SEC. 1. THE COMPOSITION AND CHARACTERS OF 



UKINE. 



320. 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 1-020, and generally holding in suspense a 

 little mucus. The mucus, when present, comes from the uri- 

 nary 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 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, and may take place on the one hand in the alimen- 

 tary canal, or during a brief transit of the substance in the 

 blood-stream, or even in the urine itself, may so to speak be 



515 



