EXCRETION BY THE KIDNEY. 421 



Abnormal Urine. —There is not a substance in the urine that 

 does not vary under disease, while the possible additions act- 

 ually known are legion. These may be derived either from 

 the blood or from the kidneys and other parts of the urinary 

 tract. The kidneys seem to take- upon themselves more readily 

 than any other organ the duty of eliminating foreign matters 

 from the body. But this aspect of the subject is too wide for 

 detailed consideration in this work.- 



The student of medicine should be thoroughly familiar with 

 the urine in its normal condition before he enters upon the 

 examination of the variations produced by disease. This is not 

 difficult, and much of it may be carried out with but a meager 

 supply of apparatus. For this purpose, however, we recom- 

 mend some work devoted to the chemical and microscopic study 

 of the urine. 



It greatly assists to remember a few points in regard to solu- 

 bilities. From a physiological point of view, the urine and its 

 variations, as the result of changes in the organism, may be ob- 

 served with advantage in one's own person — eg., the influence 

 of food and drink, tempei'ature, emotions, etc. 



Comparative. — In fl.shes, reptiles, and birds, uric acid re- 

 places urea, and is very abundant. In these animals most of 

 this substance is white. The urine is passed with the faeces. 



In certain groups of invertebrates uric acid seems to be a 

 normal excretion. 



THE SECRETION OF URINE. 



By means of apparatus adapted to register the changes of 

 voluriie the kidney undergoes, it is found that this organ not 

 only responds to every general change in blood-pressure, but 

 to each heart-beat — that is, its volume varies momentarily. 

 This shows how sensitive it is to variations in blood-pressure. 



Theories regarding the secretion of urine may be divided 

 into those that are almost wholly physical, partly physical, 

 and purely secretory: 1. To the first class belongs that of 

 Ludwig, which teaches that very dilute urine is separated from 

 the blood in the glomeruli, and by a process of osmosis and 

 absorption of water by the tubular capillaries is gradually 

 concentrated to the normal. 2. As an example of the second 

 class is that of Bowman, who maintained that the greater part 

 of the water and some of the more soluble and diffusible salts 



