a. 
1036 THE DIGESTIVE SYSTEM. 
back a second time towards the middle of the sacrum, where it joins the rectum at the usual 
level, thus making an §-shaped curve within the pelvis. On the other hand, when the 
loop is short (a not infrequent occurrence), all its curves are abridged, and it fails to pass 
over to the right side, but runs more or less directly backwards after entering the pelvis. 
From what has been said, it will be seen that the loop of the pelvic colon is subject. to 
numerous and considerable variations, which are chiefly dependent upon its length and that of 
its mesentery, and also upon the state of emptiness or distension of itself and of the other pelvic 
viscera. When the intestine is long the loop is more complex ; when short, more simple. When 
the bladder and rectum are distended, or when the pelvic colon itself is much distende d, it is 
unable to find accommodation in the true pelvis, and consequently it passes Up into the 
abdominal cavity, almost any part of the lower half of which it may occupy. But, as already 
stated, in the great majority of cases (92 per cent, according to Jonnesco) it is found after 
death lying entirely within the pelvic cavity. 
In length, the pelvic colon generally measures about 16 or 17 inches (40 to 
42°5 cm.), but it may be as short as 5 inches (12 em.), or as long as 35 inches 
(84 cm.) 
The pelvic mesocolon, which corresponds to both the sigmoid mesocolon and the meso- 
rectum, is a fan-shaped fold, short at each extremity, and long in its middle portion (Figs. 699 
and 700). Its root is attached along an inverted V-shaped line, one limb of which runs up close 
to the inner border of the left psoas, as high as the bifurcation of the common iliac artery (or 
often higher); here it bends at an acute ‘angle, and the second limb descends over the sacral 
promontory and along the front of the sacrum to the middle of its third piece, where the 
mesentery ceases, and the pelvic colon passes into the rectum. When the pelvic colon ascends 
into the abdominal cavity, this nesentery is doubled up on itself, the side, which was naturally 
posterior, becoming anterior. 
Intersigmoid Fossa (recessus intersigmoideus).—When the pelvic colon with its mesentery 
is raised upwards, a small orifice will usually be found beneath the mesentery, correspond- 
ing to the apex of the V-shaped attac hment of its root to the posterior abdominal wall. 
This orifice leads into a fossa which is directed upwards, and will often admit the last 
joint of the little finger. It is known as the cntersigmoid fossa, and is due to the imperfect 
blending of the mesentery of the descending colon of the fetus with the parietal peritoneum. 
In the foetus this mesentery is well developed, and extends from the region of the vertebral 
column out towards the descending colon. After a time it begins to unite with the underlying 
parietal peritoneum ; but in the “region of the intersigmoid fossa the union is rarely perfect, 
hence the presence of the fossa. 
In the child at birth only the terminal part of the pelvic colon lies in the pelvis. This 
is chiefly owing to the small size of the pelvic cavity in the infant. Beginning at the end 
of the iliac colon, the pelvic colon generally arches. upwards and to the right across the 
abdomen towards the right iliac fossa, where it forms one or two coils, and then passes down 
over the right side of the pelvic brim into the pelvic cavity. In cases of imperforate anus, it 1s 
important to remember, in connexion with the operation for forming an artificial anus, that, 
whilst the iliac colon is found in the left iliac region, the pelvic colon (sigmoid flexure ”) usually 
lies on the right side, and passes over the right portion of the brim to enter the pelvis. 
Structure of the Pelvic Colon.—Only the arrangement of the muscular coat need be referred 
to. As the teenie of the descending colon are followed down, it will be found that the postero- 
external band gradually passes on to the front, and unites with the anterior tenia to form a broad 
band, which occupies nearly the whole width of this bowel in its lower portion. The postero- 
internal tenia spreads out in a similar manner on the bac k ; so that in the lower half of the 
pelvic colon the longitudinal layer of the muscular coat is complete, with the exception of a 
harrow part on each side; here the circular fibres come to the surface, and the intestine presents 
a series of small sacculations. These, however, disappear, and the longitudinal fibres, although 
thicker in front and behind, form a continuous layer all round, as the rectum proper is 
approached. 
THE RECTUM. 
The rectum proper (intestinum rectum: second portion of the rectum of the 
usual descriptions) is the comparatively dilated portion of the large bowel which 
intervenes between the pelvic colon above and the anal canal—the sht-like passage 
through which it communicates with the exterior (Fig. 701). It forms a 
temporary reservoir, in which the feces accumulate a short time before they are 
discharged from the body, but as a result of habit this temporary function may be 
converted into a more or less permanent one. 
Unhke the portion of the bowel which immediately precedes it, the rectum has 
but a partial covering of peritoneum, and is entirely destitute of a mesentery ; 
sacculations, too, which are so characteristic of the large intestine, cannot properly 
be said to be present. 
