MOVEMENTS OF Till-: ALIMENTARY CANAL, ETC. 391 



arouse a conscious sensation of fulness and a desire to micturate. Under nor- 

 mal conditions the act of micturition follows. It consists essentially in a strong 

 contraction of the bladder with a simultaneous relaxation of the external 

 sphincter, if this muscle is in action, the effect of which is to obliterate more or 

 less completely the cavity of the bladder and drive the urine out through the 

 urethra. 



The, force of this contraction is considerable, as is evidenced by the height 

 to which the urine may spirt from the end of the urethra. According to 

 Mosso the contraction may support, in the dog, a column of liquid two meters 

 high. The contractions of the bladder may be and usually are assisted by 

 contractions of the walls of the abdomen, especially toward the end of the act. 

 As in defecation and vomiting, the contraction of the abdominal muscles, when 

 the glottis is closed so as to keep the diaphragm 'fixed, serves to increase the 

 pressure in the abdominal and pelvic cavities, and is thus used to assist in or 

 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 1 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 urethra, 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, indicate 

 that micturition is essentially a reflex acl with it.- centre in the lumbar cord, 

 although the same observer has shown that in dogs whose spinal cord has 

 been entirely destroyed, except in the cervical and upper thoracic region, the 

 bladder empties itself normally without the aid of external stimulation. 

 MoSSO and Pellacani 2 have made experiments upon women which ><'r\\\ to 



1 Archiv fur die gesammte Physiologie, IsTt, Bd, viii. S. 17s 



2 Archives it<tli< nii>.< ,h Biologie, L882, tome i. 



