1158 PHYSIOLOGY 



contraction of the same side of the bladder, involving all its coats and some- 

 times extending to a slight extent to the contralateral half of the bladder. 

 When both pelvic nerves are stimulated simultaneously contraction of both 

 sides of the bladder causes a considerable rise of pressure in its interior 

 (Fig. 541) which is always sufficient to overcome the resistance of the 

 sphincter and to cause a complete emptying of the bladder. There is no 

 doubt, therefore, that these nerves are the most important for the act of 

 micturition. As to the action of these nerves, however, on the sphincter the 

 results of different experimenters are somewhat at variance. In the cat 

 there seems to be no doubt that inhibition of the sphincter may result from 

 stimulation of the pelvic visceral nerves. On the other hand, Fagge, 

 working on the dog, found that although micturition was excited by the 

 stimulation of these nerves, the expulsion of urine did not occur until the 



FIG. 541. Curve showing rise of pressure in the bladder caused by stimulation 

 of s, sacral nerves ; h, hypogastric nerves. (FAGGE.) 

 The scale indicates centimetres of water. 



intravesical tension had reached the point at which the resistance of the 

 sphincter could be overcome without any alteration of its state of con- 

 traction, i.e. the point at which fluid injected into the bladder through 

 the ureter began to escape from the urethra without stimulation of any 

 nerves whatever. 



Observations on man would support the view that an active relaxation of 

 the sphincter trigoni is a necessary part of the act of micturition. Thus in 

 experiments by Reyfisch a rigid catheter was introduced into the bladder, 

 which was fully distended with fluid. On withdrawing the catheter until its 

 opening lay just outside the bladder in the posterior urethra, the flow of 

 urine stopped. The man, however, was able to micturate directly he was 

 told to, and to stop again at will. It was impossible in this case for 

 any of the urethral muscles to be concerned, since the rigid catheter 

 impeded their action. The relaxation of the sphincter must therefore 

 be brought about by impulses descending the pelvic visceral nerves, 

 which we may regard as motor to the ' detrusor ' and inhibitory to the 

 sphincter of the bladder. 



