188 ELEMENTARY PHYSIOLOGY less. 



tracked thi'ough the kidney and be seen to pass through 

 the cells of the convoluted tubules and not through the 

 walls of the glomerular blood-vessels. 



The formation of urine is therefore a double process. 

 A great deal of the water, with probably some of the 

 more soluble inorganic salts, passes by the glomeruli, 

 but the urea, the colouring matters and a great many 

 other of the constituents, are thrown into the cavities of 

 the tubules by a peculiar action of the epithelium cells. 



6. The History of Urea. — Nitrogen enters the body 

 as protein food and, practically, all of it leaves the body 

 again as urea. Somewhere or other, and by some means 

 or other, the nitrogen while in transit is turned over from 

 the proteins into urea. This change involves the whole 

 nitrogenous metabolism ' of the body and from its impor- 

 tance merits a short statement of the chief facts which 

 throws some light on the (juestion of where and how urea 

 is formed. 



In the first place the urea excreted in the urine is not 

 made in the kidney out of some other (antecedent) sub- 

 stance. The activity of the kidney con.sists entirely in 

 picking out ready made urea from the blood which passes 

 through it and discharging this urea into the channels 

 of the tubules. Hence urea must be made in tissues other 

 than the kidney and finds its way from these into the 

 blood. 



Nearly half the weight of the body is made up of 

 muscular tissue, the muscles. These muscles are the seat 

 of active oxidation even when at rest, and this activity 

 is enormously increased at times when they are contract- 

 ing. There must therefore always be a considerable wear 

 and tear going on in them, and we must suppose that this 

 leads to the formation of waste, of which some should 

 contain nitrogen, since the muscles are chiefly built up 

 of niti'Ogenous material. But this waste does not come 

 out of the muscle as ready-made urea, neither do we 



1 The word roetabotism (/ii.eTai3oA7) = change) is conveniently usei to 

 denote the sum total of those chemical changes which take place in 

 living matter, and in virtue of which we speak of it as " living." 



