SEC. 2. THE COMPOSITION AND CHARACTERS OF 



URINE. 



400. These are so fully dwelt upon in special works that we 

 may confine ourselves here to salient points. The healthy urine 

 of man is a clear yellowish slightly fluorescent fluid, of a peculiar 

 odour, saline taste, and acid reaction, having a mean specific gravity 

 of T020, 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, that is, absorbed from the alimentary canal, 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 accord- 

 ingly find that the urine contains a large number of substances, 

 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 shew that the several 

 substances present in urine must have very different histories. 

 Some of the constituents of urine appear in it in the exact form 

 in which they were introduced 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. They 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; they may so to speak be 

 superficial. Or on the other hand they 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 



