THE EVACUATION OF FECES. 285 



which the posterior roots of the lower lumbar and the sacral nerves were divided, 

 Landois observed that, while recovery was otherwise normal, the anus remained 

 open. Not rarely a portion of the fecal mass protruded for a considerable time, 

 as the sensibility in the rectum and anus was lost in such animals. Neither was 

 reflex contraction of the sphincters possible, nor could voluntary closure of the 

 anus, induced by the sense of feeling alone, take place, although this would other- 

 wise have doubtless been possible. 



An excitomotor as well as an inhibitomotor influence may be exerted 

 upon the external anal sphincter, as upon any voluntary muscle, from 

 the cerebrum. Nevertheless, closure can be maintained only for a 

 certain time if the pressure from above is considerable. Finally ener- 

 getic peristalsis overcomes even the strongest voluntary stimulation. 



FIG. 109. The Levator Ani and External Sphincter Ani Muscles. 



The evacuation of feces, which takes place habitually in human be- 

 ings at a definite interval, once or twice daily, rarely oftener, begins with 

 active peristalsis in the large intestine which passes downward to the 

 rectum. In order that the sphincter muscles may not be excited to 

 reflex activity by the advancing column of feces, it appears that an 

 inhibitory center for the sphincter-reflex, capable of voluntary stimula- 

 tion, must become active. This is situated in the brain (Masius 

 poses in the optic thalamus), whence its fibers pass through the cere- 

 bral peduncles to the anospinal center. During stimulation of 

 inhibitory apparatus, the column of feces passes through t 

 without causing its reflex closure. 



