24 ELEMENTARY PHYSIOLOGY less, 



are separated fi-om the blood by the excretory organs, 

 of which there are three — the skin, the lungs, and the 

 kidneys. 



Different as these organs may be in appearance, they 

 are constructed upon one and the same principle. Each, 

 in ultimate analysis, consists of a very thin sheet of tissue, 

 like so much delicate blotting-paper, the one face of which 

 is free, or lines a cavity in communication with the 

 exterior of the body, while the other is in contact with 

 the blood which has to be purified. 



The excreted matters are, as it were (though, as we 

 shall see, in a peculiar way), strained from the blood, 

 through this delicate layer of tissue, and on to its free 

 surface, whence they make their escape. 



Each of these organs is especially concerned in the 

 elimination of one of the chief waste products — water, 

 carbonic acid, and urea — though it may at the same time 

 be a means of escape for the others. Thus the lungs are 

 especially busied in getting rid of carbonic acid, but at 

 the .same time they give oft' a good deal of water. The 

 duty of the kidneys is to exci'ete urea (together with other 

 substances, chiefly salts), but at the same time they pass 

 away a large quantity of water and a trifling amount of 

 carbonic acid ; while thi; skin gives off' nuich water, some 

 amount of carbonic acid, and a certain (juantity of .saline 

 matter, among which a trace of urea may be, sometimes, 

 though very d(»ul)tfully, ])resent. 



10. Respiratory Organs.- Finally the lungs play a 

 douljle part, l)eing not merely eliminators of waste, or 

 excretionary products, but importers into the economy of 

 a substance which is not exactly either food or drink, but 

 something as important as either, — to wit, oxygen. 



As the carbtniic acid (and watei') is passing from the 

 blood through the lungs into the external air, oxygen is 

 passing from the air through the lungs into the blood, 

 and is carried, as Ave shall see, by tluj blood to all parts of 

 the body. We have seen (p. 6) that the waste which 



