410 THE NERVOUS SYSTEM. 



levator ani draws the borders of the relaxed orifice upward and out- 

 ward, and the feces are expelled by the muscular contraction of the 

 rectum. 



Both these actions are in some degree associated, during- health, with 

 sensation and volition. The distention of the rectum which precedes 

 evacuation is accompanied by a sensation, and the resistance of the 

 sphincter may be intentionally prolonged for a certain period. But this 

 power of control is limited. After a time the involuntary impulse, 

 growing more urgent with the increased distention, becomes irresistible ; 

 and the discharge finally takes place by reflex action of the spinal cord. 



When the irritability of the cord is exaggerated by disease, its con- 

 nection with the brain remaining entire, the distention of the rectum 

 is announced by the usual sensation ; but the impulse of evacuation is 

 so urgent that it cannot be controlled, and must take place at once. 

 The discharges are then said to be "involuntary." 



If the cord, on the other hand, be disintegrated in its middle or 

 upper portions, all sensibility and volition connected with the action of 

 the sphincter are lost. The evacuation then takes place by the ordinary 

 mechanism, as soon as the rectum is filled, but without the knowledge 

 of the patient. The discharges are then " involuntary and unconscious." 



Finally, if the lower portion of the cord, in an animal, be broken up 

 by an instrument introduced into the spinal canal, the tonic contraction 

 of the sphincter at once disappears. The same effect is produced, in 

 man, by disorganization of the lower part of the spinal cord from injury 

 or disease. The sphincter ani is then permanently relaxed, and the 

 feces are evacuated without the knowledge of the patient, as fast as 

 they descend into the rectum from the upper portions of the intestinal 

 canal. 



The urinary bladder serves also both as a reservoir and an organ of 

 evacuation, its outlet being protected by the circular muscular fibres 

 at the commencement of the urethra, known as the " sphincter vesicae." 

 While the nerves distributed to the kidneys are derived exclusively 

 from the sympathetic system, those of the bladder consist partly of 

 sympathetic filaments from the mesenteric ganglia, and partly of cere- 

 bro-spinal filaments from the lumbar portion of the spinal cord, both 

 sets being united in the hypogastric plexus. 



The tonic contraction of the vesical sphincter during health, by which 

 the urine is retained in the bladder, is a continuous, involuntary, and 

 unconscious act, like that of the sphincter ani. At the time of evacua- 

 tion, the sphincter is relaxed by a voluntary impulse, and the muscular 

 coat of the bladder contracts to expel its contents ; but although the 

 commencement of this process is voluntary, the subsequent contrac- 

 tion of the bladder continues independently of the will. According 

 to the experiments of Giannuzzi* on dogs, irritation of the lumbar 

 portion of the spinal cord, by pricking with a steel needle, causes con- 



* Journal de la Physiologie. Paris, 1863, tome vi., p. 22. 



