Posteriorly a quantity of loose cellular tissue connects the lower part of the rectum to the sacrum and coccyx; it is there related, parti cularly when distended, to the pyriformis and ischio-coccygeus muscles, and towards its anal extremity to some of the fibres of the levatores ani and the ano-coccygeal ligament.
On either side the rectum gives insertion to a portion of the recto-vesical layer of the pelvic fascia, which, though weak and cellular in that locality, nevertheless admits of being fairly traced to the walls of the gut; but the levatores ani muscles constitute the principal lateral rela tions of the intestine. In their descent they cover its surface extensively, and form in great measure the partition between the bowel and the ischio-rectal fossm.
The perineal portion of the rectum affords in some respects a striking contrast to the upper part of the same intestine; being totally devoid of serous investment, it is more fixed and (except at the anus) more dilatable than the superior division of the bowel, and its con nexions with the recto-vesical layer of the pelvic fascia, the ano-coccygeal ligament, the genito urinary passages, and the middle tendinous point of the perineum, contribute to fix it still more firmly in its position.
The coats of the rectum present certain pecu liarities interesting to the surgical anatomist. Its muscular tunic is of uncommon strength, and consists of two very distinct layers ana logous in many particulars to those of the corresponding strata in the cesophagus ; the superficial layer is formed of highly developed longitudinal fibres, florid in colour (as con trasted with those of the remainder of the large intestine), and which spread out so as to invest the whole circumference of the gut : the fibres of the deeper layer are circular, and acquire increased developement towards the anal extre rnity of the intestine, where they are continuous with the internal sphincter. The mucous mem brane is remarkable for its thickness and vascu larity and for the great laxity of its connexion with the other tissues of the gut: it adheres so loosely to the subjacent coat in the vicinity of the anus that it sometimes protrudes through that opening, and in this manner one form of prolapsus ani is produced. •
Upon the free surface of the mucous mem brane a number of longitudinal folds run down to the immediate neighbourhood of the anus ; they are called the columns of the rectum, and converge slightly as they descend; their number is variable though it seldom exceeds eight or ten, and between them inferiorly some trans verse semilunar folds may be observed, of which the free concave margins are directed upwards. In these folds of the mucous membrane the physiologist recognises a provision to facilitate the distension of the gut, and to their presence some surgeons attribute the occurrence of cer tain morbid conditions of the intestine. In addition to these folds, which are constant, others have likewise been described within the rectum ; these latter were named by the late Dr. Houston " the valves of the rectum," and appear at times remarkably distinct. When present they are each of a semilunar shape, and formed by a duplicature of mucous mem brane containing cellular tissue and a few muscular fibres between its folds. Each valve is attached by its convex margin to the walls of the gut, whilst its free edge is directed more or less inwards towards the cavity of the intestine. One of these valves is situated (according to Houston's statement) opposite to the base of the bladder, on the anterior wall of the gut and about three inches distant from the anus, whilst another is sometimes placed within one inch of the anal orifice.
That projections from the parietes of the rectum, such as have been described by IIouston, may be made apparent by a certain mode of preparation cannot be denied, but that they can or do exert much influence in supporting the weight of the column of faxes within the intes tine, or in obstructing the progress of instru ments through it, may be very fairly questioned ; for when the rectum has been removed from the dead subject and laid open, these valves are in general no longer visible, and the natural curvatures of the bowel explain sufficiently the difficulties encountered in the introduction of rectal tubes or bougies in the living.