1294 PHYSIOLOGY 



be attained with a much smaller bladder content, and if the bladder be 

 artificially distended by the injection of fluid through a catheter, 50 c.c. 

 of fluid may suffice to raise the pressure to this level. As the urine is 

 slowly secreted, the bladder wall at first gives to the incoming fluid, 

 so that a considerable amount can be stored without any marked rise 

 of pressure. Later on the pressure begins to rise more rapidly, and 

 finally attains a pressure of between 120 and 150 mm. water. At this 

 point the second effect of the stretching of the muscular wall makes 

 its appearance. A manometer connected with the bladder shows a series 

 of rhythmic contractions of the muscular wall (Fig. 538), each lasting 

 about a minute, at first slight in extent, but becoming more marked as 

 the distension of the bladder augments. In a bladder entirely cut off 

 from its connection with the central nervous system these automatic 



U.B. 



20" 



UAAAAMMAAAAAMMMMAAM/^^ 



Fiu. 538. Tracings of rhythmic contractions of urinary bladder. 



(SHBRRINGTON.) 



rhythmic contractions gradually increase in force until one of them 

 suffices to overcome the resistance presented by the tonically con- 

 tracted sphincter. A partial emptying of the bladder therefore takes 

 place, but the pressure falls below that necessary to overcome the 

 resistance of the sphincter before the bladder has been quite emptied, 

 so that there is always under these circumstances a certain amount of 

 residual urine left in the bladder. This is the condition found in 

 animals where the lower part of the spinal cord has been extirpated, 

 or in man where the same part of the central nervous system has been 

 destroyed as the result of accident or disease. 



THE MECHANISM OF EVACUATION OF THE BLADDER 



In the denervated bladder the factor finally causing partial 

 evacuation is the gradual increase in the intravesical tension from the 

 accumulation of fluid in this viscus. The same factor is prepotent in 

 determining the onset of normal micturition in an animal with the 

 nervous connections of its bladder intact. Apart from the control of 

 the higher centres, micturition will take place each time that the tension 



