ACTION AS A NERVOUS CENTRE. 467 



Another important action of the spinal cord, as a nervous centre, 

 consists in its control over the sphincters and the organs of evacuation. 



While the small intestine, the caecum, and the colon are supplied ex- 

 clusively with nerves from the abdominal plexuses of the sympathetic 

 system, the lower portion of the rectum receives branches from the 

 sacral plexus of spinal nerves, which are distributed both to its mucous 

 membrane and its muscular apparatus. The lower part of the large 

 intestine acts in great measure as a temporary reservoir, in which the 

 feces, brought down from above by peristaltic movement, accumulate 

 until the time arrives for their evacuation. The rectum, in man, is 

 usually empty, or nearly so, until shortly before evacuation ; and when 

 the feces begin to pass into it from above, it is still capable of retaining 

 them for a certain period. Their retention and discharge are provided 

 for, in this part of the alimentary canal, by two sets of muscular fibres ; 

 namely, first, the sphincter ani, which keeps the orifice of the anus 

 closed; and secondly, the levator ani and the circular fibres of the 

 rectum itself, which by their contraction open the anus and expel the 

 feces. Both these acts are regulated by the reflex influence of the 

 spinal cord. 



In the natural condition, the sphincter ani is habitually in a state of 

 contraction, thus preventing the escape of the contents of the intestine. 

 Any external irritation, applied to the verge of the anus, causes increased 

 contraction of its fibres and a more complete occlusion of its orifice. 

 This habitual closure of the sphincter is an entirely involuntary act, as 

 efficient during profound sleep as in the waking condition, and depends 

 upon the reflex action of the spinal cord. 



But when the rectum is distended to a certain point by feces passing 

 into it from above, the nervous action changes. The impression then 

 produced upon the mucous membrane of the rectum, conveyed inward 

 by its sensitive nerve fibres to the spinal cord, causes a relaxation of 

 the sphincter ani. At the same time the levator ani draws the borders 

 of the relaxed orifice upward and outward, and the feces are expelled by 

 the contraction of the muscular fibres of the rectum itself. 



Both these actions are in some degree associated, in a state of health, 

 with sensation and volition. The distension of the rectum which pre- 

 cedes an evacuation is usually accompanied by a distinct sensation, and 

 the resistance of the sphincter may be intentionally prolonged for a cer- 

 tain period. But this voluntary power over the muscular contractions 

 is limited. After a time the involuntary impulse, growing more urgent 

 with the increased distension of the rectum, becomes irresistible ; and 

 the discharge finally takes place by simple reflex action of the spinal 

 cord. 



If the irritability of the cord be exaggerated by disease, while its 

 connection with the brain remains entire, the distension of the rectum 

 is announced by the usual sensation ; but the reflex impulse to evacua- 

 tion is so urgent that it cannot be controlled by the will, and the patient 



