MICTURITION 227 



(2) by sacral automatic fibres from the 2nd and 3rd sacral 

 segments. These travel by the pelvic nerves and terminate 

 by arborising around gangha situated in the bladder wall. 

 From these gangha fibres pass to the muscles. 



Stimulation of the sympathetic causes inhibition of 

 the body of the bladder and contraction of the sphincter; 

 that of the sacral autonomic, inhibition of the sphincter 

 and contraction of the body. Both nerves contain afferent 

 fibres. 



The factors contributing to the act of micturition will 

 be best understood if we consider (1) the action of the 

 isolated bladder; (2) the action of the bladder when in 

 connection with the lumbo-sacral part of the cord ; (3) the 

 modification of (2) due to connection with the higher 

 centres. First, however, it is necessary to understand the 

 relation between the degree of distension of the bladder 

 and the pressure within the organ. 



As the bladder fills, tJie pressure ivithin it at first remains 

 practically unaltered, the wall simply giving before the 

 gradual accumulation. When distension has reached a 

 certain point, further filhng causes a rise of pressure — the 

 wall is now in a condition of increased tonus. At this 

 stage slow rhythmic contractions make their appearance. 

 These become more vigorous until eventually one occurs 

 which is sufficient to overcome the tonic contraction of the 

 sphincter. This mechanism occurs in the isolated as well 

 as in the normal bladder. 



Normally micturition occurs when the pressure is about 

 160 mm. of water. The degree to which distension occurs 

 before the pressure begins to rise depends upon the rate 

 at which the bladder fills. When this is rapid, rise of 

 pressure occurs early, so that only a small amount of urine 

 is voided. The same effect is also produced when the 

 bladder wall is unduly irritable. 



In the bladder separated from the cord the forcing open 

 of the sphincter due to the rhythmic contraction of the 

 bladder wall results in an evacuation which comes to an 



