DEFECATION. 



297 



impossible. Under these circumstances, it is probable that the faeces are passed out of 

 the rectum by antiperistaltic action. 



The condition which immediately precedes the desire for defecation is probably the 

 descent of the contents of the sigmoid flexure of the colon into the rectum. It was for- 

 merly thought that the faeces constantly accumulated in the dilated portion of the rec- 

 tum, where they remained until an evacuation took place ; but the arguments of O'Beirne 

 against such a view are conclusive. He has demonstrated, by numerous explorations in 

 the human subject, that, under ordinary conditions, the rectum is contracted and con- 

 tains neither fa3ces nor gas. It is, indeed, a fact familiar to every surgeon, ,that the rec- 

 tum usually contains nothing which can be reached by the finger in physical examina- 

 tions, and that paralysis or section of the muscles which close the anus by no means in- 

 volves, necessarily, a constant passage of faecal matter. O'Beirne not only found the rectum 

 empty and presenting a certain amount of resistance to the passage of injected fluids, but, 

 on passing a stomach-tube into the bowel, after penetrating from six to eight inches it 

 passed into a space in which its extremity could be moved with great freedom, and there 

 was instantly a rush of flatus, of fluid fasces, or of both, through the tube. In some in- 

 stances in which nothing escaped through the tube, the instrument conveyed to the hand 

 an impression of having entered a solid mass ; and on being withdrawn it contained solid 

 fseces in its upper portion. According to this observer, the sensation which leads to an 

 effort to discharge the faeces is due to the accumulation of matters in the sigmoid 

 flexure, which finally present at the contracted portion of the rectum just at its com- 

 mencement. This constriction, situated at the most superior portion of the rectum, is 

 sometimes spoken of as the sphincter of O'Beirne. 



The above is undoubtedly the mechanism of the descent of fascal matter into the 

 rectum in defaecation, as the act is usually performed ; but, under certain circumstances, 

 faeces must accumulate in the dilated portion of the rectum. Ordinarily, the discharge 

 of faeces only takes place after the efforts have been continued for a certain time ; and 

 when the evacuation is " figured," the whole length discharged frequently exceeds so 

 much the length of the rectum, that it is evident that a portion of it must have come 

 from the colon. O'Beirne states, indeed, that he has frequently examined the rectum at 

 the moment when a moderate inclination to go to stool is felt, and found it empty and con- 

 tracted. But, in cases in which the faeces are very fluid, or when the call for an evacua- 

 tion has not been regarded and has become imperative, the immediate discharge of mat- 

 ters when the sphincter is relaxed shows that the rectum has been more or less distended. 

 In many persons of constipated habit, and particularly in old subjects, the rectum may 

 become the seat of large accumulations of hardened and impacted faeces; but this is a 

 pathological condition. 



The sensation which ordinarily precedes and gives rise to the evacuation of faacal mat- 

 ter is peculiar and very variable in intensity. When this sensation is well marked but not 

 excessive, it is probably due to the presence of faacal matter in the rectum, not in suffi- 

 cient quantity, however, to press forcibly upon the sphincter. Pressure upon the rectum 

 from any cause, or irritation of its mucous membrane, is apt to give rise to this peculiar 

 sensation to a very marked degree. In some diseases, the exaggeration of this sensation, 

 then called tenesmus, is very distressing. 



In the process of defaecation, the first act is the passage, by peristaltic contractions, 

 of the contents of the sigmoid flexure of the colon through the slightly-constricted open- 

 ing of the rectum into its dilated portion below. The fgecal matter, however, is not al- 

 lowed to remain in this situation, but it passes into the lower portion of the rectum, in 

 obedience to the contractions of its muscular coat, assisted by the action of the abdomi- 

 nal muscles and the diaphragm. The circular fibres of the rectum undergo the ordinary 

 peristaltic contraction ; and the action of the longitudinal fibres is to render the rectum 

 shorter and more nearly straight. The internal and the external sphincter present a cer- 

 tain amount of resistance to the discharge of the faaces, more particularly the external 



