THE MECHANICAL PHENOMENA OF DIGESTION 333 



in the adult it may be lost by disease; in an animal it may be 

 abolished by operation, and in each case the action becomes wholly 

 reflex. Owing to the tonic contraction of the rectum and the acute 

 angle formed at the pelvi-rectal flexure, the faeces are arrested at 

 this point. In consequence the pelvic colon becomes filled with 

 faeces from below upwards, and the rectum remains empty till 

 immediately before defalcation. This has been verified in man by 

 observations with the Rontgen rays (Hertz). In persons whose 

 bowels are opened regularly after breakfast, the passage of faeces 

 into the rectum gives rise to the characteristic sensation which 

 may be termed the ' call to defaecation. ' It is the distension of the 

 rectum, and of the rectum alone, which is associated with this 

 sensation, for in persons from whom the entire rectum has been 

 removed for malignant disease the sensation is absent, and it may 

 be elicited by artificially distending the rectum, though not any 

 other part of the alimentary canal. The minimum pressure required 

 to elicit the sensation is smaller the greater the length of the gut 

 exposed to it, varying in one individual from 32 to 48 mm. of 

 mercury, according to the length of a balloon introduced into the 

 rectum. The passage of the faeces from the pelvic colon into the 

 rectum is due to the discharge of that reflex contraction of the lower 

 portion of the bowel already described (p. 332), of which the pelvic 

 nerves constitute the efferent path. This reflex peristalsis is elicited 

 by various causes, among which one of the most important is the 

 taking of food at breakfast into the empty stomach, and another 

 the muscular activity associated with getting up and dressing. 

 The desire to defalcate may for a time be resisted by the will, or it 

 may be yielded to. In the latter case the abdominal muscles, and, 

 according to Hertz, the diaphragm also, are forcibly contracted; 

 and the glottis being closed, the whole effect of their contraction 

 is expended in raising the pressure within the abdomen and pelvis, 

 and so aiding the muscular wall of the bowel itself in driving the 

 faeces from the sigmoid flexure to the rectum. The two sphincters 

 which close the anus the internal sphincter of smooth muscle, 

 and the external of striated are now relaxed by the inhibition of 

 a centre in the lumbar portion of the spinal cord, through the 

 activity of which the tonic contraction of the sphincters is normally 

 maintained. This relaxation is partly voluntary, the impulses 

 that come from the brain acting probably through the medium 

 of the lumbar centre. But in the dog, after section of the cord in 

 the dorsal region, the whole act of defaecation, including contraction 

 of the abdominal muscles and relaxation of the sphincters, still 

 takes place, and here the process must be purely reflex. Even after 

 complete destruction of the lumbar and sacral portions of the spinal 

 cord the tone of the sphincters returns after a time, and defalcation 

 is carried on as in a normal animal, the control of the sphincters 



