CHEMICAL COMPOSITION OF URINE. 397 



capillaries, is not sufficiently explained to make it sure that this 

 filtration differs from others. That the glandular epithelium does 

 take an active part in the elimination of the urea is rendered 

 almost indisputable from the researches of Heidenhain. And yet 

 there remain other parts, e.g., the loops of Henle, which are con- 

 stantly found in the kidney, and have a special vascular mechan- 

 ism, and to which none of the foregoing theories assign any spe- 

 cial or peculiar function. 



From the foregoing evidence we may fairly suppose that most 

 of the urea, and possibly some other solid constituents of the 

 urine, are selected from the blood by the epithelial cells of the 

 convoluted tubules, that the fluid part of the blood escapes at 

 the glomerulus, and flows along the varied and circuitous route 

 of the tubules, carrying with it the matters poured into the tubes 

 by the cells, and that in some part of the tubules the dilute 

 filtrate loses much of its water and all its albumin. 



CHEMICAL COMPOSITION OF URINE. 



The percentage of the various materials in urine varies as the 

 secretion differs in strength, as mentioned, but on an average 

 it may be said to contain about 4 per cent, of solids and 96 per 

 cent, water. 



The following are the more important solid matters : 

 Urea is the most important, and at the same time most abundant 

 solid constituent, commonly forming about 2 per cent, of the urine. 

 It is regarded as the chief end-product of the oxidation of the 

 nitrogenous matter in the body, so that the amount excreted per 

 diem gives us the best estimate of the amount of chemical change 

 taking place in the tissues. It is readily soluble in alcohol and 

 water, but insoluble in ether. It forms acicular crystals with a 

 silky lustre. From a chemical point of view it may be regarded 



{1ST FT 

 T\TTT 2 , or 



w ) CN ) 



H 2 y N 2 . It is isomeric with ammonium cyanate ^TT [ O, from 



H 2 J W ^ 



which it was first prepared artificially. It is also isomeric with 



