132 INTRODUCTION. 



it is the principal way by which the soluble material 

 of the blood and tissues leave the economy, we naturally 

 look to the urine to find any pathologic changes that 

 might be taking place in the body. These changes 

 are of two varieties : i . An increase in the normal 

 constituents of the urine. 2. An introduction into the 

 urine of some foreign matter that may be acting as a 

 poison ; in fact, anything introduced into urine that is 

 not normally found in it is a poison, illustrated in the 

 case of poisons, as treated in Toxicology. This variety 

 is of most interest to the specialist. The first variety 

 is oi most interest to the physician, since in ordi- 

 nary diseases it is either an increase or decrease of the 

 normal constituents of the urine. An analysis of the 

 urine not only gives us an idea of the changes going 

 on in the urinary tract, but in other places. A chem- 

 ical examination is essential, since it gives an idea of 

 the changes going on in the body a distance from the 

 urinary tract, while the microscope only detects the 

 solids in the specimen. How . are these changes de- 

 termined ? We are now going to talk about the most 

 importiant constituent of the urine, since by it the nitro- 

 genous material leaves the body. 



