THE PHYSIOLOGY OF MICTURITION 1295 



in the bladder has reached a certain height, i.e. about 15 cm. water, 

 the amount of fluid in the bladder at the time depending, on the one 

 hand on the rate at which the fluid has entered this organ from the 

 ureters, on the other hand on the irritability -of the bladder wall 

 itself and of the nervous centres concerned with its motor innervation. 

 The effect of the gradual accumulation of fluid and rise of tension is 

 twofold. In the first place, it acts on the bladder wall, causing 

 rhythmic contractions of ever-increasing intensity ; in the second 

 place, the mere stretching of the bladder originates impulses in the 

 sensory nerve-endings in its wall, which are reinforced at every 

 rise of tension caused by the rhythmic contractions. These 

 impulses travel up to the spinal centres, and are summated until 

 they result in a sudden discharge of efferent impulses of two kinds, 

 namely : 



(1) Motor impulses to the whole musculature of the fundus of the 

 bladder (the detrusor in its widest sense) ; 



(2) An inhibition of the tonic contraction of the sphincter. This 

 inhibition may be determined by inhibitory impulses travelling to 

 the sphincter and causing its relaxation, or by the central inhibition 

 of the impulses normally going to the sphincter and maintaining its 

 tonic contraction. The resultant of these two processes, the contrac- 

 tion of the detrusor and the relaxation of the sphincter, is a complete 

 emptying of the bladder, and the act is completed by the contraction 

 of the involuntary and voluntary muscles surrounding the urethra 

 and causing complete expulsion of the contents of this tube. 



THE INNERVATION OF THE BLADDER 



ACTION OF THE PELVIC VISCERAL NERVES. In all animals 

 excitation of the peripheral end of one pelvic visceral nerve causes a 

 strong contraction of the same side of the bladder, involving all its 

 coats and sometimes extending to a slight extent to the contralateral 

 half of the bladder. When both pelvic nerves are stimulated simul- 

 taneously contraction of both sides of the bladder causes a consider- 

 able rise of pressure in its interior (Fig. 539) which is always sufficient to 

 overcome the resistance of the sphincter and to cause a complete empty- 

 ing 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 experi- 

 menters 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 intra- 

 vesical tension had reached the point at which, the resistance of the. 



