468 PHYSIOLOGY CHAP. 



is made to depend on the predominance of the detrusor over the 

 sphincter. But in nine experiments Rehfisch was able to show 

 that the opening of the urethra occurred five times when the 

 curve of vesical pressure was on the down-grade, three times when 

 on the up-grade, and once when it had reached its summit. He 

 also saw that the pressure in the bladder was not maintained 

 at a constant level during the time of evacuation, but almost 

 constantly diminished. 



These results contradict the hypothesis that it is the pre- 

 ponderating force of the detrusor which opens the sphincter and 

 keeps it open for the entire period of micturition. They show, 

 on the contrary, that the moment of opening the urethra or re- 

 laxing the sphincter is independent of the contraction of the 

 detrusor, although the latter precedes the former by a shorter or 

 longer period. We may therefore conclude that in the mechanism 

 of micturition as incited by desire, the lowering of tone or active 

 voluntary relaxation of the internal sphincter of smooth muscle 

 is a main factor in the process, and that instead of opposing, it 

 promotes the contraction of the detrusor. 



Hanc (1899) obtained confirmation of this theory by experi- 

 menting with curarised dogs, on which he simultaneously recorded 

 the pressure in the bladder and the amount of flow from the 

 urethra during the reflex excited by stimulating the sciatic. At 

 the beginning of each experiment the bladder was filled with a 

 constant quantity of tepid water. He concluded from the ratio 

 between intravesical pressure and the flow of urine in thirty-two 

 experiments that they are independent of each other, i.e. that the 

 contraction of the detrusor and the expansion of the sphincter are 

 two distinct phenomena due to the activity of two separate nerves. 

 The animals being curarised, all active intervention of the will is 

 excluded in this kind of reflex micturition. 



If the animal be put under morphine before stimulating the 

 sciatic, the contraction of the detrusor persists, but there is no 

 longer dilatation of the sphincter, so that the evacuation of the 

 bladder is premature. This is probably due to the fact that the 

 two muscles are under the tonic influence of two distinct spinal 

 centres of antagonistic action. Chloral hydrate also acts like 

 morphine, but weakens the reflex of the detrusor as well. Atropine 

 and cocaine weaken both reflexes. Strychnine produces the opposite 

 effect, but in a variable degree. The same holds good for muscarine. 



It is also possible, as we stated above, to perform the act of 

 micturition perfectly, independent of any more or less urgent 

 desire, i.e. without the bladder being in such a tonic or contracted 

 state as to arouse the sense of repletion or tension. For instance, 

 micturition can be repeated a few moments after evacuation of the 

 bladder. 



This form of micturition is purely voluntary, and quite distinct 



