THE COMPOSITION AND CHARACTERS OF URINE. 405 



slightly fluorescent fluid of a peculiar odor, saline taste, and acid reaction, 

 having a mean specific gravity of 1020, and generally holding in suspension 

 a little mucus. The mucus, when present, comes from the urinary passages, 

 as do also the occasional epithelial cells. All the rest of the urine may be 

 considered as the secretion of the kidney. 



The urine, as we have said, is the chief channel by which solid matters 

 leave the body, a small quantity only passing by the skin and practically 

 none by the lungs. Hence, neglecting for the present the skin, we may say 

 that all the substances taken into the body sooner or later leave the body by 

 the urine, save the few substances which may be retained permanently within 

 the body and the substances which make up the body at the moment of its 

 death. We accordingly find that the urine contains a large number of sub- 

 stances, the exact amount of each substance present in a given quantity of 

 urine varying, in the case of every substance somewhat, and in the cases of 

 many substances very largely, from time to time. The composition of urine 

 is not only complex but extremely variable. 



Moreover, a little consideration will show that the several substances 

 present in urine must have very different histories. Some of the con- 

 stituents of urine appear in it in the exact form in which they were intro- 

 duced into the mouth ; they have been simply absorbed from the alimentary 

 canal into the blood and excreted by the kidney without undergoing change ; 

 they are derived directly and without change from the food. 



Others again are the products of changes which the food has undergone in 

 the body ; and these changes may be slight or may be extensive, and may 

 take place on the one hand in the alimentary canal, or during a brief transit 

 of the substance in the blood-stream, or even in the urine itself, may, so to 

 speak, be superficial ; or on the other hand may take place in the very depths 

 of the tissues and be closely associated with the very life of the tissues. We 

 shall, however, have to return to these matters later on, and may here briefly 

 consider what substances are, normally and abnormally, present in urine, and 

 the chief features of the fluid itself. 



337. Besides water, the constituents of urine are : 



Nitrogenous crystalline bodies. Neglecting the small proportion of these 

 bodies which, especially in the case of flesh-eaters, are introduced into the 

 economy with the food, as kreatin and the like, and so pass into the urine 

 with no or with comparatively little change, we may on the whole regard the 

 substances of this class as the products of the changes which the proteid 

 matters (and allied substances such as gelatin and the like) present in food 

 have undergone either while the food was simply food, still in the alimentary 

 canal, for instance, or after the food had been built up into the tissues of the 

 body. 



Of these by far the most important, in the urine of man and mammalia, 

 is the body urea (N 2 H 4 CO). It is the chief form in which, in these animals, 

 nitrogen leaves the body. We shall have to discuss the relations and forma- 

 tion of urea later on, but meanwhile we will simply state that it has remark- 

 able double connections with two great groups. On the one hand, it is 

 related to the ammonia group, and by hydration is readily converted into 

 ammonium carbonate (N 2 H 4 CO+2H 2 O = (NH 4 \CO 3 ). On the other hand, 

 it is related to the great cyanogen group, ammonium cyanate and urea being 

 isomeric, and the former by simple heating being converted into the latter 

 (NH 4 .CNO = N 2 H 4 CO). ' 



Though a base, forming salts with acids, such as nitrates, oxalates, etc., 

 urea occurs in urine in a free and independent condition. 



Closely allied to urea, occurring apparently as a by-product of the 

 same line of metabolism, is uric acid (C 6 HJf 4 O s ) which is found always in 



