1226 THE DIGESTIVE SYSTEM. 



are marked on the exterior by a crease, appear in the interior as three 

 prominent crescentic shelves (Fig.. 9 60), known as the plicae transversales recti 

 (O.T. Houston's valves), which help to support the faecal contents when the rectum 

 is distended. 



This folding is maintained by the arrangement of the longitudinal muscular fibres, the 

 majority of which are accumulated in the form of two wide bands, one on the front, the other on 

 the back of the bowel. These two bands, which are continuous with, and comparable in their 

 functions to, the taeniae of the colon, are shorter than the other coats of the rectum ; hence they 

 give rise, as in the case of the colon, to a folding or sacculation of the tube, which can be effective 

 only at the sides where the longitudinal fibres are fewest, for the front and back are occupied by 

 the thickened longitudinal bands (see p. 960). 



In addition to supporting the faeces, these foldings greatly increase the capacity of the rectum 

 without unduly dilating the tube. When the rectum is empty (Fig. 961) its course is comparatively 

 straight, its lateral flexure being but slightly marked, and its whole calibre very much reduced. 

 In this condition it occupies only a small portion of the posterior division of the pelvic cavity 

 near the median plane, and at each side, between it and the side^wall of the pelvis, is a large 

 fossa of the peritoneum (the pararectal fossa, p. 959), which, when the bowel is empty, contains 

 a mass of small intestine or pelvic colon (Figs. 959 and 961). When the rectum is distended the 

 lateral flexures become much more marked, and the gut, projecting alternately to each side, passes 

 out beneath the peritoneum, obliterating the pararectal fossae (Fig. 960), and fills the greater part 

 of the posterior division of the pelvis a condition which could not be brought about with a 

 straight rectum without an enormous increase in all the diameters of the tube. 



According to Jonnesco, the rectum begins that is, the pelvic mesocolon ceases most frequently 

 opposite the fibro-cartilage between the third and fourth sacral vertebrae. It is our experience 

 that the mesocolon ends more frequently above than below the third sacral vertebra often, indeed, 

 at the level of the second (Birmingham). 



At its superior end the rectum, following the curve of the sacrum, slopes downwards and at the 

 same time slightly backwards ; its middle portion is practically vertical, but the terminal third 

 or more is directed downwards and forwards at an angle varying from 45 to 60 with the horizontal. 

 The pelvic floor, upon which this latter part rests, forms here a similar angle with the horizontal. 

 The bend which the bowel makes behind the inferior end of the prostate, where the rectum passes 

 into the anal canal, is, as pointed out above, abrupt, and usually approaches a right angle, so 

 that the anal canal itself slopes downwards and backwards at an angle of nearly 45 with the 

 horizontal. 



Not uncommonly the abrupt curve, at the junction of the rectum with the anal canal, presents 

 in front a knuckle-like projection (well seen on median section), immediately above the canal. It 

 is most marked in females, and sometimes appears as if the bowel were doubled back upon itself 

 at this point. The floor of the pouch thus formed may dip down in front, even below the level 

 of the upper aperture of the anal canal. This condition is most common in multiparae, and is 

 evidently due to the relaxed condition of the pelvic structures, and the slight support afforded 

 by the perineal body to this part of the gut in these, and the great capacity and shallowness of 

 the pelvis in the female. 



In length the rectum usually measures about 5 or 6 inches (12*5 to 15'0 cm.), but 

 it may be much longer. 



Its diameter is smallest above, near the junction with the pelvic colon, and is 

 greatest below, near the anal canal, where there is a special enlargement known as 

 the ampulla recti (rectal ampulla). When empty the rectum measures little over 

 an inch (2*5 cm.) in diameter, but in a state of extreme distension it may be as 

 much as 3 inches (7*5 cm.) in width. 



Peritoneal Relations of the Rectum (Figs. 959, 961). As a rule the superior 

 two-thirds of the rectum has a partial covering of peritoneum anteriorly and at the 

 sides at first, lower down anteriorly only whilst the lowest third has no peritoneal 

 investment whatsoever. When the mesocolon ceases at the end of the pelvic colon, 

 its two layers separate and leave the posterior aspect of the rectum destitute of 

 peritoneum. Very soon the membrane quits its sides also, and is then found on 

 the front only ; so that the greater part of the rectum lies behind or beneath the 

 pelvic peritoneum, as it were, and is capable of expanding and contracting without 

 being in any way hampered by its partial peritoneal coat. 



From the front of the rectum the peritoneum is carried forwards to the base of 

 the bladder in the male, forming the floor of the excavatio recto-vesicalis (recto- 

 vesical or recto-genital pouch, Fig. 961). In the female it passes to the superior part 

 of the posterior wall of the vagina, forming the floor of the excavatio recto-uterina 

 [cavum Douglasi] (O.T. pouch of Douglas, Fig. 961). At each side, in both sexes, 

 it passes from the front of the rectum on to the posterior wall of the pelvis, forming 

 the bottom of a large fossa, seen at the sides of the rectum when that bowel 



