41-i THE SPINAL CORD. 



even fairly understand the cause of the involuntary act. If the 

 body by any accident suddenly and unexpectedly loses its balance, 

 the limbs are thrown into a position calculated to protect the ex- 

 posed parts, and to break the fall, by a similar involuntary and in- 

 stantaneous movement. The brain does not act in these cases, for 

 there is no intentional character in the movement, nor even any 

 complete consciousness of its object. Everything indicates that it 

 is the immediate result of a simple reflex action of the spinal cord. 



The cord exerts also an important and constant influence upon 

 the sphincter muscles. The sphincter ani is habitually in a state of 

 contraction, so that the contents of the intestine are not allowed to 

 escape. When any external irritation is applied to the anus, or 

 whenever the feces present themselves internally, the sphincter 

 contracts involuntarily, and the discharge of the feces is prevented. 

 I This habitual closure of the sphincter depends on the reflex action 

 of the spinal cord. It is entirely an involuntary act, and will con* 

 tinue, in the healthy condition, during profound sleep, as complete 

 and efficient as in the waking state. 



When the rectum, however, has become filled by the accumula- 

 tion of feces from above, the nervous action changes. Then the 

 impression produced on the mucous membrane of the distended 

 rectum, conveyed to the spinal cord, causes at the same time re- 

 laxation of the sphincter and contraction of the rectum itself; so 

 that a discharge of the feces consequently takes place. 



Now all these actions are to some extent under the control of 

 sensation and volition. The distended state of the rectum is usually 

 accompanied by a distinct sensation, and the resistance of the 

 sphincter may be voluntarily prolonged for a certain period, just as 

 the respiratory movements, which are. usually involuntary, may be 

 intentionally hastened or retarded, or even temporarily suspended. 

 But this voluntary power over the sphincter and the rectum is 

 limited. After a time the involuntary impulse, growing more 

 urgent with the increased distension of the rectum, becomes irre- 

 sistible ; and the discharge finally takes place by the simple reflex 

 action of the spinal cord. 



If the spinal cord be injured in its middle or upper portions, the 

 sensibility and voluntary action of the sphincter are lost, because its 

 connection with the brain has been destroyed. The evacuation 

 then takes place at once, by the ordinary mechanism, as soon as 

 the rectum is filled, but without any knowledge on the part of the 



