; and the skin, 

 which relieves it of its superfluous watery and some small proportion of its 

 solid particles. Experiment shows that the retention of carbon, or urea, in 

 the blood is speedily followed by death ; while the non-secretion of bile, if 

 entire, poisons the system ; and in milder cases, its absence from the 

 alimentary canal interferes with the due elaboration of the chyle. 



