412 MICTURITION. [BOOK n. 



water is the pressure of the urine upon the vesical walls, the 

 quantity needed to produce fulness being dependent on the 

 amount of tonic contraction of the muscular fibres existing at the 

 time. 



Micturition as sketched above seems at first sight, and espe- 

 cially when we appeal to our own consciousness, a purely voluntary 

 act. A voluntary effort throws the bladder into contractions, an 

 accompanying voluntary effort throws the ejaculator and abdominal 

 muscles also into contractions, and, the resistance of the urethra 

 being thereby overcome, the exit of the urine naturally follows. If 

 we adopt the view of a sphincter vesicaB being relaxed at the same 

 time, we have to add to the above simple statement the supposition 

 that the will, while causing the detrusor urinse to contract, also 

 lessens the tone of the sphincter, probably by inhibiting its centre 

 in the lumbar cord. 



There are facts however which prevent the acceptance of so 

 simple a view. In the first place, in cases of urethral obstruction, 

 where the bladder cannot be emptied when it reaches its ac- 

 customed fulness, the increasing distension sets up fruitless but 

 powerful contractions of the vesical walls, contractions which are 

 clearly involuntary in nature, which wane or disappear, and 

 return again and again in a rhythmic manner, and which may 

 be so strong and powerful as to cause great suffering. It seems 

 that the fibres of the bladder, like all other muscular fibres, have 

 their contractions augmented in proportion as they are subjected 

 to tension. Just as a previously quiescent ventricle of a frog's 

 heart may be excited to a rhythmic beat by distending its cavity 

 with blood, so the quiescent bladder may, quite independent of 

 the will, be excited, by the distention of its cavity, to a peristaltic 

 action which in normal cases is never carried beyond a first effort, 

 since with that the bladder is emptied and the stimulus is removed, 

 but which in cases of obstruction is enabled clearly to manifest its 

 rhythmic nature. 



In the second place it has been shewn that quite normal mictu- 

 rition may take place in a dog in which the lumbar region of the 

 spinal cord has been completely and permanently separated by 

 section from the dorsal region. In such a case there can be no 

 exercise of volition, and the whole process appears as a reflex 

 action. When under these circumstances the bladder becomes 

 full (and otherwise apparently the act fails) any slight stimulus, 

 such as sponging the anus or slight pressure on the abdominal 

 walls, causes a complete act of micturition : the bladder is entirely 

 emptied, and the stream of urine towards the end of the act 

 undergoes rhythmical augmentations due to contractions of the 

 ejaculator urinse. These facts can only be interpreted on the view 

 that there exists in the lumbar cord (of the dog) what we may 

 speak of as a micturition centre capable of being thrown into 

 action by appropriate afferent impulses, the action of the centre 



