The Colon, commencing at the ileo-caecal valve, and terminating at the junction with the rectum, opposite the front of the body of the third sacral vertebra, is fully 3 feet long, and has a diameter ranging from 2 inches at the caecum to 1 inches at the sigmoid flexure, its general shape being that of a capital M. The ascending and descending colon may, or may not, have a mesocolon, the proportions being about equal, while a mesocolon is rather more common on the left than on the right. As a mesocolon renders the operation of lumbar colotomy difficult, this would be one reason for avoiding that operation.

The ascending colon, about 8 inches long, ascends in front of the fascia iliaca, the fascia over the quadratus lumborum, and the lower and outer portion of the right kidney to the under surface of the right lobe of the liver, where is situated the hepatic flexure, at which the ascending communicates with the transverse colon. As it ascends it curves slightly with the concavity to the left, and while it is quite superficial near its commencement, the hepatic flexure is deeply placed, and is therefore not easily palpable. Where the caecum has not descended, the ascending colon is absent.

The hepatic flexure is generally an acute bend, situated between the lateral abdominal wall externally and the descending duodenum internally, which rests on the kidney posteriorly, and impresses the liver above. It may be supported by the hepato-colic ligament, a peritoneal band, which is occasionally given off to it from the right extremity of the gastro-hepatic omentum.

Commencing at the hepatic flexure, which, as stated, is deeply placed, the transverse colon, which is generally about 20 inches long, runs forward and to the left, thus once more becoming superficial and therefore palpable. While the general direction of the transverse colon is upwards towards the splenic flexure, which is placed on a higher level than the hepatic, its shape varies considerably, its right side portion being comparatively fixed while the left portion is largely influenced by the movements of the stomach. Normally, the umbilicus should indicate the lower border of the transverse colon, which, however, sometimes descends much below this, and even below the pubis, where it may contract adhesions to the pelvic organs (ptosis). It is not infrequently found in umbilical hernice, and in hernise through the foramen of Winslow, and has been present in a left inguinal hernia, while its presenting through incisions made for removal of the appendix or left inguinal colotomy has led to considerable confusion. At the hepatic end it is in relation to the liver and gall-bladder, and hepatic abscesses have discharged into the transverse colon, and fistulae have occurred between the gall-bladder and transverse colon from large gall-stones ulcerating through. Posteriorly, also, at this part it is related to the descending duodenum, to the front of which, as well as the head of the pancreas, it is generally fairly fixed either by short mesentery or areolar tissue. Beyond this point the transverse mesocolon develops, and then the bowel becomes free, until close to the splenic flexure. Owing to its comparative freedom of movement, the transverse colon may occasionally, and particularly when distended with gas, lie in front of both stomach and liver, thus tending to obliterate the area of liver dulness.

At the splenic flexure the bowel runs upwards and backwards once again, becoming deeply placed, until the base of the spleen is reached, when it bends sharply downwards into the descending colon. At this point the colon lies deeply behind the stomach and under the spleen, being kept in position by the phreno-colic ligament which runs to it from the diaphragm. Dragging upon this ligament is supposed to be the cause of pain referred to the left scapular region in some cases of chronic constipation. The diaphragm is affected, and so impressions are conveyed to the cervical cord by the left phrenic nerve, whence they are referred to the distribution of the descending supra-acromial nerves.

The transverse mesocolon conveys the blood-supply to the transverse colon, and also forms a diaphragm at the level of the descending duodenum and pancreas, which extends laterally to the kidneys and anteriorly to the transverse colon, dividing the abdominal cavity into two compartments, the lower of which contains the whole of the small intestine.

The descending colon, some 9 or 10 inches long, extends from the splenic flexure to the inner border of the psoas muscle at the brim of the pelvis. Curving at first downwards and inwards along the outer border of the kidney, it runs vertically to the iliac crest (this portion being about 4 to 5 inches long : descending colon proper), and then passes downwards and inwards in front of the iliacus muscle, crosses the psoas a little above the level of Poupart's ligament, and terminates at the pelvic brim in the pelvic colon (this iliac colon portion is about 5 inches long). The descending colon, like the iliac, is generally devoid of mesentery, being only covered by peritoneum on the front and sides. Both portions are generally separated by loops of small bowel from the anterior abdominal wall, and they are narrower than the ascending colon.

The pelvic colon, which is continuous above with the iliac colon and below with the rectum, constitutes the sigmoid flexure proper. It commences at the inner border of the left psoas by crossing the external iliac vessels and dipping over the pelvic brim, and then runs in the pelvis from left to right, resting on the bladder or uterus. It then turns backwards along the light posterior wall of the pelvis till it reaches the middle line, where, at the level of the third piece of the sacrum, it becomes continuous with the rectum. It thus forms a loop which is supplied with a complete mesentery, which permits of considerable movement, its position varying with the amount of distension of the bladder, etc. This portion varies in length from foot to nearly 3 feet, the average being about 17 inches When very short, its course is more simple, while,-when long, it may describe an S. When of normal length and shape, the two ends of the sigmoid are some 3 inches apart. Sometimes, however, they may, from natural or pathological causes, become approximated, and then the condition known as volvulus, in which the two ends of the loop become twisted upon one another, is apt to occur. Here the bowel becomes enormously distended, while its vascular supply is cut off, and gangrene may supervene. Faecal accumulations, carcinoma recti, etc., may also cause distension, the loop resembling a greatly dilated stomach. In some cases of great distension it may rise quite out of the pelvis, and even cause palpitation and dyspnoea by pressing upon the diaphragm. The mesentery of the loop is fan-shaped, its attachment, having the shape of an inverted V, running up the inner border of the left psoas to the bifurcation of the common iliac, and then turning sharply downwards to descend over the sacral promontory and front of the sacrum to the third piece, where it ends. At the apex of the inverted angle there is a small 1 intersigmoid fossa,' which lies under the mesentery, admits the tip of the little finger, and has rarely been the seat of strangulated hernia. On account of its freedom of movement, this portion of bowel is generally selected in the operation for inguinal colotomy. (This operation has almost entirely displaced the older operation of lumbar colotomy, in which the bowel is opened from behind through an incision parallel to the last rib, and passing through a point midway between the centre of the crest of the ilium and the tip of the last rib. The structures cut through are the latissimus dorsi and external oblique, internal oblique, transversalis muscle, fascia lumborum, and transversalis fascia, and the bowel is exposed between the psoas and quad-ratus lumborum.)

In inguinal colotomy an incision about 2 inches long is made at right angles to a line joining the anterior superior spine and umbilicus, and 1 inches from the anterior superior spine. The two obliques and transversalis are split, the peritoneum opened sufficiently to admit the finger, and the sigmoid is drawn up and secured. Generally a glass rod is passed through its mesentery, partly to keep it up, and partly to form a spur, which will tend to prevent faecal matter passing beyond that point. It is generally wise to stitch the serous coat of bowel, peritoneum, and skin together to prevent leakage either into the peritoneal cavity or the muscular layers, when, subsequently, the bowel is opened, forming an artificial anus. In picking up a portion of bowel for this operation, it should be remembered that large intestine is distinguished from small intestine not merely by its greater size (which is variable) and thicker walls, but also by the longitudinal bands and the appendices epiploicae. Sometimes it is necessary, from the extent of disease of the bowel on the left side, to do a colotomy on the right side. The operation is more difficult, the caecum not coming so readily into the wound, and as the faecal material is much more fluid and irritating at this point, the patient has difficulty in keeping the parts clean and the skin free from irritation. In connection with the statement made by physiologists that the large intestine only absorbs water and some salts, it is important to note that where the colon is opened on the right side the patient generally emaciates rapidly, whereas, when opened on the left, the patient may rapidly regain an excellent bodily condition.

Dysenteric ulcers occur generally in the large intestine, particularly toward the rectum and anus, and frequently give rise to stricture.