428 THE ELIMINATION OF WASTE PRODUCTS. 



traction of an obscurely peristaltic nature, the force of which is more than, 

 sufficient to overcome the resistance offered by the urethra, and the urine 

 issues in a stream, the sphincter vesicse externus being at the same time either 

 relaxed after the fashion of the sphincter ani, or at least overcome. In its 

 passage along the urethra, the exit of the urine, at all events of the last por- 

 tions, is forwarded by irregularly rhythmic contractions of the bulbo-caver- 

 nosus or ejaculator urinse muscle, the contractions of which compress the 

 urethra ; and the whole act is further assisted by pressure on the bladder 

 exerted by means of the abdominal muscles, very much the same as in 

 defecation. 



In the case of the rectum we were able ( 237) to distinguish between the 

 actions of the longitudinal and of the circular coats, and we said that the 

 two coats had distinct nervous supplies (Fig. 89). The bladder has, as we 

 have said, a similar nerve supply, and it is very probable, but not yet dis- 

 tinctly proven, that this like double supply has a like double action. Stimu- 

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

 it throws the longitudinal coat of the rectum into contractions, 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 

 longitudinal coat. Stimulation of the hypogastric nerves carrying fibres 

 from the dorsal and upper lumbar cord (see Fig. 109), while throwing the 

 circular coat of the rectum into strong contractions, gives rise to vesical con- 

 tractions, 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 chiefly 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 the chief influence. 



362. 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 sometimes 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 ob- 

 scure continuous contraction which we speak of as " tone ;" and, further, that 

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

 ently 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. 



363. Micturition as sketched above seems at first sight, and especially 

 when we appeal to our own consciousness, a purely voluntary act. A volun- 

 tary 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. 



