464 PHYSIOLOGY CHAP. 



which fluid could be injected as required into the bladder. He 

 then ascertained the height to which the internal pressure of the 

 bladder must be raised in order to see the first drop of fluid flow 

 from the catheter introduced into the urethra at different depths. 

 He found that this pressure was approximately the same, either 

 when the catheter was pushed as far as the prostate, so as to 

 hinder the action of Wilson's muscle, or when it was in the free 

 part of the urethra, on which the action neither of this muscle 

 nor of the compressor urethrae was hindered. 



While Versari restored to the internal sphincter its anatomical 

 function as distinct from that of the detrusor muscle, these 

 important experiments of Rehfisch prove it to be the chief 

 physiological factor in the closure of the urethra, and therefore in 

 the normal retention of urine. 



X. The mechanism of micturition or urinary excretion is 

 certainly more complex than that of retention. It should be stated 

 at the outset that three different kinds of Micturition can be 

 distinguished and must be considered separately : (a) wholly 

 involuntary micturition, (&) micturition produced by a desire to 

 pass urine ; (c) voluntary micturition, i.e. independent of desire. 



Involuntary micturition is a constant physiological phenomenon 

 in infants at the breast, both in sleeping and waking ; it is often 

 observed in young children during teething, particularly when 

 tedious and painful ; it sometimes occurs with older children in 

 sleep, up to the age of puberty. In all these cases micturition is 

 a purely reflex, unconscious phenomenon, discharged by the great 

 excitability of the efferent and afferent nerves to the bladder, by 

 which the tone of the detrusor muscle, and therefore the tension 

 of the bladder walls, is reflexly exaggerated. It is probable that 

 in these cases the tone of the internal sphincter increases simul- 

 taneously, so that there is a conflict between the two antagonist 

 muscles, the sphincter which closes the urethral orifice, and the 

 detrusor which tends to open it by an outward pull (as assumed 

 by Kohlrausch), which is due more particularly to the contraction 

 of the fibres inserted radially between the muscular bundles of the 

 sphincter, to which Versari has lately called attention. Since the 

 struggle ends with emission of the urine contained in the bladder, 

 the expulsive force of the detrusor ultimately prevails over the 

 retentive force of the sphincter. 



Experimental confirmation of this very simple mechanism of 

 involuntary micturition is afforded by an experiment of Mosso and 

 Pellacani (1882) on chloroformed and curarised dogs. They 

 divided the ureter close to the neck of the bladder, opened the 

 abdominal cavity, introduced a cannula into one of the ureters 

 joined up with a Mariotte's bottle, and determined the pressure 

 necessary to obtain a flow from the urethra. The tone of the 

 detrusor muscle was then artificially raised by mechanical, 



