DEFECATION. 621 



is usually accompanied by a manifest upward return of the faecal mat- 

 ters from the sigmoid flexure into the colon. 



In evacuating the faeces, the object to be accomplished is, that the 

 contents of the large intestine shall be pressed upon with" a force supe- 

 rior to the resistance presented by the annulus or upper extremity of 

 the contracted rectum, and the muscles of the anus. The contraction 

 of the rectum is generally insufficient to effect this last object, notwith- 

 standing the great thickness of its muscular layer. In cases, however, 

 of irritability of the rectum, the sphincter is incapable of resisting the 

 force developed by the proper muscular fibres of the gut. Under ordi- 

 nary circumstances, the aid of the diaphragm and abdominal muscles 

 is invoked, and it is chiefly through these muscles, that volitidn influ- 

 ences the act of defecation, suspending, deferring, or accelerating it, 

 as the case may be. After a full inspiration, the muscles that close the 

 glottis ; and the expiratory muscles, especially those of the anterior 

 part of the abdomen, contract simultaneously. The air cannot escape 

 from the lungs; the diaphragm is depressed upon the abdominal viscera, 

 and the whole thorax presents a resisting body; so that all the expira- 

 tory power of the muscles bears upon the viscera, and presses them 

 against the vertebral column. In this way, considerable force is 

 exerted upon the contents of the colon and rectum; the resistance of 

 the sphincter, already diminished by the direct exercise of volition, 

 is surmounted; it yields, and the faeces are extruded. The levator ani 

 and ischio-coccygeus, aided by the transversus perinei muscle, support 

 the anus during the expulsory efforts, and restore it to its place after 

 the efforts have ceased. Whilst straining is effected by the diaphragm 

 and abdominal muscles, the longitudinal muscular fibres of the rectum 

 contract, so as to shorten the intestine, and, consequently, the space 

 over which the faeces have to pass. At the same time, the circular 

 fibres contract from above to below, so as to propel the excrement 

 downwards, and to cause the mucous membrane to extrude, and form a 

 ring or bourrelet, like that which occurs at the cardiac orifice of the 

 stomach, when the food is passing from the oesophagus into that organ. 

 If this extrusion occurs to a great extent, it constitutes the disease 

 called prolapsus ani. 



Dr. O'Beirne 1 of Ireland, guided by the following facts and argu- 

 ments ; that great irritation would be produced in the sphincter ani, 

 and bladder, if the faeces descended readily into the rectum ; that the 

 difficulty experienced in throwing up an injection is inconsistent with 

 the idea of the gut being open, and proves that it is firmly contracted 

 and closed; that when the surgeon has occasion to pass his finger up 

 the rectum, he rarely encounters either solid or fluid fasces ; that the 

 two sphincter muscles of the anus are weakened in certain diseases, and 

 divided in operations, and yet it rarely happens, that the power of 

 retaining the faeces is destroyed ; that on passing a stomach-tube to 

 the height of half an inch up the rectum, in a number of healthy per- 

 sons', it was found, that nothing escaped, and that the tube could be 



1 New Views of the Process of Defecation, &c., Dublin, 1S33; reprinted in this country, 

 Washington, 1834. 



