490 DEFECATION. [BOOK n. 



may remain wholly efficient when separated from the brain, the 

 paralysis of the sphincter which occurs in certain cerebral diseases 

 is, we may infer (unless we suppose that the human spinal cord 

 has lost functions present in that of the lower animals, a matter 

 which we shall discuss in treating later on of the spinal cord), due 

 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 defaecation 

 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. 



Defaecation 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 faeces 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 defaecation, 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 thoracic cord. In such cases the 

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

 in the rectum. 



276. The 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 

 plain muscular fibres. The alimentary canal therefore, like the 

 heart, though to a less degree, possesses within itself such 

 mechanisms as are requisite for carrying out its own movements ; 

 and, as in the case of the heart, there is no adequate evidence 



