546 . MICTURITION. [BOOK n. 



When the bladder has become full, we feel the need of making 

 water, the sensation being heightened if not caused Jby the trick- 

 ling of a few drops of urine from the full bladder into the ure- 

 thra. We are then conscious of an effort ; during this effort 

 the bladder is thrown into a long-continued contraction 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 portions, is forwarded 

 by irregularly rhythmic contractions of the bulbo-cavernosos 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 defsecation. 



Experiments on cats, dogs and other animals shew that con- 

 tractions of the bladder can be brought about by stimulation of 

 the anterior roots of certain lumbar nerves chiefly the third and 

 fourth, and of the first three sacral nerves ; stimulation of the 

 anterior roots of the nerves between these two sets does not give 

 contractions of the bladder. The sacral roots seem to have 

 more powerful and more distinctly unilateral effects than have 

 the lumbar roots, and the movements brought about have not 

 exactly the same character in the two cases, though it cannot 

 be said that the contraction is in the former case strictly longi- 

 tudinal and in the latter case circular. The nerve fibres issuing 

 by the lumbar nerves pass into the sympathetic chain and thence 

 by the inferior mesenteric ganglion and hypogastric nerves to the 

 hypogastric plexus ; the nerve fibres issuing by the sacral pass 

 more directly to the hypogastric plexus. 



346. 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 con- 

 trary, 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 obscure continuous contraction which we speak of 

 as 4 tone ' ; and further that the amount of its tone is exceed- 

 ingly 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 pro- 

 duce the necessary fulness is dependent on the amount of tonic 



