328 DIGESTION 



being due either to a property of the muscular tissue itself or to 

 local ganglia. The contraction of the levatores ani helps to resist 

 over distension 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. If the call to defalcation is neglected, the desire 

 passes away. . This is not due to the faeces being carried back into 

 the pelvic colon by antiperistalsis, as has generally been stated. 

 The faeces which have passed into the rectum remain there, as can 

 be shown by examination with the finger after the desire to empty 

 the bowels has disappeared. The reason for the disappearance 

 of the sensation is the relaxation of tone which occurs in the 

 muscular coat of the rectum after a period of distension. It is not 

 till it has been again distended by the entrance of a further portion 

 of faeces that the call to defaecation is again experienced. When 

 the call is repeatedly neglected, the sensibility of the rectum to dis- 

 tension becomes blunted, and this is a common cause of constipation. 



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 pro- 

 gress 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 eighth hour no further 

 trace of them was detected. About the eighth hour they re- 

 appeared in the caecum, where they remained with little or no 

 onward movement till the fourteenth hour. From the fourteenth 

 to the sixteenth hour they travelled along the ascending colon, and 

 tarried a long time at the left angle of the colon. From the nine- 

 teenth to the twenty- second or twenty-fourth hour they slowly 

 passed downward in the descending colon, and stopped at the sig- 

 moid flexure till their expulsion in defaecation. In some subjects 

 the entire passage of the capsules was complete in sixteen hours, in 

 others not until after thirty hours. A one cent piece swallowed by 

 a healthy child four years old was recovered in the faeces 52 hours 

 later, and a button, slightly larger, swallowed by the same child, 

 appeared after almost exactly the same interval. 



Vomiting. We have seen that under normal conditions the 

 movements of the alimentary canal always tend to carry the food 

 in one definite direction, along the tube from the mouth to the 

 rectum. The peristaltic waves generally run only in this direction, 

 and, further, regurgitation is prevented at three points by the 



