The relations of each of these segments to the adjacent textures and organs may be easily deduced from their course as described above.
Thus the ascending colon lies on the right kidney and quadratus lumborum muscle, from which it is only separated by loose areolar tissue. On its left side, is the psoas muscle ; and above it, the vertical portion of the duo denum. In front, it is covered by coils of in testine ; or, if sufficiently distended to thrust these away, by the anterior wall of the belly.
The transverse colon is almost always in contact with the omentum and abdominal pa rietes, which it touches in the horizontal line that marks the mutual limit of the umbilical and epigastric regions. Above it, is the first portion of the duodenum ; with the stomach, liver, gall bladder, and spleen. Below it, are the coils of the small intestine. Behind it, lie the second and third portions of the duo denum,—the latter covering the aorta. To its posterior surface is attached the transverse meso-colon; which connects this part of the in testine with the wall of the belly, by a double fold of peritoneum, that splits to enclose the tube. The double lamina formed by the re union of these two layers of serous membrane in front of the bowel, is continuous, at the lower border of the great omentum, vvith the similar process that descends from the great curvature of the stomach.* The descending colon, like the ascending, lies on the left kidney and the left quadratus lumborum muscle, and is covered by a va riable quantity of the small intestine.
The sigmoidfiexure is much more frequently in contact with the abdominal wall than the preceding portion of the colon. And its freedom of movement, to which this contact is partly due, also allows the bowel to deviate considerably- from its ordinary curvature and position. The shape of this bend corresponds so exactly to its name, that it is scarcely necessary to bestow any further description upon it. Its curve is chiefly in the vertical plane ; though a slight lateral curvature is almost always present, and is easily exag gerated into a much more distinct bend by the mobility of this segment of the canal.
The use of the sigmoid flexure seems to be that of forming a receptacle for the fxces :— a receptacle of' which the shape and arrange ment are such as to spare the rectum and its sphincter from much of the pressure and weight against which they would otherwise constantly have to contend. When full, the convexity of its lower bend often projects below the iliac fossa, so as to descend into the pelvis. Indeed, the whole of the colon is very liable to displacernent from the various po sitions just assigned to it :— prolonged dis tention by its accumulated contents, or the mechanical force exerted by the pressure of stays externally, or of tumours internally — being all capable of altering its relations, and even confusing its different parts* with each other.
The colon retains the sacculated shape as sumed by the cmcurn. Its size undergoes a progressive though slight decrease, from its commencement in the cwcum to its termina tion in the rectum. Its peritoneal coverings reach their minimum in the ascending and descending portions ; where they only cover about two-thirds of the moderately distended bowel, and leave its posterior or attached third quite unoccupied by this mernbrane, and con nected by loose areolar tissue to the subjacent parts. Hence it is these portions of the bowel which are selected in the operation for artificial anus. But, just as great distention can always increase this uncovered portion, so, vice versa, excessive contraction may reduce it to a mere line, or may even develope a kind of short meso-colon in connection with either of these parts. The muscular strata which cause its sacculated shape, also retain the arrangement existing in the ccecum. But on the transverse colon, the internal longitudinal band becomes inferior. And on the sigmoid flexure, this and the posterior band generally merge into a single one. The latter change is accompanied by an indistinctness of the transverse sacculi themselves.