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much in size and in the appearance presented by their lining mucous membrane that they can be distinguished from one another without difficulty.

Both the jejunum and ileum are irregularly disposed in the form of crowded loops or coils (Fig. 913) which are connected to the posterior abdominal wall by a great fan-shaped fold of peritoneum, containing their vessels and nerves, and known as the mesentery. Hence the name of intestinum tenue mesenteriale is applied to them. The mesentery is of such a length that the coils are able to move about freely in the abdominal cavity, and consequently the position occupied by any portion of the tube, with the exception of the beginning of the jejunum and the ending of the ileum, can never be stated with certainty. Nevertheless, it may be said that, in general, the jejunum occupies the superior and left portions of the cavity below the stomach, the ileum the inferior and right divisions, its termina! part almost always lying in the pelvis, just before it joins the large gut.

According to Mall, the most usual arrangement is to find the proximal coils of the jejunum on the left side, and high up. Then the tube crosses the vertebral column below the duodenum, and a few coils are placed on the right side. It then crosses to the left side again, and severi coils are formed, some of which may descend into the pelvis. Thence it passes again to the

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FIG. 928.-A PORTION OF SMALL INTESTINE, WITH MESENTERY AND VESSELS. The peritoneal coat has been removed from the right half, and the two layers of the muscular coat exposed.

right side, where it is coiled up, and then finally descends into the pelvis. The terminal portion almost always lies in the pelvis, just before it ascends to join the large intestine.

As the coats of the large and small intestine agree in many particulars, it will be convenient to describe the general structure of the intestines here. Subsequently, any peculiarities of structure in particular regions will be described with the corresponding division of the tube.

STRUCTURE OF THE INTESTINES.

The wall of the intestines, like that of the stomach, is made up of four coats, which are named from without inwards-tunica serosa, tunica muscularis, tela submucosa, and tunica mucosa (Figs. 928 and 929).

1. Tunica Serosa. The serous coat is formed of peritoneum, and confers on the intestines their smooth and glossy appearance. It varies in the extent to which it clothes the different divisions of the tube, giving the duodenum, the ascending, descending, and iliac colons, and the rectum only a partial covering; whilst it clothes the jejunum and ileum, the cæcum, the transverse and the pelvic colons completely. The detailed arrangement of this coat will be given with the description of each division of the intestinal tube.

2. Tunica Muscularis.-This consists of unstriped muscle arranged in two layers

-an outer stratum longitudinale, in which the fibres run longitudinally, and an inner stratum circulare, in which they are circularly disposed. The muscular coat is thicker in the duodenum than in any other part of the small intestine, and it gradually diminishes in thickness until the end of the ileum is reached. On the other hand, in the large intestine, it is thickest in the rectum and thinner towards the beginning of the colon.

The stratum longitudinale of the muscular coat is much thinner than the underlying stratum circulare. In the small intestine it forms a complete sheet, continuous all round the gut (Fig. 928), but thickest at its free margin; whilst in the large intestine it is divided up into three longitudinal bands known as the tonic coli, which will be more fully described in connexion with the colon.

The stratum circulare, much thicker than the longitudinal layer, is composed of bundles of muscular fibres arranged circularly round the tube (Fig. 929), and forming in all parts a continuous sheet. Unlike the longitudinal fibres, those of the circular layer take part in the formation of the valves of the pylorus and colon.

3. Tela Submucosa.-The submucous coat is a loose but strong layer of areolar tissue connecting the muscular and mucous coats, on which chiefly depends the strength of the intestinal wall. In addition to forming a bed in which the vessels break up before entering the mucous coat, it contains the glandulae duodenales (Brunneri) (Fig. 929); and, in both small and large intestines, the bases of the solitary lymph nodules lie in it (Fig. 929).

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4. Tunica Mucosa.-The mucous membrane constitutes the inner coat of the intestine. It is everywhere composed (Fig. 929) of the following parts:-(1) A layer of striated, columnar, epithelial cells, resting on (2) a basement membrane. Outside this lies (3) a layer of retiform tissue, containing a considerable number of scattered lymph cells. This layer is limited towards the tela submucosa by (4) an extremely thin sheet of unstriped muscle, the lamina muscularis mucosa, which is not visible to the naked eye. The mucous membrane is very vascular, particularly in the small intestine. It is thicker in the duodenum than in the jejunum, and thicker in the jejunum than in the ileum. Throughout both the small and large intestines the substance of the mucous membrane is closely set with innumerable (small microscopic) tubular glands, known as the glandulæ intestinales [Lieberkühni] (O.T. glands or follicles). In shape they are minute straight tubes, like diminutive testtubes. Their mouths open on the free surface of the mucous membrane: their closed ends lie in the deeper part of the mucous coat, and their cavities are lined with columnar epithelium. They open on the surface between the bases of the villi of the small intestine, and in the large gut their orifices are found all over the surface of the nonvillous mucous membrane.

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SMALL INTESTINE

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FIG. 929.-DIAGRAM to show the structure of the large intestine, the duodenum, and the jejunum.

Certain special developments of the mucous coat, found in particular regions of the intestinal tube, must next be considered: these are the (1) villi intestinales; (2) plicæ circulares [Kerkringi] (O.T. valvulæ conniventes); (3) noduli lymphatici solitarii (O.T. solitary glands); and (4) noduli lymphatici aggregati [Peyeri] (O.T. Peyer's patches).

Villi Intestinales. If the mucous membrane of any part of the small in

testine is examined, it is seen to present a soft, velvety, or fleecy appearance
(Fig. 930, B); this is due to the presence of an enormous number of minute
processes, known as villi, which cover its surface.

They are minute cylindrical or finger-like projections of the tunica mucosa

(Fig. 929) about 3th orth of an inch (1-2 to 1.6mm.) in height, and barely visible to the naked eye,hout which are closely set all over the surface of the lining membrane of the small intestine. Begin-ported by a ning at the edgeded by a L of the pyloric the lymp valve, they arection of 1 broad but short are usual in the duodenum,de end and grow narpiercing rower as they are ow end pr followed down of the mu through the intestine to the A, as seen in a portion of jejunum which has been filled with alcohol and hardened; valvula coli, at B, a portion of fresh intestine spread out under water. the edge of which they cease. They are found, not only on the general surface of the mucous membrane, but also upon the plicæ circulares, and, while they are not present over the solitary lymph nodules, they are found in the intervals between the individual nodules of the aggregated nodules.

B

FIG. 930.-PLICE CIRCULARES (natural size).

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They play an important part in the absorption of the products of digestions memb
which takes place in the small intestine.

Plica Circulares [Kerkringi].-When the intestine is empty and contracted.
its mucous membrane is thrown into effaceable folds or ruga, which disappearte
on distension. But in addition to these, there are found in certain portions of
the small intestine a series of large, permanent transverse folds, which are not
effaceable; these are known as plicæ circulares (Fig. 930). These are usually more
or less crescentic in shape, and resemble a series of closely placed shelves running
transversely around the gut. They rarely form more than two-thirds of a circle;
sometimes, however, they present a circular or even a spiral arrangement, the spiral
extending little more than once round the tube, as a rule. Occasionally they
bifurcate at one or both ends; sometimes, too, short irregularly directed branches
pass off from them. They are usually about 2 to 3 inches (5 to 7.5 cm.) in length,
and their breadth, that is their projection into the cavity, may be as much as 3rd
of an inch (8 mm.), whilst in thickness, as seen when cut across, they measure about
4th inch (3 mm.).

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They are composed of two layers of the tunica mucosa, with a prolongation from the tela submucosa between, to bind the two together. They are covered with villi, and are permeated by intestinal glands. Their use is to increase the amount of surface available for secretion and absorption.

Plicæ circulares are not found in the upper part of the duodenum. They begin at a distance varying from 1 to 2 inches (2.5 to 5 cm.) from the pylorus. At first they are small, irregular, and scattered; but they are larger lower down, and at the opening of the bile duct (4 inches from the pylorus) they are distinct and prominent. In the rest of the duodenum, and in the superior half of the jejunum, they are highly developed, being large, broad, and closely set. In the inferior half of the jejunum they become gradually smaller and fewer. Passing down into the ileum, they

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become still smaller and more irregular, and, as a rule, they practically cease a little below the middle of the ileum.

Often patches of plicæ circulares, much reduced in size, can be traced to within a short distance of the valvula coli. According to Sappey, Luschka, and others, they usually reach to within two or three feet of the end of the ileum.

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Noduli Lymphatici Solitarii.-The solitary lymph nodules are minute masses of lymph tissue, opaque and of a whitish colour, found Aggregated lymph projecting on the surface of the mucous membrane Two solitary glands nodule throughout the whole length of both the small and large intestines.

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Isolated lymph cells are found in abundance scattered through the connective-tissue layer of the intestinal mucous membrane generally; in places these cells are gathered together to form little nodules, supported by a framework of retiform tissue, and surrounded by a lymph space which communicates below with the lymph vessels of the tela submucosa. Such a collection of lymph cells constitutes a solitary nodule. They are usually of a rounded or oval shape (Fig. 931), the wide end resting in the tela submucosa, the nodule itself piercing the lamina muscularis mucosa, and the narrow end projecting slightly above the general surface of the mucous membrane. In size they vary from

th to th of an inch (6 to 30 mm.), but their average bulk is about that of a small grain of sago, to which they bear some resemblance.

As already mentioned, they are present throughout the small and large intestines, being particularly abundant in the vermiform process and the cæcum. In the small intestine they are found on the plica circulares, as well as upon the general surface of the Solitary gland mucous membrane between them.

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Intermediate form

FIG. 931. NODULI LYMPHATICI AGGREGATI and SOLITARII, from intestine of child two years old (natural size).

few small patches made up of two or three lymph nodules; they are marked "intermediate form."

Noduli Lymphatici Aggregati.-These lymph nodules (O.T. Peyer's patches) consist of a large number of minute lymph nodules grouped closely together so as Near the lower border are seen a to form a slightly elevated area, usually of an oblong form, on the surface of the mucous membrane (Fig. 931). In length they vary from half an inch (12 mm.), or less, to three or four inches (100 mm.), and in width they commonly measure from a third to half an inch (8 to 12 mm.). Their number is variable, but in the average condition about 30 or 40 are found. They are best marked in young subjects, where they form considerable elevations above the general surface, and may be as many as 45 in number. After middle life they atrophy, and in old age, although usually present, they are indistinet, occasionally being marked by little more than a dark discoloration of the mucous membrane. They are invariably situated along the surface of the intestine opposite the line of mesenteric attachment, with their long axis corresponding to that of the bowel. Consequently, in order to display them, the tube must be slit up along its attached or mesenteric border.

These aggregated nodules are entirely confined to the small intestine, being largest and most numerous in the ileum, particularly in its inferior part, where they usually assume an oblong shape; in the inferior half of the jejunum they are small, circular, and few in number; in its superior part they are rare; and, although their presence has been noted in the inferior portion of the duodenum, they may be said to be absent, as a general rule, from this division of the intestine.

The plica circulares stop at the margins of the aggregated nodules, and are not continued across them; but villi are found on the surface of the aggregated nodules, in the intervals between the individual lymph nodules.

The chief bowel lesion in typhoid fever is found in these aggregated and in the solitary nodules.

When the surface of one of these nodules from a child's intestine (in which these structure are particularly well developed) is carefully examined, it is seen to be made up, not of a seri of separate, rounded nodules grouped together, but rather of a number of wavy, irregular, al branching ridges connected with one another by cross branches (Fig. 981), the whole recalling in miniature the appearance of a raised map of a very mountainous district in which the chief chains run irregular courses, and are joined to one another by connecting ridges.

Small patches, intermediate in form between solitary and aggregated nodules, and consisting of two or three lymph nodules, are also usually present.

DUODENUM.

The duodenum, the portion of the digestive tube which immediately succeeds the stomach, is the first part of the small intestine, and differs from the rest of that tube in having no mesentery, and hence it is closely fixed to the posterior abdominal wall. The ducts of the liver and pancreas open into it, and some special glands are found in its wall, known as the duodenal glands of Brunner.

Shape and Divisions.-The duodenum begins at the pylorus, about the level of the first lumbar vertebra, and ends at the left side of the first or second lumbar vertebra (Fig. 932). Between those two points it pursues an irregular course which has some resemblance to the outline of a horse-shoe. It is made up o: three main parts, namely:-(1) The pars superior, which begins at the pylorus. passes posteriorly and to the right beneath the liver, and ends at the neck of the gall-bladder by turning down, forming the flexura superior, and joining (2) the pars descendens. This begins at the neck of the gall-bladder, runs down on the posterior abdominal wall, on the right of the vertebral column, behind the transverse colon (Fig. 932), and ends opposite the third or fourth lumbar vertebra There it turns to the left, and passes into (3) the pars inferior. This portion at first runs more or less transversely to the left, across the vena cava, aorta, and vertebral column (pars horizontalis), and then ascends as far as the inferior surface of the pancreas (pars ascendens). There, at the level of the first or second lumbar vertebra, it bends abruptly forwards, forming the duodeno-jejunal flexure (Fig. 932), and passes into the jejunum. The junction of the pars descendens and pars inferior constitutes the flexura inferior.

Taking the whole of the duodenum together, it forms an irregular horseshoeshaped curve, with the opening directed upwards and to the left, and the ends reaching to within about two inches of one another. Within the concavity of the curve the head of the pancreas is placed.

The incomplete ring which the duodenum makes does not all lie in the same plane; for, whilst its greater part is placed in a frontal plane, the superior part, and the commencement and termination of the inferior part, lie more in a sagittal plane (Fig. 932).

Position and Size.-As a rule, a little more than half of the duodenum lies in the epigastrium; the remainder-namely, about the inferior third of the descending portion and the adjoining two-thirds of the inferior portion-are placed in the umbilical region. With the exception of. the terminal ascending portion of the third part, the whole of the duodenum lies to the right of the median plane.

Its length is usually about 11 inches (27-5 cm.), its first portion being the shortest and its third portion the longest. Its diameter varies considerably, and may be stated to average about 11 inches when empty, but it may be as much as two inches when distended.

Relations. Pars Superior.-The superior part (O.T. first portion) begins at the pylorus, opposite the first lumbar vertebra. From there it runs to the right, and then posteriorly, beneath the liver, when the stomach is empty, but directly backwards when it is full; and ends at the neck of the gall-bladder by turning downwards and passing into the descending part. Its length varies from about 1 to 2 inches (3-7 to 50 cm.), and is said to be greater when the stomach is empty than when distended.

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