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 1/020, 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 sub- 

 stances 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 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 ; 



