464 DEFECATION. [BOOK 11. 



to inhibition of this lumbar centre, and not to paralysis of any 

 cerebral centre. 



Thus a voluntary contraction of the abdominal walls, accom- 

 panied by a relaxation of the sphincter, might press the contents 

 of the descending colon into the rectum and out at the anus. 

 Since however, as we have seen, the pressure of the abdominal 

 walls is warded off the sigmoid flexure, such a mode of defecation 

 would always end in leaving the sigmoid flexure full. Hence the 

 necessity for these more or less voluntary acts being accompanied 

 by an involuntary augmentation of the peristaltic action of the 

 large- intestine, sigmoid flexure and rectum. 



In the movements of the rectum we can trace out more 

 distinctly than in other regions of the alimentary canal the 

 separate actions of the longitudinal and circular fibres. The 

 former, by means of contractions travelling from above downwards, 

 shorten the rectum, and since the anus affords a more or less fixed 

 support pull the reetum and its contents down; the latter, by 

 means of contractions travelling from above downwards but taking 

 plaee somewhat later, narrow the rectum and so squeeze the 

 contents onwards and "inwards. 



Defecation then appears to take place in the following manner. 

 The large intestine and sigmoid flexure becoming more and more 

 full, stronger and stronger peristaltic action is excited in their 

 walls. By this means the feces are driven into the rectum and 

 so, by a continuance of the movements increasing in vigour, 

 against the sphincter. Through a voluntary act, or sometimes at 

 least by a simple reflex action, the lumbar sphincter centre is 

 inhibited and the sphincter relaxed. At the same time the 

 contraction of the abdominal muscles presses firmly on the descend- 

 ing colon, and thus, contractions of the levator ani assisting, the 

 contents of the rectum are ejected. 



It must however be remembered that, while in appealing to 

 our own consciousness, the contraction of the abdominal walls and 

 the relaxation of the sphincter seem purely voluntary efforts, the 

 whole act of defecation, including both of these seemingly so 

 voluntary components, may take place in the absence of conscious- 

 ness, and indeed, in the case of the dog at least, after the complete 

 severance of the lumbar from the dorsal cord. In such cases the 

 whole act must be purely reflex, excited by the presence of feces 

 in the rectum. 



276. TJte nervous mechanisms of gastric and intestinal 

 movements. Both the stomach and intestines when removed 

 from the body and thus wholly separated from the central nervous 

 system may, by direct stimulation, be readily excited to move- 

 ments; and indeed in the absence of all obvious stimuli, movements 

 which seem to be spontaneous may at times be observed. The 

 movements of which we are speaking are orderly movements of a 

 peristaltic nature, not mere local contractions of a few bundles of 



