CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 715 



the amount of its tone is exceedingly variable, the organ, quite 

 independently of distinct efforts at micturition, being at one time 

 contracted and at another flaccid and distended. When it is in a 

 contracted state, a small quantity of fluid may exert the same 

 effect on the vesical walls as a larger quantity when the bladder is 

 flaccid. Hence while the determining cause of the desire to make 

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

 quantity needed to produce the necessary fulness is dependent on 

 the amount of tonic contraction of the muscular fibres existing at 

 the time. And we have evidence that this tone is regulated by the 

 nervous system. 



429. 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 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 pcwerful 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 

 micturition 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 thoracic 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 



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