CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 683 



centre in the spinal cord, while other voluntary efforts throw the 

 ejaculator and abdominal muscles into contractions, and, the 

 resistance of the urethra being thereby overcome, the exit of the 

 urine naturally follows. 



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 distension 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 upper 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 lower spinal 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 

 being such as to cause a contraction of the walls of the bladder and 

 of the ejaculator urinse, and at the same time to suspend the tone 

 of the sphincter vesicse externus. Clinical experience also goes 

 to shew the existence of a similar micturition centre in man, 

 placed higher up in the cord than the corresponding ' genital ' 

 centre governing the genital organs. 



Moreover we have, in the case both of man and of other 

 animals, experimental and other evidence that contraction of the 

 bladder is frequently brought about by reflex action. Thus the 

 pressure within the bladder when observed for any length of time 

 is found to be subject to considerable and manifold variations. 



