The large intestine is an approximately cylindrical tube from 120 to 150 cm. in length and of variable width. It is composed of two main portions: the ccecum, with the vermiform appendix, and the colon. These two portions, exactly alike and not sharply demarcated, are arranged in a large horseshoe loop about the small intestine, the large intestine becoming continuous with the rectum on the left.
The general characteristics of the large intestine are as follows: It is the widest segment of the gut (in the restricted sense of the latter word); its caliber is greatest at the caecum and diminishes toward the rectum; its wall when viewed from without is not smooth, like that of the small intestine, but usually exhibits sacculations (haustra) due to constrictions produced by three longitudinal muscular bands, known as the bands of the colon, which extend throughout the entire length of the large intestine (with the exception of the rectum). They begin in the caecum at the base of the appendix and extend along the colon at about equal distances from each other, as smooth glistening bands about 8 mm. wide, which do not follow the configuration of the sacculations. The band situated at the place corresponding to the mesenteric insertion is termed the mesocolic band, the opposite onethe free band, and the third the omental band, since it corresponds to the site of the adhesion of the great omentum to the transverse colon. The sacculations disappear after the removal or complete relaxation of the bands.
The outer surface of the large intestine is further characterized by subserous accumulations of fat which hang down from the region of the omental and free bands as pedunculated irregular lobulated appendages enveloped by serous membrane. They are known as the epiploic appendages and are subject to manifold individual variations in number, size, and shape.
The interior of the large intestine exhibits transverse semicircular folds, the semilunar jolds, which correspond to the constrictions between the sacculations. As a rule, their length corresponds to the distance between two bands, but they may be somewhat longer. In contrast to the circular folds of the small intestine, these folds include the entire thickness of the intestinal wall, since the constrictions contain circular muscular fibers, and since the sacculations gradually disappear during relaxation of the bands the semilunar folds may become correspondingly indistinct, portions of the colon thus occasionally coming to resemble the small intestine in appearance.
The mucous membrane of the large intestine is smooth and has no villi. The small circular orifices of the intestinal glands, which are especially large, increase in size toward the rectum, and are not visible without the aid of a lens. Solitary lymph follicles are also present in moderate numbers.
The muscular coat of the large intestine consists of an outer longitudinal layer, which is markedly thickened at the bands and but feebly developed between them, and of a continuous circular layer.
The caecum is that portion of the large intestine which is situated below the orifice of the ileum. It is about 7 cm. long, and equally as wide (6 to 8 cm.), consequently presenting an approximately spherical shape. It is the widest portion of the entire large intestine and upon its inner wall presents a valve at the orifice of the ileum, the valvula coli, ileocecal valve (valve of Bauhin, of Tulp.), formed by two folds of mucous membrane which are termed the upper and the lower lip of the valve. They are formed not only of the mucous membrane but also of the two muscular layers of the ileum, and are so placed that they project into the caecum. Their surfaces, which are directed toward the large intestine, have the character of the mucous membrane of that portion of the intestine, while those which are turned toward the lumen of the small intestine are covered with villi to the edge of the valvular orifice. When open the lips of the valve are separated by an elongated slit pointed at either end, and when in contact they form a tolerably complete closure between the large and small intestine. From the lips of the valve semicircular folds radiate to the inner surface of the anterior and posterior walls of the caecum, resembling the semicircular folds, except that they are longer. They are termed the frenula (anterior and posterior) of the valve and form the boundary between the caecum and the colon.
The caecum lies in the right iliac fossa with its lower extremity at the level of the center of the inguinal (Poupart's) ligament; when distended it is in contact with the anterior abdominal wall.
It usually possesses only a short mesentery known as the mesocaecum, and is consequently almost completely enveloped by peritoneum and is somewhat more movable than the adjacent ascending colon. It may, however, like the ascending colon, be broadly attached to the posterior abdominal wall. The omental band is situated upon the right side of the caecum, and all three bands converge toward the base of the appendix.
The processus vermijormis or appendix of the human subject, a small, slender portion of the large intestine, is a blind rudimentary structure of very variable development. Its length varies between 3 and 20 cm. (although these extremes may in rare cases be exceeded), the average being about 9 cm. When the caecum is empty its slightly conical apex is continuous with the appendix; but when it is distended, as in its normal condition, the appendix arises from the inner or posterior wall, and is usually curved in a most variable manner, being sometimes slightly and sometimes markedly convoluted or partly coiled up, sometimes hanging down into the pelvis or at other times situated in front of the caecum. At its origin from the caecum it is funnel-shaped, and its orifice is guarded by a sickle-shaped fold of variable development which is directed downward and to the right and is termed the valve of the vermiform process (appendix).
The lumen of the appendix is normally very small ; the weak muscular wall is rich in lymphoid nodules and poor in intestinal glands. Indeed, the lymphoid tissue of the appendix is so dense that it seems to form a circular Peyer's patch, and the mucous membrane in consequence is relatively thick.
The colon is the longest portion of the large intestine and is composed of four parts: the ascending colon, the immediate upward continuation of the caecum; the transverse colon; the descending colon; and the transition to the rectum or the sigmoid colon (s. romanum, sigmoid flexure). At the junction of the ascending and transverse colons the gut is bent at a right angle, the right (hepatic) flexure of the colon, and at the junction of the transverse and descending colon there is an acute left (splenic) flexure.
The ascending colon passes almost vertically upward, in front of the right quadratus lumborum and in immediate contact with the right kidney, to the lower surface of the right lobe of the liver, where it produces the colic impression. It lies chiefly in the right lumbar, lateral abdominal, and hypochondriac regions, is usually distended in the cadaver, and has very well-marked sacculations. When markedly distended , only a portion of the ascending colon is in contact with the anterior abdominal wall. Its free band is on its anterior surface, its omental band on its lateral, and its mesocolic band upon its medial surface. Only the anterior and lateral walls of the ascending colon possess a peritoneal investment.
The transverse colon has a relatively long mesentery and is freely movable. It passes almost transversely across the upper part of the umbilical region from the right to the left hypochondriac region, immediately behind the anterior abdominal wall, from which it is separated only by the adherent great omentum. Since the left flexure is higher than the right, the transverse colon ascends slightly from right to left, and at the same time it describes an arch, the convexity of which is anterior, since the transverse mesocolon is longest in the middle and shorter at the ends toward the flexures.
The upper border of the transverse colon is in contact with the liver, the gall-bladder, the greater curvature of the stomach, and the spleen. The coils of the small intestine are below it, while behind it are situated the duodenum (inferior and descending portion) and part of the pancreas. It also is usually distended in the cadaver and frequently so markedly so that it overlaps the stomach and displaces the large omentum. Its free band is on its inferior surface, its omental band on its upper anterior, and its mesocolic band on its upper posterior surface.
Epiploic appendages are usually found only along the free band and its sacculations are very distinct, although occasionally not so well developed as in the ascending colon.
The descending colon commences with the splenic flexure at the lower pole of the spleen and in front of the left kidney. It is situated in the left hypochondriac, lumbar, and lateral abdominal regions in front of the left quadratus lumborum and at the outer border of the left kidney; it has no mesentery and is in immediate contact with the posterior abdominal wall. Unlike the ascending colon the descending colon as well as the splenic flexure is usually empty in the cadaver. Above it is in relation with the tail of the pancreas, and below, in the left iliac fossa, it passes in front of the external iliac vessels to become directly continuous with the sigmoid colon. In the cadaver it is usually separated from the anterior abdominal wall by coils of the small intestine. The arrangement of its bands is precisely like that in the ascending colon, its caliber is considerably smaller, and its sacculations are not so pronounced and may be even almost entirely absent for a certain distance.
The sigmoid colon is the immediate continuation of the descending colon, from which it is distinguished by the possession of a broad mesentery, the sigmoid mesocolon, and by its consequent mobility. In other respects it is exactly like the descending colon except that the sacculations continue to become more scanty and the bands become broader as the rectum is approached. It usually consists of two parallel limbs which are slightly curved and pass transversely in front of the left psoas, and below the promontory it passes quite gradually into the rectum. Its position is variable, but is usually arranged in a loop which is directed upward, although one of the branches of the loop may lie in the true pelvis. When markedly distended a portion of varying length is frequently in contact with the anterior abdominal wall.
From Human Anatomy (1909) by DR. Johannes Sobotta (1869-1945) Professor of Anatomy in the university of Wurzburg.