466 PHYSIOLOGY CHAP. 



a very different volume of fluid, and the need of micturition 

 always arises under the same pressure (in a dog at a pressure of 

 20 cm., in a girl at a pressure of 18 cm. water) whatever may -be 

 the volume of fluid contained in the bladder. These authors 

 concluded that micturition is excited by the pressure to which 

 the walls of the bladder are subjected, and not by the varying 

 degree of their distension. 



If when the desire to micturate arises, the act of evacuation 

 is delayed by the voluntary mechanism above described, the 

 desire may lessen, or even disappear after a certain time. This 

 phenomenon depends on a lowering of tone in the bladder and 

 the consequent reduction of vesical pressure, although the quantity 

 of urine has not been diminished, but even increases, whicl 

 confirms the variability of the tone of the bladder and the 

 dependence of the desire to micturate upon vesical pressure, i.t 

 on a certain active reaction of the muscles of the bladder to its 

 contents. 



When, on the contrary, the need to micturate increases and 

 becomes imperative, the flow of urine must be given free vent, 

 and may in this case be termed voluntary, but only because it 

 commences with the relaxation of Wilson's muscle (external 

 sphincter), i.e. with the removal of the obstacle .opposed by the 

 will to the action of the detrusor urinae. It is true that in order 

 to reinforce the expulsory effort of the latter, and to empty the 

 bladder as completely as possible, abdominal compression, i.e. the 

 repeated voluntary contraction of the abdominal muscles and 

 diaphragm, plays an active part. This intervention, however, 

 is not necessary, because the detrusor is in itself capable (after 

 the animal's body has been opened) of developing sufficient force 

 to hold up a column of water 1*5-2 m. in height (as observed by 

 Mosso and Pellacani), and micturition normally takes place easily, 

 even if more slowly, without any intervention of abdominal 

 compression, i.e. with no perceptible modification of respiratory 

 rhythm. 



The mechanism of micturition aroused by consciousness of 

 tension in the bladder accordingly differs from that of involuntary 

 micturition only in the previous, voluntary relaxation of the ex- 

 ternal sphincter. On this assumption both involuntary micturition 

 and that preceded by the desire to micturate would be essentially 

 reflex acts, independent of the direct exercise of the will. 



This theory, which at first sight appears simple and satisfactory, 

 was overthrown by an ingenious experiment of Rehfisch. 



By perfecting a method already employed by v. Zeissl in the 

 study of vesical iunervation (infra), Rehfisch introduced a catheter 

 into the human bladder, provided with a two-way tap by which 

 boracic acid solution could be injected in order to increase the 

 content and distension of the bladder. By a turn of the tap he 



