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The villi are also said to be shorter and broader in the jejunum, more slender and filiform in the ileum (Rauber).
The terminal portion of the ileum, after crossing the margin of the superior aperture of the pelvis minor, runs upwards, and also slightly backwards and to the right, in close contact with the cæcum, until the ileo-cæcal orifice is reached.
Diverticulum Ilei (O.T. Meckel's Diverticulum).-This is a short finger-like protrusion which is found springing from the lower part of the ileum in a little over 2 per cent. of the bodies examined. It is usually about 2 inches long, and of the same width as the intestine from which it comes off. Most commonly it is found about 2 feet from the valvula coli, and opposite the original termination of the superior mesenteric artery. As a rule, its end is free; bat occasionally it is adherent either to the abdominal wall, the adjacent viscera, or the mesentery, and in such cases it may be the cause of strangulation of the intestine.
The diverticulum is due to the persistence of the proximal portion of the vitelline (or vitellointestinal) which connects of with the yolk sac. In shape it may be cylindrical, conical, or cord-like, and it may present secondary diverticula near its tip. It arises most frequently from the free border of the intestine, but it sometimes comes off from the side. It runs at right angles to the gut most commonly, but it may assume any direction, and it is often provided with a mesentery. In 3302 bodies specially examined with reference to its existence, it was present in 73, or 2-2 per cent., and it appeared to be more commen in the male than in the female. In 59 out of the 73 cases its position with reference to the end of the ileum was examined: its average distance from the ileo-cæcal valve was 32 inches measured along the gut, the greatest distance being 12 feet, and the smallest 6 inches. In 52 specimens the average length was 2.1 inches, the longest being 5 inches, the shortest inch. The diameter usually equals that of the intestine from which it springs; but occasionally it is cord-like, and pervious only for a short way; on the other hand, it may attain a diameter of 3 inches.
Vessels and Nerves of the Jejunum and Ileum.-The arteries for both the jejunum and ileum—the jejunal and ileal-come from the superior mesenteric, and are contained between the two layers of the mesentery. After breaking up and forming three tiers of arches, the terminal branches (Fig. 772, p. 931) reach the intestine, where they bifurcate, giving a branch to each side of the gut. These latter run transversely round the intestines, at first under the peritoneal coat; soon, however, they pierce the muscular coat and form a plexus in the submucosa, from which numerous branches pass to the mucous membrane, where some form plexuses around the intestinal glands whilst others pass to the villi. The veins are similarly disposed, and the blood from the whole of the small intestine beyond the duodenum is returned by the superior mesenteric vein, which joins with the splenic to form the portal vein.
The lymph vessels of the small intestine (known as lacteals) begin in the villi, and also as lymph sinuses surrounding the bases of the solitary nodules; a large plexus is formed in the submucosa, a second between the two layers of the muscular coat, and a third beneath the peritoneum. The vessels from all these pass up in the mesentery, being connected on the way with the numerous (from 40 to 150) mesenteric glands, and finally unite to form the truncus intestinalis, which opens into the cisterna chyli.
The nerves come from the coeliac plexus, through the superior mesenteric plexus, which accompanies the superior mesenteric artery between the layers of the mesentery, and thus reaches the intestine. Some of the fibres are derived ultimately from the right vagus. The nerve-fibres are non-medullated, and form, as in other parts of the canal, two gangliated plexuses the myenteric in the muscular coat, and the submucosal in the submucosa.
Structure. The tunica serosa is complete in all parts of the jejunum and ileum. The tunica muscularis is thicker in the jejunum, and grows gradually thinner as it is traced down along the ileum. The tela submucosa contains the bases of the solitary nodules (Fig. 929), but otherwise calls for no special remark. The tunica mucosa is thicker and redder above in the jejunum, thinner and paler in the ileum. It is covered through out by villi intestinales, which are shorter and broader in the jejunum, longer and narrower in the ileum. In its whole extent it is closely set with intestinal glands, and numerous solitary nodules are seen projecting on its surface. Aggregated lymph nodules are particularly large and numerous in the ileum; they are fewer, smaller, and usually circular in the jejunum. Finally, the mucous membrane forms plica circulares, which are much more prominent in the jejunum; they are smaller and fewer in the superior part of the ileum, and usually disappear a little below its middle.
The ileum is succeeded by the intestinum crassum (large intestine), which begins on the right side, some 2 inches below the ileo-cæcal junction, and comprises the following parts:
1. Cæcum. The cæcum is a wide, short cul-de-sac, consisting of the portion of the large bowel below the valvula coli. It lies in the right iliac region, and from its medial and posterior part a worm-shaped outgrowth, the vermiform process, is prolonged (Fig. 951).
2. Colon Ascendens.-The ascending colon ascends vertically in the right lumbar region as far as the inferior surface of the liver: here the gut bends to the left, forming the flexura coli dextra (O.T. hepatic flexure), and then passes transversely across the abdomen, towards the spleen, as the transverse colon.
3. Colon Transversum.-The transverse colon, a loop of intestine which passes across the abdominal cavity in an irregular looped manner. It ends at the inferior extremity of the spleen. There it turns downward, forming the flexura coli sinistra (O.T. splenic flexure), and passes into the descending colon.
FIG. 950.-LARGE INTESTINE.
A piece of transverse colon from a child two years old. The three chief characteristics of the large intestinesacculations, tæniæ, and appendices epiploica-are shown.
4. Colon Descendens.-The descending colon runs down on the left side, from the splenic flexure to the rectum.
It is usually divided into the following parts:
(a) Descending colon, which extends down to the crest of the ilium.
(b) The iliac colon extends from the crest of the ilium to the superior aperture
of the pelvis, where it is succeeded by the pelvic colon.
(c) The pelvic colon is a large loop of intestine which is usually found in the pelvis. The iliac and pelvic portions of the colon taken together are sometimes described as the colon sigmoideum.
5. Intestinum Rectum.-The rectum, the terminal part of the large bowel, succeeds the pelvic colon, and ends in the anal canal, which opens on the surface at the anal orifice.
In its course the large bowel is arranged in an arched manner around the small intestine, which lies within the concavity of the curve (Fig. 912).
In length, the great intestine is equal to about one-fifth of the whole intestinal canal, and usually measures between 5 and 5 feet (180 to 195 cm.). Its breadth is greatest at the cæcum, and from this-with the exception of a dilation at the rectum-it gradually decreases to the anus. At the cæcum it measures, when distended, about 3 inches (75 mm.) in diameter; beyond this it gradually diminishes, and measures only 1 inches (37 mm.) or less in the descending and iliac divisions of the colon.
The large intestine, with the exception of the rectum and vermiform process, may be easily distinguished from the regularly cylindrical small intestine by (a) the presence of three longitudinal bands-the tæniæ coli-running along its surface (Fig. 950); (b) by the fact that its walls are sacculated; and (e) by the presence of numerous little peritoneal processes, known as appendices epiploicæ, projecting from its serous coat. In addition, the larger intestine is usually wider than the small, but reliance cannot be placed on this character, for the jejunum is oftenindeed, generally-wider than the empty and contracted descending colon.
Tæniæ Coli. In the large bowel, unlike the small, the longitudinal fibres of the muscular coat do not form a complete layer, continuous all round the tube,
but, on the contrary, are broken up (Fig. 950) into three bands, known as the tæniæ coli. These bands, which are about inch (6 mm.) wide, begin at the base of the vermiform process, and run along the surface of the gut at nearly equal distances from one another until the rectum is reached. There they spread out and form a layer of longitudinal muscular fibres, which is continuous all round the tube (see p. 1229). The bands are about one-sixth shorter than the intestine to which they belong; consequently, in order to accommodate the bowel to the length of the tæniæ, the gut is tucked up, giving rise to a sacculated condition (Fig. 950) Three rows of pouches or saccules are thus produced, along the length of the tube, between the tæniæ. If the tæniæ are dissected off, the sacculations largely disappear, the intestine becomes cylindrical, and at the same time about one-sixth longer.
The appendices epiploica (Fig. 950) are little processes or pouches of peritoneum, generally more or less distended with fat, except in emaciated subjects, which project from the serous coat along the whole length of the large intestine, with the exception of the rectum proper.
When the interior of a piece of distended and dried large intestine is examined, its saccules appear as rounded pouches, haustra, separated by crescentic folds, plica semilunares coli, corresponding to the creases on the exterior separating the saccules from one another.
The position of the three taniæ on the intestines is as follows:-On the ascending, descending, and iliac colons one tænia lies on the anterior aspect of the gut, and two on the posterior aspect, namely, one to the lateral side (postero-lateral), the other to the medial side (postero-medial). It is chiefly along the first of these (the anterior) that the appendices epiploicæ are found. On the transverse colon their arrangement is different, but is rendered exactly similar by turning the great omentum, with the colon, up over the thorax. On the transverse colon in the natural position, the anterior tænia of the ascending and descending colons becomes the posterior (or postero-inferior) termed tania libera, the postero-lateral becomes anterior or tania omentalis and the postero-medial becomes superior in position and is termed tania mesocolica. The anterior and postero-lateral tæniæ of the iliac colon pass below on to the front of the pelvic colon and
In formalin-hardened bodies portions of the large intestine, but particularly of the descending and sigmoid colons, are often found fixed in what appears to be a state of contraction, when they are reduced to a diameter of about or of an inch (16 to 19 mm.). Under similar conditions parts of the small intestine are found correspondingly reduced.
The appendices epiploicæ, although generally said to be absent in the foetus, can be distinctly seen as early as the seventh month, but at this time they contain no fat.
Structure of the Large Intestine. The tunica serosa is complete on the vermiform process, cæcum, transverse colon, and pelvic colon; incomplete on the ascending, descending, and iliac divisions of the colon and on the rectum. It will be described in detail with each of these portions of the intestine.
The tunica mucosa is of a pale, or yellowish, ash colour in the colon, but becomes much redder in the rectum. Unlike that of the small intestine, its surface is smooth, owing to the absence of villi, but it is closely studded with the orifices of numerous large intestinal glands. Solitary lymph nodules are also numerous, particularly in the vermiform process (Fig. 955).
Vessels and Nerves.-The cæcum and vermiform
receive their blood from the
(ileo-colic artery; the ascending colon from the right colic artery;
and the transverse colon from
the middle colic artery, which lies in the transverse mesocolon. These are all branches of the superior mesenteric. The descending colon is supplied by the left colic, and the iliac and pelvic colons by the sigmoid arteries, branches of the inferior mesenteric. The rectum derives its blood from the three hæmorrhoidal arteries, which will be described with that division of the gut.
The veins correspond largely to the arteries, and join the inferior and superior mesenteric vessels, which send their blood into the portal vein.
The lymph vessels of the large intestine arise from plexuses in the submucous and subperitoneal coats, as in other parts of the alimentary canal.
The deeper vessels escape chiefly along the entering blood-vessels, those from the lateral aspects passing behind the intestine.
The vessels pass medially to a series of glands lying along the medial border of the intestine ("paracolic" glands (Jamieson)); thence they pass along the lines of the main arteries, passing then to glands disposed at intervals about these vessels (intermediate and main glands) The lymph vessels from the lower half of the descending colon, and from the iliac and pelvic colons, join the left lymph trunk of the lumbar glands. Those of the rectum and cæcum will be described
Nerves. The nerves come from the superior mesenteric plexus, an offshoot of the cœliac plexus, and from the inferior mesenteric, a derivative of the aortic plexus. The arrangement is similar to that of the nerves of the small intestine.
INTESTINUM CECUM AND PROCESSUS VERMIFORMIS.
Intestinum Cæcum.-After leaving the pelvic cavity, as already described, the
FIG. 951.-THE CECUM AND VERMIFORM PROCESS FROM THE FRONT.
Its asymmetrical form is due to the fact that the lateral and medial portions of the organ undergo an unequal development in the child. The medial (or medial and posterior) section lags behind, whilst the lateral (or lateral and anterior) division grows much more rapidly, and, projecting downwards, soon comes to form the inferior end or fundus of the cæcum. As a result the original extremity of the gut, with the vermiform process springing from it, is hidden away behind and to the
medial side of the fundus.
In length the distended cæcum usually measures about 2 inches (60 mm.); whilst its breadth is usually more, and averages about 3 inches (75 mm.).
Position. It is usually situated almost entirely within the right iliac region of the abdomen, immediately above the lateral half or third of the inguinal ligament; but its inferior end projects medially in front of the psoas major and reaches the hypogastrium (Fig. 951). On the other hand, it is sometimes found high up in the right lumbar region (owing to the persistence of the foetal position), or hanging over the pelvic brim and dipping into the pelvic cavity to a varying extent.
In the great majority of cases the cæcum is completely covered with peritoneum on all aspects, and lies quite free in the abdominal cavity. small proportion, namely, about 6 or 7 per cent. of bodies, the posterior surface (probably as a result of adhesions) is not completely covered, but over a greater or less portion of its extent is bound down to the posterior abdominal wall by connective tissue.
Relations. Posteriorly, the cæcum rests on the ilio-psoas muscle; generally, too, on the vermiform process and the femoral nerve. Anteriorly, it usually lies in contact with the omentum and anterior abdominal wall; but when the cæcum is empty, the small intestine intervenes. Its lateral side is placed immediately above the lateral half or third of the inguinal ligament (Fig. 951), whilst the medial side has the termination of the ileum lying in contact with it. On the medial and posterior aspect, but more on the former than the latter, the small intestine joins the cæcum. On the same aspect, and usually from 1 to 1 inches (25 to 37 mm.) lower down, the vermiform process comes off.
The interior of the cæcum corresponds in general appearance to that of the large intestine; but it presents two special features on the posterior part of its medial wall, namely, the ileo-cæcal orifice, guarded by the valvula coli (O.T. ileocæcal valve), and below that the small opening of the processus vermiformis, both of which call for further notice.
Orifice of vermiform process
As seen from the interior, in specimens which have been distended and dried (Fig. 953), the valve is made up of two crescentic segments-a superior, labium superius, in a more or less horizontal plane, forming the superior margin of the aperture; and an inferior, labium inferius, which is larger, placed in an oblique plane, and sloping upwards and inwards (i.e., towards the cavity of the cæcum). Between the two segments is situated the slit-shaped opening, which runs in an almost anteroposterior direction, with a rounded anterior and a pointed posterior extremity (Fig. 952). At each end of the orifice the two segments of the valve meet, unite, and are then prolonged around the wall of the cavity as two prominent folds-the frenula valvula coli. It is thought that when the cæcum is distended, and its circumference thereby increased, these frenula are put on the
stretch, and, pulling upon the two segments of the valve, they bring them into apposition, and effect the closure of the orifice.
The position of the valvula coli, in the average condition, may be indicated on the surface of the body by the point of intersection of the intertubercular and vertical lateral lines. A point 1 to 1 inches (2.5 to 37 cm.) lower down would correspond to the orifice of the vermiform process.
Orifice of vermiformi
FIG. 952. -THREE FORMS OF VALVULA COLI, from
bodies hardened by intra-vascular injections of
Valvula Coli (O.T. Ileo- cæcal Valve). Where the ileum enters the large intestine, the end of the small gut is, as it were, thrust through the wall of the large bowel, carrying with it certain layers of that wall, which project into the cæcum in the form of two folds, lying respectively above and below its orifice, and constituting the two segments of the valve (Fig. 952). The condition may be compared to a partial inversion or telescoping of the small into the large intestine: it must be added that the peritoneum and longitudinal muscular fibres of the bowel take no part in this infolding; on the contrary, they are stretched tightly across the crease produced on the exterior by the inversion, and thus serve to preserve the fold and the formation of the valve.
The hardening was not so complete in the case of the highest of the three valves represented. In each a bristle is passed through the orifice of the vermi
In bodies hardened in situ with formalin, the valve and orifice present an entirely different appearance (see Fig. 952, in which three different forms of hardened valves are shown), suggesting, much more closely than in the dried state, the appearance of telescoping or inversion mentioned In them also the two segments of the valve are much thicker and shorter, but they can