522 EVACUATION OF THE URINE. 



alone suffices. When the distention of the bladder is only moderate or 

 slight, the sensory, excito-reflex nerves of the bladder must first be stimu- 

 lated, and either through irritation of the sensory nerves by voluntary 

 contraction of the striated muscles of the urethra and the floor of the 

 pelvis, or of the nerves of the bladder as a result of abdominal pressure. 



As electric stimulation from the cerebral peduncle downward through 

 the motor paths of the spinal cord to the motor nerves of the unstriated 

 musculature of the bladder causes contraction of the bladder, many 

 investigators have concluded that the will is capable of exciting spon- 

 taneous contractions of the bladder directly in this way. The author con- 

 siders this view as incorrect. In his opinion, voluntary evacuation of 

 urine is always induced by reflex influences, in the excitation of which 

 the will participates only in a secondary manner. With the vesical 

 center situated in the spinal cord still other nervous apparatus cooper- 

 ates. As painful irritation of sensory nerves in different parts of the 

 body also is capable of causing reflex contraction of the bladder the 

 involuntary discharge of urine that occurs frequently in children suffer- 

 ing from disorders of dentition may be of this character; as, further, as 

 has already been pointed out, sensory nerves situated at a higher level, 

 even cerebral nerves, are capable of exciting the vesical reflex, it must be 

 concluded that the vesical center extends for a considerable distance up- 

 ward, perhaps to the anterior portion of the optic thalamus, and that 

 from these higher levels descend motor paths that are susceptible of 

 possibly reflex stimulation in the spinal cord. Irritation of the medulla 

 from the cerebral peduncle downward causes contraction of the walls 

 of the bladder. 



With respect to the mechanism for the retention of the urine in the 

 bladder through the sphincter muscle of the urethra, consideration 

 should be given to the following facts : 



4. The motor nerves for the striated sphincter muscle are con- 

 tained in the pudendal nerve, derived from the anterior roots of the 

 third and fourth sacral nerves. Irritation causes contraction of the 

 muscle; paralysis, inability to close the urethra, with the result that 

 dribbling or incontinence of urine takes place. The nerves maybe both 

 stimulated voluntary interruption of the stream of urine and in- 

 hibited through the action of the will. 



5. The sensory nerves of the urethra pass into the spinal cord 

 through the posterior roots of the third, fourth, and fifth sacral nerves. 

 These stimulate, on the one hand, the reflex for the urethral sphincter, so 

 that as soon as urine escapes from the bladder into the commencement 

 of the urethra the sphincter muscle contracts; as, for instance, in adults, 

 during sleep, when the bladder becomes distended. On the other hand, 

 they transmit sensory impressions from the urethra, particularly also 

 when urine forces its way into the canal. 



6. The center for the urethral -sphincter reflex urethrospinal cen- 

 ter is situated, in the dog, at the level of the fifth, and, in the rabbit, 

 at the level of the seventh lumbar vertebra. 



7. From the cerebral cortex the voluntary motor paths course down- 

 ward through the spinal cord to the sphincter muscle of the urethra, 

 within the pyramidal tracts. 



8. The inhibitory paths for this muscle likewise pass from the brain 

 through the spinal cord, and through them the muscle may voluntarily 



