DIGEST TO X 311 



it may be abolished by operation, and in each case the action 

 becomes wholly reflex. In the normal course of events, the pres- 

 sure of the faeces accumulating in the sigmoid flexure at last begins 

 to elicit the discharge of that reflex contraction of the lower 

 portion of the bowel already described (p. 310), of which the pelvic 

 nerves constitute the efferent path. At the same time, the sensa- 

 tions set up by the presence of faeces in the rectum, the lower 

 part of which, at any rate, is empty and quiescent in the intervals 

 of defaecation, give rise to the characteristic desire to empty the 

 bowels. This desire may for a time be resisted by the will, or it 

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

 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 contrac- 

 tion 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 defaecation is carried on as in 

 a normal animal, the control of the sphincters being due either 

 to a property of the muscular tissue itself or to local ganglia. 

 The contraction of the levatores ani helps to resist overdistension 

 of the pelvic floor and to pull the anus up over the faeces as they 

 escape. The nervi erigentes carry efferent constrictor fibres, and 

 the hypogastrics, as a rule, efferent dilator fibres, to the sphincters. 

 While the internal sphincter is by itself capable of maintaining a 

 tonus of considerable strength, the external sphincter contributes 

 an important share (30 to 60 per cent.) to the closure of the 

 rectum. 



The time of passage of substances through the alimentary 

 canal has been studied by administering collodion capsules 

 filled with subnitrate of bismuth to human beings, and observing 

 their progress by taking shadow pictures of them at intervals 

 with the Rontgen rays. During the first twenty minutes two 

 such capsules swallowed at the same time by a healthy young 

 man were clearly seen in the greater curvature of the stomach, 

 but in the interval between the first half-hour and the seventh or 



