MOVEMENTS OF THE ALIMENTARY CANAL, ETC. 329 



complete the emptying of the bladder. It is, however, not an essential part of 

 the act of micturition. The last portions of the urine escaping into the urethra 

 are ejected, in the male, in spirts produced by the rhythmic contractions of the 

 bulbo-cavernosus muscle. 



Considerable uncertainty and difference of opinion exists as to the physio- 

 logical mechanism by which this series of muscular contractions, and especially 

 the contractions of the bladder itself, is produced. According to the frequently 

 quoted description given by Goltz l the series of events is as follows : The dis- 

 tention of the bladder by the urine causes finally a stimulation of the sensory 

 fibres of the organ and produces a reflex contraction of the bladder musculature 

 which squeezes some urine into the urethra. The first drops, however, that 

 enter the urethra stimulate the sensory nerves there and give rise to a conscious 

 desire to urinate. If no obstacle is presented the bladder then empties itself, 

 assisted perhaps by the contractions of the abdominal muscles. The emptying 

 of the bladder may, however, be prevented, if desirable, by a voluntary con- 

 traction of the sphincter urethrse, which opposes the effect of the contraction of 

 the bladder. If the bladder is not too full and the sphincter is kept in action 

 for some time, the contractions of the bladder may cease and the desire to 

 micturate pass off. According to this view the voluntary control of the 

 process is limited to the action of the external sphincter and the abdominal 

 muscles ; the contraction of the bladder itself is purely an unconscious reflex 

 taking place through a lumbar centre. 



The experiments of Goltz and others, upon dogs in which the spinal cord 

 was severed at the junction of the lumbar and the thoracic regions, prove 

 that micturition is essentially a reflex act with its centre in the lumbar cord, 

 but a number of physiologists have concluded that the contractions of the 

 bladder itself, in spite of its involuntary musculature, is also under control of 

 the will. Mosso and Pellacani 2 have made experiments upon women which 

 seem to show that this is the case. In these experiments a catheter was intro- 

 duced into the bladder and connected with a recording apparatus to measure 

 the volume of the bladder. It was found that, in some cases at least, the 

 woman could empty the bladder at will without using the abdominal muscles. 

 The same authors adduce experimental evidence to show that the sensation of 

 fulness and desire to micturate come from sensory stimulation in the bladder 

 itself caused by the pressure of the urine. They point out that the bladder is 

 very sensitive to reflex stimulation ; that every psychical act and every sensory 

 stimulus is apt to cause a contraction or increased tone of the bladder. The 

 bladder is, therefore, subject to continual changes in size from reflex stimula- 

 tion, and the pressure within it will depend not simply on the quantity of 

 urine but on the condition of tone of the bladder. At a certain pressure 

 the sensory nerves are stimulated and under 'normal conditions micturition 

 ensues. We may understand, from this point of view, how it happens that we 

 have sometimes a strong desire to micturate when the bladder contains but little 



1 Pftuyer's Archivfilr die gesammte Physiologie, 1874, Bd. viii. S. 478. 



2 Archives italienne de Biologie, 1882, tome i. 



