THE URINARY SYSTEM. 399 



the bladder may be quickly and greatly increased by the presence of 

 a comparatively small quantity of urine. In man, micturition usually 

 occurs when the pressure within the bladder is about 150 mm. of 

 water ; and if the pressure is suddenly raised to this level by injecting 

 fluid through the urethra into the bladder the desire for micturition 

 is experienced. 



Transection of the spinal cord in the upper lumbar region does not 

 destroy the .reflex mechanism, though it severs the path by which 

 sensory impulses reach the brain and voluntary impulses pass to the 

 sacral centre, By means of voluntary impulses the emptying of the 

 bladder can be inhibited in the adult, but in the infant such im- 

 pulses are lacking and the act of micturition is purely reflex. The 

 emptying of the bladder is usually assisted by voluntary contraction of 

 the abdominal muscles, and in man such a contraction, by increasing 

 the pressure within the bladder, frequently initiates the reflex action. 



When the centre in the sacral region is destroyed, for instance as 

 the result of injury in man, the bladder still reacts like plain muscle 

 elsewhere, and increased tension causes it to contract. As soon as the 

 pressure within the bladder falls below a certain level, however, it fails 

 to overcome the resistance offered by the sphincter muscles, and the 

 bladder is not completely emptied. 



