1034 THE DIGESTIVE SYSTEM. 
Descending Colon (colon descendens).—This is much narrower and less obtrusive 
than the ascending colon. It begins in the left hypochondrium at the splenic 
flexure, passes down on the left side of the abdomen, and ends in the lumbar region, 
opposite the crest of the ilium, by passing into the iliac colon. Its course is not 
quite straight, for it first curves downwards and inwards along the outer side of the 
left kidney, and then descends almost vertically to the iliac crest (Fig. 699). 
Its length is usually from 4 to 6 inches (10 to 15 em.), and its width, which is 
very much less than that of the ascending colon, about 14 inches (37 mm.). 
Relations.—The descending colon first lies in contact with the outer aspect of 
the left kidney; below this it descends, like the colon of the opposite side, in the angle 
between the psoas and quadratus lumborum muscles. Behind, it rests wpon the 
lower part of the diaphragm above, and on the quadratus Iumborum below. Jn 
front (and somewhat to the outer side also, except when the bowel is distended) 
are placed numerous coils of small intestine, which hide the colon completely from 
view, and compress it against the posterior abdominal wall in such a way that, at 
first sight, it may be overlooked in a fat body. To its inner side lies the lower 
part of the kidney above, the psoas below. 
In the great majority of bodies only the front and sides of the descending 
colon are covered by peritoneum (Fig. 709); the posterior surface, being destitute 
of a serous coat, is connected to the posterior wall of the abdomen by areolar 
tissue. In a small proportion of eases, on the other hand, the serous coat is com- 
plete, and the colon is furnished with a short mesentery. 
Up to the fourth or fifth month of foetal life the descending colon has a complete investment 
of peritoneum and a long mesentery. After the fifth month the mesentery adheres to, and soon 
blends with, the parietal peritoneum on the posterior abdominal wall, and is completely lost as a 
rule, The persistence of this mesentery, in a greater or less degree, explains the occasional 
presence of a descending mesocolon in the adult. 
Sigmoid Flexure and Rectum.—lt has been customary to divide the remaining portion 
of the large intestine into sigmoid flexure and rectum. The former was said to begin at the 
crest of the ilium, to lie in the iliac fossa, and to end at the brim of the pelvis. Or, in later 
years, the “sigmoid colon” was described as “ that part of the colon which is attached to the 
left iliac fossa, from the iliac crest to the brim of the true pelvis” (Symington). Its upper part 
was said to be covered by peritoneum on the anterior and lateral surfaces only, its lower part 
to form a large loop with a complete serous coat and a long mesentery, which hung down into 
the pelvic cavity when the bladder and rectum were empty, and passed up out of it when 
these were distended. 
The rectum was described as beginning at the brim of the pelvis, opposite the left sacro-iliae 
joint, and as ending at the anus. It was divided into three portions, of which the first extended 
from the brim of the pelvis to the middle of the third piece of the sacrum, had a complete 
covering of peritoneum, and was connected to the pelvic wall by a mesentery—the mesorectum. 
The second and third parts of the reetum we may pass over for the present, as they agree in 
general with the description of the rectum given below. 
Treves in 1885, and Jonnesco in 1889, directed attention to the fact that no such loop as the 
classical sigmoid flexure, lying in the iliae fossa, was to be found in nature; and also, that the 
separation of the first portion of the rectum from the sigmoid flexure—so-called—was both 
artificial and inaccurate. They pointed out that the “first part of the rectum” really belongs 
to the sigmoid flexure, with which it has everything in common, and that on no grounds ean it 
be properly assigned to the rectum. 
An unbiassed study of the parts concerned, particularly in bodies the viscera of which have 
been hardened an sctu, will leave little doubt on an unprejudiced mind that the old descriptions 
are not only artificial but erroneous. Consequently, the admirable account of this part of the 
intestine, given by Jonnesco, will be followed in its main features in describing the divisions of 
the bowel heretofore known as the sigmoid flexure and first part of the rectum. 
Jonnesco, recognising that this portion of the intestine lies partly in the iliac fossa and 
partly in the pelvis, very appropriately calls the former the iliac colon and the latter the pelvic 
colon. The ihac colon includes the portion of the “sigmoid flexure” which extends from the 
crest of the ilium to the inner side of the psoas muscle (that is practically the brim of the 
pelvis), and is usually destitute of a mesentery. The pelvic colon embraces the remainder of the 
“sigmoid colon” and the first part of the rectum, both of which are attached by a continuous 
mesentery, and form one large loop lying in the pelvic cavity, and ending at the level of the 
third sacral vertebra by passing into the rectum proper. 
Tliac Colon (colon iliacum).—This corresponds to the portion of the “sigmoid — 
flexure ” which les in the iliac fossa, and it has no mesentery. It is the direct 
continuation of the descending colon, with which it agrees in every detail, except 
as regards its relations. Beginning at the crest of the ilium, it passes downwards 
