Zz _ THE RECTUM. . 103! 
In this condition it occupies only a small portion of the posterior division of the pelvic cavity 
near the mesial plane, and at each side, between it and the lateral wall of the pelvis, is a large 
fossa of the peritoneum (the pararectal fossa, p. 1051), which, when the bowel is empty, contains 
a mass of small intestine or pelvic colon (Figs. 701 and 703). 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 fossee (Fig. 702), 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—imost frequently 
opposite the disc 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. 
At its upper 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 lower 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 reetum with the anal canal, presents 
in front a knuckle-lke projection (well seen on mesial 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 (Fig. 744). This condition is most common in multi- 
pare, 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 em.), 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 rectal ampulla (ampulla recti). When empty the rectum measures little over 
an inch (2°5 em.) in diameter, but in a state of extreme distension it may be as 
much as 3 inches (7°5 em.) in width. 
Peritoneal Relations of the Rectum (Figs. 705 and 749).—As a rule the upper 
two-thirds of the rectum has a partial covering of peritoneum—in front and at 
the sides at first, later on in front only—whilst the lower third has no peritoneal 
investment whatsoever. When the mesocolon ceases at the end of the pelvic 
colon, its two layers separate and leave the back 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 les behind or beneath the 
pelvic peritoneum, as 1t 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 recto-vesical or recto-genital pouch 
(excavatio recto-vesicalis, Fig. 749). In the female it passes to the upper part 
of the posterior wall of the vagina, forming the floor of the pouch of Douglas 
(excavatio recto-uterina Douglasi, Fig. 744). Whilst at each side, im 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 
is empty, and known as the pararectal fossa. With distension this fossa is 
encroached upon by the enlarging bowel, and soon is obliterated. 
The level at which the reflection of the peritoneum takes place from the front of 
the rectum is of considerable practical importance in connexion with operations in 
this region. Asa general rule this reflection, that is the bottom of the recto-vesical 
pouch, is placed at a distance of 1 inch (2:5 cin.) above the base of the prostate, or 
about 3 inches above the anus, but the level is subject to considerable variation, 
being as a rule relatively much higher in well-developed muscular or fatty subjects, 
whilst it is usually lower in emaciated bodies, owing to the thinness of the structures 
forming the pelvic floor. 
The bottom of the recto-vesical fossa may reach down in an extreme case to within an inch 
(2°5 cm.) of the anus, whilst it is not at all rare to find it within 2 inches (5:0 cm.) of that orifice ; 
