280 DIGESTIVE SYSTEM. 



rectum alone can expel them, without the will having any 

 other share in the matter than that of not offering any 

 obstacle to their passage. In ordinary cases, however, the 

 Bolid state of the excrement requires the intervention of 

 more numerous and considerable forces, which act principally 

 under the influence of the will : the first is the phenomenon 

 of straining, by means of which the larynx closes, causing the 

 walls of the thoracic cavity, which is filled with air, to sup- 

 ply a fulcrum to the muscles which are about to act ; all 

 those muscles which can compress the abdomen, that is, the 

 muscles of the abdominal coat, the diaphragm, and the 

 muscles of the perineum (levator am), then contract, pro- 

 ducing compression on every side. The levator ani, while 

 compressing the viscera from bottom to top, brings just in 

 front of the excrement the orifice through which it must 

 pass. The longitudinal fibres of the rectum, which are so 

 largely developed, act for the same purpose, and this is only 

 one mode of the mechanism which we studied when analyz- 

 ing the peristaltic movement (see Deglutition, p. 225). More- 

 over, these longitudinal fibres terminate below by folds, 

 which disappear more or less distinctly in the perineum, 

 forming a convex curve directed towards the centre of the 

 anus ; whence it follows that during their contraction their 

 curve straightens, and, consequently, dilates the orifice 

 through which the excrement is to pass. 



