CHAP. XVII.] ELIMINATION. 209 



the bladder is overcome, and the urine is ejected through the 

 urethra. Involuntary micturition may occur as a result of 

 spinal injury involving the nerve centres which send nerves 

 to the bladder. It may be due to a want of " tone " in the 

 muscular walls, or it may result from some abnormal irritation. 



General characters of the urine. Normal urine may be de- 

 scribed as a transparent watery fluid, of a pale yellow colour, 

 acid reaction, specific gravity of 1020, and possessing an odour 

 which can only be described as " characteristic " or " urinous." 

 Each one of these characters is liable to some variation within 

 the limits of health as well as in disease. 



The transparency of urine may be diminished in health by the 

 presence of mucus, derived from the genito-urinary tract, or by 

 the deposit of salts. In disease the urine may become clouded 

 by the presence of pus. 



The colour of urine depends mainly upon the amount of water 

 it contains; also upon a diminution or increase of colouring 

 matters. In the copious urine of hysteria the colour is very 

 light, while in the diminished flow in fevers it is very high. 

 Abnormal colouring matters are derived from food or medicine, 

 or result from some diseased condition. 



The reaction of urine should always be tested from a collec- 

 tion of urine passed during twenty-four hours as it is affected 

 by diet and exercise. To test the reaction of urine, litmus 

 paper is used. Acid urine turns blue litmus paper red; alka- 

 line urine turns red litmus paper blue. When the colour of 

 the paper remains unchanged the urine is said to be neutral. 

 The reaction of mixed urine is normally acid. 



The specific gravity depends upon the amount of solid waste 

 matters present in the urine. In health, it may vary from 1015 

 to 1025. When the solids are dissolved in a large amount of 

 water, the specific gravity will naturally be lower than when, 

 from a deficiency of water, the urine is more concentrated. It 

 is notably heightened by the presence of sugar in the disease 

 called Diabetes Mellitus. 



The composition of urine. The chief constituents of normal 

 urine are water, urea, uric acid, colouring matters, and salts. Of 

 these constituents, urea is by far the most important, for it is 

 the chief solid waste product of the body. To eliminate urea is 

 the special work of the kidneys, and if for any reason they fail 



