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which lies between the right and left flexures. It begins at the end of the right flexure, at the point where the colon passes forwards from the anterior surface of the kidney, and, turning to the left, crosses the descending duodenum (Fig. 957 It runs at first transversely to the left, and for the first few inches is comparatively fixed, being united to the front of the descending part of the duodenum and the head of the pancreas either by a very short mesentery or by areolar tissue. Immediately to the left of the head of the pancreas a long mesentery is developed, which allows the colon to hang down in front of the small intestine, at a considerable distance from the posterior abdominal wall. The portion of the colon so suspended is therefore very movable, and consequently its position is very variable. and is influenced by posture and by the condition of the other viscera. Towards its left extremity the mesentery shortens again, thus bringing the gut towards the tail of the pancreas (Fig. 957), along which it runs upwards into the left hypochondrium, under cover of the stomach, as far as the inferior end of the spleen. where it passes into the left (O.T. splenic) flexure (Fig. 942). Its two ends lie in the right and left hypochondriac regions respectively, whilst its middle portion hangs down into the umbilical, or even the hypogastric region.
Its average length is about 19 or 20 inches (47.5 to 500 cm.), that is, more than twice the distance, in a direct line, between its two extremities. This great length is accounted for by the curved and somewhat irregular course which the bowel pursues.
Relations. The greater part of the transverse colon lies behind the greater omentum, which must consequently be turned upwards in order to expose Above, it is in contact, from right to left (Fig. 957), with the liver and gall-bladder (which also descend in front of the colon), the stomach, and, near its left end, with the tail of the pancreas and inferior end of the spleen (Fig. 947). Anteriorly are placed the omentum and the anterior abdominal wall; towards its termination the stomach also is anterior. Posteriorly, it first lies in contact with the descending duodenum and head of the pancreas; further to the left, where it hangs down, the small intestine is placed below and posteriorly, and it is connected to the posterior abdominal wall (more correctly, to the anterior border of the pancreas) by the transverse mesocolon. It is also loosely connected to the stomach by the gastro-colic ligament which is attached to its anterior surface. The transverse mesocolon and the gastro-colic ligament are described with the peritoneum, p. 1242.
The transverse colon is completely covered with peritoneum, with the exception of the first few inches of its posterior surface, which are often, if not usually uncovered.
The state of the peritoneal covering on the posterior surface of the first part of the transvers colon would seem to depend, in some degree, on the extent to which the liver passes downwards on the right side. With a small, high liver no mesentery is present, and the posterior surface is devoid of peritoneum; on the other hand, when the liver is enlarged in the vertical direction, it pushes the colon downwards before it, and brings the upper line of the peritoneal reflection from its back, into contact with the lower, thus giving rise to the mesentery. In the foetus of three or four months every part of the colon is supplied with a long mesentery; subsequently this, as a rule, disappears at the beginning of the transverse colon, but it may be reproduced in the manner stated.
Flexura Coli Sinistra (Left Flexure of the Colon (O.T. Splenic FlexureThe terminal portion of the transverse colon runs upwards (also posteriorly and to the left) until the inferior end or base of the spleen is reached; here it bends sharply, forming the left flexure, and runs down into the descending colon. The flexure is placed deeply in the left hypochondrium, posterior to the stomach, and in contact with the base of the spleen. It lies at a higher level than the right colic flexure, and is connected to the abdominal parietes by the phrenico-col ligament, which helps to maintain it in this position.
Ligamentum Phrenicocolicum (Fig. 947). This is a triangular fold of peritoneum with a free anterior border, which is attached medially to the left flexure and laterally to the diaphragm opposite the ninth to the eleventh rib. (Owing to the fact that the base of the spleen rests upon it, the ligament has also received the older name of sustentaculum lienis.)
The phrenico-colic ligament is formed in the foetus from the left margin of the greater omentum (Jonnesco).
The peritoneal covering of the left colic flexure is similar to that of the descending colon.
Colon Descendens.-The descending colon is much narrower and less obtrusive than the ascending colon: indeed in a large number of cases it is found firmly contracted. It begins in the left hypochondrium at the left 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 medially along the lateral border of the left kidney, and then descends almost vertically to the iliac crest (Fig. 957).
Its length is usually from 4 to 6 inches (10 to 15 cm.), and its width, which is less than that of the ascending colon, about 1 inches (37 mm.).
FIG. 957. THE ABDOMINAL VISCERA AFTER THE REMOVAL OF THE JEJUNUM AND ILEUM (from a photograph of the same body as depicted in Fig. 942). The transverse colon is much more regular than usual.
Relations. The descending colon first lies in contact with the lateral border of the left kidney; below that it is placed, like the colon of the opposite side, in the angle between the psoas and quadratus lumborum muscles. Posteriorly, it rests upon the lower part of the diaphragm above, and on the quadratus lumborum below. Anteriorly (and somewhat laterally 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. To its medial side lies the inferior part of the kidney above, the psoas major below.
In the great majority of bodies only the front and sides of the descending colon are covered with peritoneum (Fig. 968); 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 cases, on the other hand, the serous coat is complete, 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 a a rule. The persistence of this mesentery, in a greater or less degree, explains the occasiona presence of a descending mesocolon in the adult.
Iliac Colon. This corresponds to the portion of the "sigmoid flexure" which lies 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 and somewhat medially, lying in front of the iliacus muscle. A little way above the inguinal ligament it turns medially over the psoas major, and ends at the medial border of this muscle by dipping into the pelvis and becoming the pelvic colon (Fig. 958). It usually measures about 5 or 6 inches (12.5 to 15 cm.) in length, but it varies considerably in this respect.
Relations. Posteriorly, it lies upon, and, as a rule, is connected by areolar tissue to, the front of the ilio-psoas muscle. It also crosses the left ureter, the left internal spermatic vessels, and the femoral nerve. Anteriorly, it is usually covered by coils of small intestine, which hide it from view; but when distended, or when it occupies a lower position than usual, it comes into direct contact with the anterior abdominal wall. As a rule (90 per cent. of bodies-Jonnesco), it is covered with peritoneum only on its sides and anterior surface. Occasionally (10 per cent. of cases) it is completely covered, has a short mesentery (1 inch, 2 to 3 cm.), and is slightly movable.
In its course it passes down over the iliac fossa near its middle, generally forming a curve with its concavity directed medially and upwards, and having reached a point 1 or 2 inches (4 to 5 cm.) above the inguinal ligament, it turns medially across the psoas major towards the pelvic cavity. Occasionally the iliac colon occupies a lower position than this, and runs along the deep surface of the inguinal ligament, immediately behind the anterior abdominal wall.
Pelvic Colon. The pelvic colon is a large coil of intestine, which begins at the medial border of the left psoas major muscle, where it is continuous with the iliac
colon, 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 variously 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 be 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; in 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 mesorectum, 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 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 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 tæniæ of the descending colon are followed down, it will be found that the posterolateral band gradually passes on to the front, and unites with the anterior tania to form a broad band, which occupies nearly the whole width of this bowel in its lower portion. The posteromedial tænia 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
a series of small sacculations. The sacculations disappear, and the longitudinal fibres, although thicker in front and behind, form a continuous layer all round, as the rectum proper approached.
Intestinum Rectum. The rectum is the portion of the large bowel which intervenes between the pelvic colon above and the anal canal-the slit-like passage through which it communicates with the exterior (Fig. 961).
Unlike 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.
The rectum begins at the termination of the pelvic mesocolon, namely, about the
The sacrum has been sawn across through the 4th sacral vertebra, and its inferior part removed with the cocey1. The posterior portions of the coccygei, levatores ani, and of the external sphincter have been cut away. The "pinching in" of the lower end of the rectum by the medial edges of the levatores ani, resulting in the formation of the flattened anal canal, is suggested in the illustration, which has been made from formalin-hardened male body, aged 30. The lateral inflexions of the rectum, corresponding to the plica transversales recti, are also shown.
level of the third sacral vertebra, and ends, where the bowel pierces the pelvic floor, opposite the inferior and posterior part of the prostate in the male, or at a point 11⁄2 inches (3.7 cm.) in front of, but at a more inferior level than, the tip of the coccyx in both sexes. It first descends along the front of the sacrum and coccyx, following the curve of these bones; beyond the coccyx, it rests, for about 11 inches (37 cm.), on the posterior part of the pelvic floor, there formed by the union of the two levatores ani; and finally, having reached the inferior part of the prostate. it bends rather abruptly backwards and downwards, pierces the pelvic floor. and passes into the anal canal (Fig. 959).