88 PHYSIOLOGY FOR NURSES 



and retains the urine until distended moderately when 

 there is a desire to urinate. The neck of the bladder is 

 surrounded by a thickening of plain muscle fiber form- 

 ing a spliincter, or ring-like, muscle, whose constant con- 

 traction prevents the dribbling away of urine as fast as 

 it enters the bladder. 



Micturition is the act of emptying the bladder. The 

 urine is forced along the ureters by contraction of the 

 muscular coats of those tubes every ten or twenty sec- 

 onds so that it enters the bladder in small jets and not as 

 a steady flow. When the bladder is being filled, the pres- 

 sure causes a stimulation of the fibers of the sphincter, 

 through the sensory nerves, and the contraction of this 

 muscle prevents the escape of the urine. A further ac- 

 cumulation of urine increases the pressure on the sen- 

 sory nerves and, by a reflex act, causes a contraction of 

 the muscular coats of the bladder, an inhibition of the 

 center in the lumbar part of the spinal cord which con- 

 trols the sphincter, which is consequently relaxed, al- 

 lowing the urine to flow freely along the urethra. 



Fullness of the bladder is not the only stimulus that 

 causes a desire to urinate. An irritating quality in the 

 urine itself, which may be caused by some drugs, men- 

 tal conditions, like anxiety or other emotion, may create 

 a desire to void the urine when the bladder is not only 

 not full, but when nearly empty. This seems to be due 

 to changes of tone in the bladder muscle itself. 



The urethra in the female is a much shorter channel 

 than in the male. As the catheter is an instrument very 

 frequently used by the nurse, she should be familiar 

 with the normal urethra. In woman the canal is so short 

 and wide that the passage of the catheter is usually very 

 easy. In man, however, the contraction of the muscular 

 fibers around the bulbous part of the urethra frequently 



