. 



COLON. 1223 



Ion, and ends at the level of the third sacral vertebra by passing into the rectum. 

 Between those two points it has a well-developed mesentery, and forms a large and 

 iously shaped coil, which usually lies in the cavity of the pelvis (93 per cent.). 

 Whilst the loop of the pelvic colon is very irregular in form, the following may 

 given as perhaps its most common arrangement. Beginning at the medial margin 

 of the left psoas major, it first plunges over the brim into the pelvis minor, and 

 crosses that cavity from left to right ; it next bends backwards and then returns 

 along the posterior wall of the pelvis towards the median plane, where it turns 

 down and passes into the rectum (Figs. 957 and 958). 



Relations. In its passage into the pelvis it crosses the external iliac vessels ; 



running from left to right across the cavity, it rests on the bladder or uterus, 

 according to the sex ; whilst the coils of the small intestine lie above it. 



It is completely covered by peritoneum, and is furnished with an extensive 

 mesentery the pelvic mesocolon which permits of considerable movement. 



In cases where the pelvic colon is unusually long (Fig. 957), in returning from the right 

 side of the pelvis it crosses the median plane, going even as far as the left wall, and then turns 

 back a second time towards the middle of the sacrum, where it joins the rectum at the usual 

 level, thus making an S-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 dependent chiefly 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 distended, it is 

 unable to find accommodation in the pelvis minor, 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 cm.), 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. 957 

 and 958). Its root is attached along an inverted V-shaped line, one limb of which runs up close 

 to the medial border of the left psoas major, 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 mesentery is doubled up on itself, the side which was naturally 

 posterior becoming anterior. 



Recessus Intersigmoideus. When the pelvic colon with its mesentery is raised upwards, 

 a small orifice will usually be found beneath the mesentery, corresponding to the apex of 

 the V - shaped attachment 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 intersigmoid fossa, and is due to the imperfect blending of the 

 mesentery of the descending colon of the foetus with the parietal peritoneum. The ureter is 

 found lying behind the apex of this fossa. In the foetus this mesenterv 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 arius, it is 

 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 tseniae of the descending colon are followed down, it will be found that the postero- 

 lateral band gradually passes on to the front, and unites with the anterior taenia to form a broad 

 band, which occupies nearly the whole width of this bowel in its lower portion. The postero- 

 medial tsenia spreads out in a similar manner on the back ; so that in the inferior half of the 

 pelvic colon the longitudinal layer of the muscular coat is complete, with the exception of a 

 narrow part on each side ; there the circular fibres come to the surface, and the intestine presents 



