Chap. XVII] URINARY SYSTEM 333 



bladder through the urethra. The act by which the urine is ex- 

 pelled is called micturition. It occurs normally as the result of 

 irritation due to the accumulation of urine within the bladder. 

 The accumulation stimulates the muscular walls to contract, and 

 the resistance of the sphincter at the neck of the bladder is over- 

 come. The action is involuntary, but it may be controlled by vol- 

 untary effort. 



Involuntary micturition. — Involuntary micturition may occui 

 as the result of lack of consciousness ; and as the result of spinal 

 injury involving the nerve centres, which send nerves of control 

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

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



Retention of urine. — When the kidneys secrete urine, but it is 

 retained within the bladder, we speak of it as retention. Reten- 

 tion or failure to void urine may be due to : (1) dulling of the 

 senses so that there is no desire to void, (2) nervous contraction of 

 the urethra, and (3) some obstruction in the urethra or in the 

 neck of the bladder. 



In some cases the bladder may become so fully distended that 

 the retention of urine may be accompanied by more or less con- 

 stant voiding of small amounts of urine. 



Suppression of iirine. — When the kidneys fail to secrete urine, 

 it is spoken of as suppression, and is a far more serious condition 

 than retention, as it is usually due to disease of the kidneys. 



GENERAL CHARACTERS OF THE URINE 



Normal urine may be described as a transparent, amber-colored 

 liquid, with a characteristic odor, an acid reaction, and a specific 

 gravity of about 1.020. 



Transparency. — 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. 



Color. — The color of urine depends upon the quantity voided 

 and the relative amounts of water and coloring matters. If the 

 quantity is abnormally increased, it is usually more dilute and of 

 a paler color; as, for instance, the copious light-colored urine of 

 hysteria or diabetes insipidus. One exception to this is diabetes 

 mellitus, where the quantity is increased, but the color is dark 



