CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 547 



contraction of the muscular fibres existing at the time. And we 

 have evidence that this tone is regulated by the nervous system. 



347. Micturition as sketched above seems at first sight, 

 and especially when we appeal to our own consciousness, a purely 

 voluntary act. A voluntary effort throws the muscular fibres 

 of the bladder into contractions, an accompanying voluntary 

 effort lessens the tone of the sphincter externus, probably by 

 inhibiting its centre in the spinal cord, while other voluntary 

 efforts throw the ejaculator and abdominal muscles into con- 

 tractions, and, the resistance of the urethra being thereby over- 

 come, 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 ob- 

 struction, where the bladder cannot be emptied when it reaches 

 its accustomed fulness, the increasing distension sets up fruit- 

 less but powerful contractions of the vesical walls, contractions 

 which are clearly involuntary in nature, which wane or disap- 

 pear, 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 ven- 

 tricle 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 

 micturition may take place in a dog in which the lumbar region 

 of the spinal cord has been completely and permanently sepa- 

 rated 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 mic- 

 turition : 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 



