THE PHYSIOLOGY OF MICTURITION 1157 



increase in force until one of them suffices to overcome the resistance pre- 

 sented by the tonically contracted sphincter. A partial emptying of the 

 bladder therefore takes place, but the pressure falls below that necessary to 

 overcome the resistance of the sphincter before the bladder has been quite 

 emptied, so that there is always under these circumstances a certain amount 

 of residual urine left in the bladder. This is the condition found in animals 

 where the lower part of the spinal cord has been extirpated, or in man where 

 the same part of the central nervous system has been destroyed as the result 

 of accident or disease. 



THE MECHANISM OF EVACUATION OF THE BLADDER 



In the denervated bladder the factor finally causing partial evacuation 

 is the gradual increase in the intravesical tension from the accumulation of 

 fluid in this viscus. The same factor is prepotent in determining the onset 

 of normal micturition in an animal with the nervous connections of its 

 bladder intact. Apart from the control of the higher centres, micturition 

 will take place each time that the tension 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 in- 

 hibition 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 contraction 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 excita- 

 tion of the peripheral end of one pelvic visceral nerve causes a strong 



