382 DEFECATION. [BOOK n. 



broken, relaxation takes place. If the spinal cord be divided 

 somewhat higher up, for instance in the dorsal region, the 

 sphincter, after the depressing effect of the operation, which may 

 last several days, has passed off, regains and subsequently main- 

 tains its tonicity, shewing that the centre is not placed higher up 

 than the lumbar region of the cord. The increased or diminished 

 contraction following on local stimulation is probably due to reflex 

 augmentation or inhibition of the action of this centre. The. 

 centre is also subject to influences proceeding from higher regions 

 of the cord, and from the brain. By the action of the will, 

 by emotions, or by other nervous events, the lumbar sphincter 

 centre may be inhibited, and thus the sphincter itself relaxed ; or 

 augmented, and thus the sphincter tightened. A second item 

 therefore of the voluntary process in defecation is the inhibition of 

 the lumbar sphincter centre, and consequent relaxation of the 

 sphincter muscle. Since the lumbar centre may remain wholly 

 efficient when separated from the brain, the paralysis of the 

 sphincter which occurs in certain cerebral diseases is probably 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 defsecation 

 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 aftbrds a more or less fixed 

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

 means of contractions travelling from above downwards but 

 taking place somewhat later, narrow the rectum and so squeeze 

 the contents onwards and outwards. 



Defsecation then appears to take place in the following man- 

 ner. 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 fseces 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 con- 

 traction 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. 



