The nature of the contents of the rectum greatly, affects the degree in which the muscles of the abdomen are made to assist in their expulsion. When these contents consist only of gases and liquids, they require so little of this aid, as to be sometimes expelled without it. But the extrusion of hard scy balous evacuations often demands the help of ab dominal pressure, to an extent such as in volves all the viscera of the trunk, and seriously obstructs the flow of blood in the larger veins of the head and thorax.
As regards the levator ani muscle, its origin and insertion, together with the course taken by its fibres, leave no doubt as to what must be the direct effect of its contraction. It raises the end of the rectum, together with the ligamentous structures of the perinmum anteriorly, and the coccyx posteriorly'. But as the time of this action seems exactly to coincide with the exertion of the abdominal pressure just alluded to, the degree in which it really raises these structures can only equal the surplus of its force over that of the mus cles of the belly. Hence it may be doubted whether the muscle generally does more than fix the bowel : an effect which is, however, of the highest importance to the mechanism of defxcation. The influence of the neigh bouring perinmal muscles is still more obscure.
Such being the known agents of the pro cess of defxcation, we may next attempt to sketch the course of its phenomena.
The ordinary peristalsis of the large in testine propels into the rectum a variable quantity of fwces. These, after a longer or shorter sojourn in its first or second por tions, excite an active peristaltic contraction of its muscular coat. In general it is only when they reach the lower extremity of the bowel, that the abdominal pressure adds to this peristalsis its far more powerful expulsive force. The combined effect of both these actions urges the fmcal niass ag,ainst the ex ternal sphincter, which relaxes at this in stant, by a voluntary effort, so as to permit the extrusion of the descending niass : a small portion of the loose mucous membrane being at the same time generally everted around it.
A variable length of flEcal substance thus passes through the orifice of the anus. The continuity of the descending mass being finally interrupted, the act of respiration*isresumed ; the abdominal pressure ceases; and at the same time, the contraction of the levator ani, aided by that of the rectum itself, returns the projecting extremity of the bovvel into the pelvis, by a kind of sudden and forcible re traction. The latter act, in which both of
the sphincters may be presumed to play an important part, often subdivides a continuous fmcal mass ; returning the upper segment thus cut off into the cavity of the rectum which it was just leaving. The total du ration of the expulsive act appears to be chiefly determined by the consistence of the fmces, the velocity of their transit, and the exigencies of the suspended process of respi ration itself.
The mucous membrane of the rectum is con nected with its muscular coat much more loosely than that of the colon. Owing to this circumstance, it generally exhibits nume rous folds. Most of these are mere tempo rary results of the passive contraction of the muscular coat. And in correspondence with such an origin, they are very irregular in size, number, and position. They are, how ever, more frequently found occupying the dilated lower end of the bowel, where they take what is usually a longitudinal direction. .4 But in addition to such casual and tem porary folds, Mr. Houston* has described others, which he states to have a definite direction and situation, as well as a more permanent character. According to this anatomist, three is the number of these folds or valves usually present. The largest and most constant of the three projects from the anterior wall of the rectum, opposite to the base of the bladder, and about three inches above the anus. The valve next above this springs from the left wall of the bowel, about midway between the last and the third or up permost fold. This latter projects from the right wall of the upper end of the rectum. The shape of all three is nearly semilunar their depth about half an inch ; and they are fixed by a convex border to about half the circumference of the intestine. And Kohl rausch t has described a fold which tolerably answers to the fourth and least constant of those mentioned by Mr. Houston. He states it to be always present, as a transverse projection from the posterior wall of the rectum opposite the middle of the coccyx. In general it con tains no muscular fibres : but rarely these may even encircle the bowel, as a continuous ring or third sphincter, which forms the loss er boundary of a dilated and sacciform segment of the rectum.