THE MUSCULAR MECHANISMS OF DIGESTION. 301 



caecum, not only does distention of the csecum, by stretching the valve along 

 the line of the lips, bring them into apposition, but the pressure exerted by 

 the peristaltic movement has the same effect. In this way any return of the 

 contents from the large to the small intestine is prevented. 



Arrived at the sigmoid flexure, the contents, now more or less solid feces, 

 are supported by the bladder and the sacrum, so that they do not press on 

 the sphincter ani. 



236. Defecation. This is a mixed act, being superficially the result of 

 an effort of the will, and yet carried out by means of an involuntary mechan- 

 ism. Part of the voluntary effort consists in producing a pressure-effect, by 

 means of the abdominal muscles. These are contracted forcibly as in ex- 

 piration, but the glottis being closed and the escape of air from the lungs 

 prevented, the whole force of the pressure is brought to bear on the abdo- 

 men itself, and so drives the contents of the descending colon onward toward 

 the rectum. The sigmoid flexure is by its position sheltered from this pres- 

 sure ; a body introduced per anum into the empty rectum is not affected by 

 even forcible contractions of the abdominal walls. 



The anus is guarded by the sphincter ani, which is habitually in a state 

 of normal tonic contraction, capable of being increased or 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 connection of the sphincter with 

 the spinal cord be 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 maintains its tonicity, showing 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 prob- 

 ably 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 effi- 

 cient when separated from the brain, the paralysis of the sphincter which 

 occurs in certain cerebral diseases is probably due to inhibition of this lum- 

 bar centre, and not to paralysis of any cerebral centre. 



Thus a voluntary contraction of the abdominal -walls, accompanied 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 intes- 

 tine, 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 downward, shorten the rectum, and, since the anus affords a more or 

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

 means of contractions travelling from above downward but taking place 

 somewhat later, narrow the rectum and so squeeze the contents onward and 

 outward. 



