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3. The tela submucosa is composed of loose areolar tissue, which allows of a free movement of the mucous layer on the muscular coat, and which also admits, under certain abnormal conditions, of a prolapse of the mucous membrane through the ana orifice. The hæmorrhoidal plexus of veins is contained in this layer.
4. The tunica mucosa must be considered separately in the rectum and pars analis rect. That of the rectum is redder in colour than the mucous membrane of the colon, as a result of its greater vascularity. It is also thicker, and owing to the looseness of the underlying submucosa, is thrown into numerous irregular ruga when the rectum is empty; these disappear when the bowel is distended, and there then become evident three (sometimes more, sometimes less) crescentic folds, which are much less noticeable in the empty state. and which have been already referred to as the plicæ transversales recti. Lymph nodules and intestinal glands are present; but these latter are not so numerous as in the colon. although their calibre is greater.
The mucous membrane of the pars analis recti presents a number of vertical ridges known as the columnæ rectales (Morgagnii); between the lower ends of these are found a series of small semilunar folds which connect the lower ends of these columns together and are called the anal valves (Fig. 962). Above the level of the anal valves the canal is lined by a modified mucous membrane resembling that of the rectum; the portion below the valves (i.e. the lower 12 to 16 mm. of the canal) is covered by modified skin, continuous with that around the anus.
The term annulus hæmorrhoidalis is applied to the segment enclosed by the sphincter externus muscle. It extends from the region of the rectal sinuses to the anal orifice. and in its wall is found the large venous plexus, termed the plexus hæmorrhoidalis.
The mucous membrane of the rectum presents a characteristic punctated appearance, which is due to the presence of a considerable number of rounded depressions, such as might be made by firmly pressing a finely pointed pencil against the membrane. These rectal pits are tubula in form, and have an accumulation of lymph tissue at the bottom of each, the whole appear ance being such as might be produced if a small solitary nodule were drawn down from the surface into the intestinal wall.
Plicæ Transversales Recti (O.T. Rectal Valves, or Valves of Houston).-These are crescentic shelf-like folds which project into the cavity of the rectum from its sides (Fig. 960). They are composed of an infolding of the mucous, submucous and greater part of the circular muscular coats, and their form is preserved by the relative shortness of the anterior and posterior bands of longitudinal muscular fibres. They are produced, as pointed out above, by the projection, into the interior of the bowel, of the creases on the exterior which result from the lateral inflections of the rectum. In the majority of cases three are present (there may be four, five, or, it is said, even more), but often the lowest of the three is smal or absent; or all the valves may be ill-developed and indistinct. When median sections of the empty rectum are examined, the valves are not easily seen, as they then project but slightly, and are almost completely hidden amongst the numerous ruge of the mucous coat. They are most evident in a distended rectum which has been hardened in situ; they can also be seen during life, per anum, with the aid of a rectal speculum.
As a rule two valves are found on the left and one on the right side; this latter is generally the largest, and is situated a little above the level of the peritoneal reflection, viz., 3 or 3 inches (7.5 to 8.7 cm.) above the anus; the other two valves are found about 1 to 1 inches (2·5 to 37 cm.) higher up and lower down respectively. The valves are distinctly marked in the foetus, and seem to constitute an essential part of the huma rectum, their use being to support the contents of the rectum, which they break up inte segments, each supported by a valve. They are said to interfere sometimes with the introduction of an enema tube.
Columnæ Rectales [Morgagnii].-The mucous membrane of the pars analis recti, or lower portion of the anal canal, presents a number (5 to 10) of permane! vertical folds, separated by grooves, and known as the columnæ rectales (O.T. columns of Morgagni) (Fig. 962). They are usually to inch (8 to 12 mm.) in length, to
inch (3 to 6 mm.) in width, and they extend down to within or inch (12 to 20 mm.) of the anal aperture. They are formed by infoldings of the mucous membrane, containing in their interior some bundles of longitudinal muscle and also, as a rule, an artery and a vein.
Very often the contained vein presents an enlargement, or a knob-like tortuous plexus in the lower part of the column; below this the plexus is continued down external to the mucous membrane of the lower zone of the anal canal into the anal veins. This portion has accordingly been described hæmorrhoidal zone of the anal canal. Sometimes the columns are very indistinct; occasionally no trace of them can be found, although in the fœtus they are usually well marked.
Obt. vessels and nerve
Pelvic plexus of nerves and
Anal Valves. If a probe is passed downwards along the groove which separates two adjacent columnæ rectales (Fig. 962), its point will usually catch in a small crescentic fold which joins the lower ends of the two columns. These little folds, which resemble in miniature the segments of the semilunar valves of the heart, are the anal valves. They project inwards and upwards, and behind each is found a little pocket-like sinus (sinus rectalis).
FIG. 962.-THE INTERIOR OF THE ANAL CANAL AND LOWER PART OF RECTUM, Showing the columnæ rectales, and the anal valves between their lower ends. The columns were more numerous in this specimen than usual.
FIG. 963.-DISSECTION OF THE RECTUM FROM THE FRONT IN A SPECIMEN HARDENED BY FORMALIN
These valves were first described by Morgagni. Recently the view has been advanced by Ball that they are the remains of the embryonic cloacal or anal membrane; and he explains the production of "painful fissure of the anus" by the tearing down of one of them during defæcation by hardened masses of fæces.
The epidermis is continued in a thin and modified form from the exterior up along the anal canal as far as the superior end of the columnæ rectales; and the view is pretty generally held that
only this lower portion of the anal passage is formed from the proctodæum in the embryo. The junction of the skin with the mucous membrane is indicated by a fine wavy line ("white line" of Hilton-ano-cutaneous line of Hermann) which runs around the bowel at the level indicated. The mucous membrane of the region immediately above the anal valves is of a more or less transitional nature; glands are absent from it, and over the columnæ rectales it is covered with stratified epithelium, the superficial cells of which are flattened, whilst in the grooves between the columns the epithelium is columnar. In the upper zone of the anal canal the mucous membrane gradually approaches to the rectal type, but the intestinal glands and lymph nodules are few and scattered.
Anus or Anal Orifice. At the inferior aperture of the anal canal, the modified skin of its lower zone passes into the ordinary skin. A little way outside the orifice, hairs, sebaceous glands, and large modified sweat-glands appear, the last being termed glandulæ circumanales.
Action of the Sphincters. In connexion with the anal canal are found three musclesnamely, the paired levatores ani, the external sphincter, and the internal sphincter-the action of which may be briefly referred to here.
Levator Ani. The fibres of the levator which arise from the pubis (pubo-coccygeus or sphincter recti portion) pass backwards on each side of the beginning of the anal canal, and, in great part, meet behind the passage. These two muscular bands-which are but a little distance apart at their origin, and are actually united behind the bowel-are closely approximated during the contraction of the muscles, like the limbs of a clamp, and, pressing on the sides of the anal canal, they assist in closing the upper part of that passage, whilst at the same time drawing it slightly towards the pubes. There is little doubt that the levator ani in this way acts as one of the chief sphincters of the bowel; and it should be noticed that it is placed where its action would be most effective, namely, opposite the point at which the rectum is narrowed or "pinched in" to form the anal canal. In addition to its sphincter action the muscle supports the expanded bowel immediately above the anal canal, and in this way sustains the weight of the fæces when the rectum is distended. It is probably relaxed during defæcation, except perhaps at the completion of the act. The muscle is under the control of the will.
The m. sphincter ani externus forms a muscular cylinder around the inferior two-thirds of the anal canal, with (except in the case of some of its inner fibres) an anterior and a posterior attachment. When the muscle contracts, its fibres are tightly stretched between its two attachments, and the space between them is reduced to a narrow antero-posterior slit. By this action the anal canal is flattened from side to side and closed, so that, whilst the levator ani is the sphincter of the upper aperture of the anal canal, the external sphincter closes its inferior and greater part It is under the control of the will, but under ordinary circumstances it is in a state of tone
The m. sphincter ani internus is merely a thickening of the circular muscular coat at the inferior end of the bowel. It is continuous with the circular fibres of the gut, not only in structure, but probably also in action, its chief use being to empty the anal canal completely, after the passage of each fæcal mass. Owing to the fact that the canal is an antero-posterior slit, not a circular orifice, and that the internal sphincter forms a muscular ring around it, acting alone, it is scarcely competent to keep the sides of the canal in apposition, and probably it acts rather as a detrusor than a true sphincter of the anal passage.
Vessels. The rectum and anal canal receive their blood supply from three chief sources, namely, the three hæmorrhoidal arteries; to these another less important, though constant, source may be added-the middle sacral artery.
1. The superior hæmorrhoidal artery, the principal artery of the rectum, is the prolongation of the inferior mesenteric. At first it descends in the root of the pelvic mesocolon until the rectum is reached. Here it divides into two chief branches which run downwards and forwards around the sides of the rectum-the right, usually the larger, lying more posteriorly, the left more anteriorly, and the two, as it were, embracing the bowel between them. From these two arteries come off secondary branches (about five to eight in all), which pierce the muscular coat about the middle of the rectum, and then descend in the submucosa as a series of longitudinally running "terminal branches" as far as the anal valves, above the level of which one is usually found beneath each of the rectal columns. These terminal branches give off numerous twigs in their course, which form a hæmorrhoidal plexus in the submucosa by anastomosing with one another, and also with branches of the middle, and, in the inferior part of the bowel, of the inferior hæmorrhoidal artery.
2. The middle hæmorrhoidal arteries, two in number-one on each side-are usually branches of the hypogastric or of the internal pudendal; they run on the wall of the inferior part of the rectum, and each breaks up into four or five small branches, some of which supply the muscular wall of the inferior part of the rectum, whilst the others pierce the muscular coat near the superior end of the anal canal, and join in the submucosa with the plexus formed by the superior hæmorrhoidal artery already described.
3. The inferior hæmorrhoidal arteries, generally two or three in number on each side, arise at variable levels from the internal pudendal. They are distributed to the levatores an and the sphincters. Other branches pierce the sphincters and break up in the submucosa into a close network which supplies the inferior part of the anal canal, and communicates above with the plexus formed by the superior and middle hæmorrhoidal arteries. The inferior ha mor
rhoidal artery is distributed chiefly on the posterior, and the middle hæmorrhoidal chiefly on the anterior aspect of the lower part of the bowel.
4. One or more small branches of the middle sacral artery reach the posterior surface of the rectum, where they are distributed chiefly, if not solely, to the muscular coat.
Anastomosis of the Hæmorrhoidal Arteries. The superior and middle hæmorrhoidal arteries anastomose freely in the hæmorrhoidal plexus of the submucosa, and also by a few large branches on the exterior of the bowel: some perforating branches of the middle sacral and inferior hæmorrhoidal arteries also join the plexus in the submucous layer at the lower part of the rectum. In addition, small branches of these several arteries unite with one another in the muscular coat. It should be remarked that the superior hæmorrhoidal artery supplies both the muscular and mucous coats in the superior part of the rectum, but the muscular coats in the inferior part are supplied by the middle and inferior hæmorrhoidal vessels only.
Veins of the Rectum and Anus.-These form two chief plexuses of large vessels devoid of valves, namely, the internal hæmorrhoidal plexus situated in the submucous coat, and the external hæmorrhoidal plexus in the outer coat. The internal hæmorrhoidal plexus takes origin near the margin of the anus in a number of small (anal) veins, which are radially disposed beneath the skin of the anus, and communicate below with the rootlets of the inferior hæmorrhoidal vein over the external sphincter. These anal veins, traced upwards, join together, and are joined by others from the surrounding parts to form larger and often tortuous vessels, which ascend in the columnæ rectales, where they frequently present ampullary enlargements, varying in size up to that of a small pea, which are said to be the starting-points of hæmorrhoids. Passing upwards, the veins are known as the "terminal veins"; they communicate freely with one another, forming the plexus, and unite into still larger vessels, which pierce the muscular coat about the middle of the rectum, and join to form the superior hæmorrhoidal vein.
From the inferior part of the internal hæmorrhoidal plexus numerous vessels pass through the external sphincter to join a venous network on the outer surface of that muscle, from which the inferior hæmorrhoidal veins arise. This network, as pointed out above, also communicates with the internal hæmorrhoidal plexus, through the anal veins which descend from the latter beneath the skin of the anal canal, to the exterior of the sphincter.
The various veins which pass out through the walls of the rectum unite freely on its exterior to form a rich venous plexus (external hæmorrhoidal plexus), through which the three hæmorrhoidal vessels are brought into free communication with one another. Passing off from this plexus, the superior hæmorrhoidal joins the left colic vein and forms with it the inferior mesenteric vein, which opens into the splenic; the middle hæmorrhoidal joins the hypogastric, from which the blood passes through the common iliac to the vena cava inferior; and the inferior hæmorrhoidal joins the internal pudendal, a tributary of the hypogastric vein. Thus, on the rectum, a free anastomosis is established between the veins of the portal and systemic circulations.
Lymph Vessels.-Most of the lymph vessels of the rectum pass to the lymphoglandulæ sacrales, of which some lie close to the muscular coat on the side of the rectum along the superior hæmorrhoidal vessels, while others, four or five in number, and of a larger size, lie in front of the promontory of the sacrum, between the layers of the pelvic mesocolon. The glands of opposite sides are connected with one another by the middle sacral plexus and with the hypogastric and mesocolic lymph glands. The efferent vessels from these pass to the lumbar glands. Some of those from the lower part of the anal canal join the cutaneous lymph vessels round the anus, and pass with them to the inguinal and subinguinal glands. A few of the lymph vessels from the lower portion of the rectum are said (by Quenu) to join the hypogastric glands.
Nerves. The nerves of the rectum come partly from the sympathetic and partly from the cerebro-spinal system. The sympathetic fibres are derived from the inferior mesenteric plexus, through the superior hæmorrhoidal nerve and the corresponding plexus, and from the upper and lower divisions of the hypogastric plexus, the former accompanying the superior hæmorrhoidal, the latter the middle hæmorrhoidal vessels, to the rectum. The cerebro-spinal fibres arise from the second, third, and fourth sacral nerves soon after these leave the sacral foramina (and constitute the "pelvic splanchnics" of Gaskell). They run forward in the pelvic connective tissue, and joining the pelvic plexuses, reach the side of the rectum. Fibres of the inferior hæmorrhoidal branches of the pudendal nerve (third and fourth sacral) are also distributed to the lower part of the anal canal as well as to the external sphincter.
It has been shown by experiments on animals, that the cerebro-spinal nerves (from the second, third, and fourth sacral) convey motor impulses to the longitudinal fibres, but inhibitory impulses to the circular muscular fibres. In like manner the branches from the sympathetic convey motor fibres (derived from some of the lumbar rami communicantes) to the circular muscle, and inhibitory fibres to the longitudinal muscle of the rectum.
The reflex centre which governs the action of the sphincters and the muscular fibres of the rectum ("defæcation centre") is situated in the lumbar region of the spinal medulla, and appears to be capable of carrying out the whole act of defæcation even when separated from the brain. Variations.The best known anomalies found in connexion with the rectum are those classed under the term imperforate anus or atresia ani. The atresia may be simply due to a partial or complete persistence of the anal membrane (see p. 42), which separates the proctodæum from the hind-gut in the embryo (atresia ani simplex); or the hind-gut may be deficient in its lower part, when there is a considerable interval between the proctodæum and the gut (defectus recti partialis, vel totalis); or the rectum may open into the vagina, the uterus, the
bladder, or the ureters, when usually no anus is evident; or finally the cloaca may persist Other forms are also described, but the foregoing are those most commonly found.
For the development of the rectum and anus, see pp. 39 and 42.
An introductory sketch of the peritoneum was given on p. 1160; subsequently, when describing the abdominal viscera, an account of its detailed relations to each of them was included. We shall here consider the membrane and its folds as parts of one continuous whole; and we shall also describe its arrangement as seen on horizontal and vertical sections of the abdomen.
As already explained, the peritoneum is the serous membrane which, on the one hand, lines the abdominal cavity, and on the other forms a more or less complete covering for the contained viscera. The portion which lines the walls of the cavity is known as the parietal peritoneum; that which clothes the viscera is called the visceral peritoneum. The membrane is connected to both walls and viscera by a layer of areolar tissue-tela subserosa, the extra or subperitoneal connective tissue-which is considerable in amount in certain regions, whilst it is reduced to a mere trace in others, particularly on the viscera. (The subperitoneal tissue is described at p. 475.)
The peritoneal cavity is described as consisting of two portions-the general peritoneal or great sac and the bursa omentalis. The great sac is opened when the anterior abdominal wall is removed or incised, and the peritoneum which encloses it lines the greater portion of the wall of the abdominal cavity, and invests most of the abdominal viscera; the omental bursa lies chiefly on the posterior aspect of the stomach, and is much smaller. It must be clearly understood that these two sacs are not two separate cavities, but simply subdivisions of one great cavity; for the omental bursa is merely a recess of the greater sac, from which it has become partly shut off largely by changes that take place in the position of the adjacent viscera during their development. If the general peritoneal cavity is compared to a bag, the bursa omentalis might be represented as a pocket lying behind, and opening into it by a narrow orifice the foramen epiploicum [Winslowi].
Speaking generally, the great sac lines the walls of the abdominal cavity, and it also covers the various organs which receive a peritoneal investment, except the back of the stomach, the caudate lobe of the liver, the left supra-renal gland, the upper surface of the pancreas, and also parts of the spleen, left kidney, and transverse colon; all of these, as well as the parietes behind the caudate lobe, are clothed by the bursa omentalis.
The general peritoneal sac is placed between the parietes anteriorly and the abdominal viscera posteriorly. It is composed of two layers: an anterior, which lines the anterior abdominal wall; and a posterior, which mainly covers the viscera; but this posterior layer is carried forwards by the viscera, so that the two layers come in contact, and the cavity of the sac is practically obliterated.
The anterior layer of the peritoneum covers the anterior abdominal wall completely, from the diaphragm above to the pelvis below. Over the greater part of its extent the connexion of the serous membrane to the wall is by a small amount of fatty extra-peritoneal connective tissue; but below, near the pubic region, the fat is more abundant, and the connexion between the two becomes much looser. This is to allow of the movement of the peritoneum which takes place there during distension of the bladder. As the bladder enlarges it passes up in the extraperitoneal tissue of the lower part of the anterior abdominal wall, off which it raises the peritoneum, so that, in the fully distended condition, the anterior surface of the bladder is in contact with this wall, without the interposition of peritoneum, for a distance of two inches (50 cm., or occasionally more) above the pubes (Fig. 964).
Running up in the fatty subserous tissue are found five cord-like structures, one placed in the median plane, and two at each side. These are (a) the lig. umbili cale medium (O.T. urachus)-the remains of the allantois of the foetus-which in the adult is a slender fibrous band connected to the umbilicus above, and to the