CHAP, i.] DIGESTIOX. 239 



diminished by a stimulus applied, either internally or externally, 

 to the anus. The tonic contraction is in part at least due to the 

 action of a nervous centre situated in the lumbar spinal cord. If 

 the nervous connexion of the sphincter with the spinal cord be 

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

 the dorsal region, the sphincter, after the depressing effect of the 

 operation, which may last several days, has passed off, still 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 is 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 

 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 entirely involuntary augmentation of the peristaltic action 

 of the large intestine and sigmoid flexure. Or rather, to describe 

 matters in their proper order, defaecation takes place in the 

 following manner. The sigmoid flexure and large intestine be- 

 coming more and more full, stronger and stronger peristaltic action 

 is excited in their walls. By this means the fseces are driven 

 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 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 defsecation, 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 



F. 19 



