682 MICTURITION. [BOOK n. 



between the actions of the longitudinal and of the circular coats, 

 and we said that the two coats had distinct nervous supplies 

 (Fig. 70). The bladder has, as we have said, a similar double 

 nerve supply, and it is very probable, but not yet distinctly proven, 

 that this like double supply has a like double action. Stimulation 

 of the branches coming from the sacral nerves, at the same time 

 that it throws the longitudinal coat of the rectum into con- 

 tractions brings about in the dog, in which the longitudinal fibres 

 of the bladder are much more pronounced than the circular, 

 powerful vesical contractions. Moreover, stimulation of the sacral 

 nerves on one side produces unilateral contraction of the bladder. 

 From this we may infer that the sacral nerves govern the longi- 

 tudinal coat. Stimulation of the hypogastric nerves carrying fibres 

 from the dorsal and upper lumbar cord, see Fig. 70, while throwing 

 the circular coat of the rectum into strong contractions, gives rise 

 to vesical contractions, but these are by no means so marked as 

 those which appear when the sacral nerves are stimulated. We 

 may probably conclude that the more important fibres in the 

 fundus of the bladder, which are for the most part longitudinal, 

 are to be regarded as governed for the most part by the sacral 

 nerve-fibres, while the circular muscular fibres around the neck of 

 the bladder, whose contraction completes as it were the emptying 

 of the bladder, are those on which the hypogastric nerve-fibres 

 have chief influence. 



428. We said just now " when the bladder has become full," 

 but this must not be understood to mean, " when the bladder has 

 received a certain quantity of fluid." On the contrary, it is a matter 

 of common experience that we feel the desire to make water some- 

 times when a large quantity and sometimes when a small quantity 

 of urine has accumulated in the bladder. We have evidence that 

 the bladder possesses to a very high degree that obscure con- 

 tinuous contraction which we speak of as ' tone ' ; and further that 

 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 



