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Its general direction is downwards, but this varies at its two extremities, being downwards and backwards above, downwards and strongly forwards below.
Curvatures.-The rectum is far from straight, notwithstanding its name, for it is curved in both the antero-posterior and the transverse planes. Viewed from the side, it forms a gentle curve, with the convexity posteriorly, which extends from the beginning of the rectum to the back of the prostate, and fits into the hollow of the sacrum and coccyx (flexura sacralis). At the back of the prostate a second curve (flexura perinealis) is formed where the rectum joins the anal canal. The convexity of the perineal flexure is directed forwards,
FIG. 960.-DISTENDED RECTUM IN SITU.
From a formalin-hardened male body, age 56. The peritoneum and extra-peritoneal tissue were removed, after the pelvis had been sawn along a plane passing through the superior part of the symphysis pubis in front and the lower part of the second sacral vertebra behind. The bladder, which was empty and contracted, has also been removed, but its form is shown by a dotted line. The rectum was very much distended, and almost completely occupied the pararectal fossæ.
whilst its concavity embraces the ano-coccygeal body-the mass of muscular and connective tissue which lies between the tip of the coccyx and the anal canal.
When viewed from the front the rectum is seen to be regularly folded from side to side in a zigzag fashion, the folding being slightly marked when the rectum is empty, but becoming much more distinct with distension (Figs. 960 and 961). In other words, when viewed from this aspect it presents, in the majority of cases, three more or less distinct lateral flexures or inflexions. Of these the upper and lower have their concavities directed to the left as a rule; the third flexure, which is the best marked, lies between the other two, but on the right side. Not infrequently, however, two are found on the right and one on the left side. The flexures, which
are marked on the exterior by a crease, appear in the interior as three prominent crescentic shelves (Fig. 960), known as the plicæ transversales recti (O.T. Houston's valves), which help to support the fæcal contents when the rectum is distended.
This folding is maintained by the arrangement of the longitudinal muscular fibres, the majority of which are accumulated in the form of two wide bands, one on the front, the other on the back of the bowel. These two bands, which are continuous with, and comparable in their functions to, the tæniæ of the colon, are shorter than the other coats of the rectum; hence they give rise, as in the case of the colon, to a folding or sacculation of the tube, which can be effective only at the sides where the longitudinal fibres are fewest, for the front and back are occupied by the thickened longitudinal bands (see p. 960).
In addition to supporting the fæces, these foldings greatly increase the capacity of the rectum without unduly dilating the tube. When the rectum is empty (Fig. 961) its course is comparatively straight, its lateral flexure being but slightly marked, and its whole calibre very much reduced. In this condition it occupies only a small portion of the posterior division of the pelvic cavity near the median plane, and at each side, between it and the side wall of the pelvis, is a large fossa of the peritoneum (the pararectal fossa, p. 959), which, when the bowel is empty, contains a mass of small intestine or pelvic colon (Figs. 959 and 961). When the rectum is distended the lateral flexures become much more marked, and the gut, projecting alternately to each side, passes out beneath the peritoneum, obliterating the pararectal fossa (Fig. 960), and fills the greater part of the posterior division of the pelvis-a condition which could not be brought about with a straight rectum without an enormous increase in all the diameters of the tube.
According to Jonnesco, the rectum begins that is, the pelvic mesocolon ceases-most frequently opposite the fibro-cartilage between the third and fourth sacral vertebræ. It is our experience that the mesocolon ends more frequently above than below the third sacral vertebra—often, indeed, at the level of the second (Birmingham).
At its superior end the rectum, following the curve of the sacrum, slopes downwards and at the same time slightly backwards; its middle portion is practically vertical, but the terminal third or more is directed downwards and forwards at an angle varying from 45° to 60° with the horizontal. The pelvic floor, upon which this latter part rests, forms here a similar angle with the horizontal The bend which the bowel makes behind the inferior end of the prostate, where the rectum passes into the anal canal, is, as pointed out above, abrupt, and usually approaches a right angle, so that the anal canal itself slopes downwards and backwards at an angle of nearly 45° with the horizontal.
Not uncommonly the abrupt curve, at the junction of the rectum with the anal canal, presents in front a knuckle-like projection (well seen on median section), immediately above the canal. It is most marked in females, and sometimes appears as if the bowel were doubled back upon itself at this point. The floor of the pouch thus formed may dip down in front, even below the level of the upper aperture of the anal canal. This condition is most common in multiparæ, and is evidently due to the relaxed condition of the pelvic structures, and the slight support afforded by the perineal body to this part of the gut in these, and the great capacity and shallowness of the pelvis in the female.
In length the rectum usually measures about 5 or 6 inches (12.5 to 150 cm.), but it may be much longer.
Its diameter is smallest above, near the junction with the pelvic colon, and is greatest below, near the anal canal, where there is a special enlargement known as the ampulla recti (rectal ampulla). When empty the rectum measures little over an inch (2.5 cm.) in diameter, but in a state of extreme distension it may be as much as 3 inches (7·5 cm.) in width.
Peritoneal Relations of the Rectum (Figs. 959, 961).-As a rule the superior two-thirds of the rectum has a partial covering of peritoneum-anteriorly and at the sides at first, lower down anteriorly only-whilst the lowest third has no peritoneal investment whatsoever. When the mesocolon ceases at the end of the pelvic colon, its two layers separate and leave the posterior aspect of the rectum destitute of peritoneum. Very soon the membrane quits its sides also, and is then found on the front only; so that the greater part of the rectum lies behind or beneath the pelvic peritoneum, as it were, and is capable of expanding and contracting without being in any way hampered by its partial peritoneal coat.
From the front of the rectum the peritoneum is carried forwards to the base of the bladder in the male, forming the floor of the excavatio recto-vesicalis (rectovesical or recto-genital pouch, Fig. 961). In the female it passes to the superior part of the posterior wall of the vagina, forming the floor of the excavatio recto-uterina [cavum Douglasi] (O.T. pouch of Douglas, Fig. 961). At each side, in both sexes, it passes from the front of the rectum on to the posterior wall of the pelvis, forming the bottom of a large fossa, seen at the sides of the rectum when that bowel
is empty, and known as the pararectal fossa. As the rectum becomes distended this fossa is encroached upon by the enlarging bowel, and soon is obliterated.
The level at which the reflection of the peritoneum takes place from the front of the rectum is of considerable practical importance in connexion with operations in this region. As a general rule that reflection, that is, the bottom of the recto-vesical pouch, is placed at a distance of 1 inch (2-5 cm.) above the base of the prostate, or about 3 inches above the anus, but the level is subject to considerable variation, being as a rule relatively much higher in well-developed muscular or fatty subjects, whilst in emaciated bodies, owing to the thinness of the structures forming the pelvic floor, it is usually lower.
The bottom of the recto-vesical pouch may reach down in an extreme case to within an inch (2-5 cm.) of the anus, whilst it is not at all rare to find it within 2 inches (50 cm.) of that orifice; on the other hand, it may be considerably higher than normal, sometimes being placed at a dis
Ending of pelvic mesocolon
Rectum traint nd Pararectal fossa
Second sacral vertebra
Median umbilical ligament (urachus)
FIG. 961. THE PERITONEUM OF THE PELVIC CAVITY.
The pelvis of a thin male subject, aged 60, was sawn across obliquely. Owing to the absence of fat the various pelvic organs are visible through the peritoneum, though not quite so distinctly as presented here. The urinary bladder and rectum are both empty and contracted; the paravesical and pararectal fossæ, as a result, are very well marked.
tance of 4 or 4 inches (100 to 11.2 cm.) from the anus. It should also be added that the level is generally believed to be somewhat raised by distension of the rectum and bladder, and lowered when they are empty.
In the child at birth, the peritoneum extends down to the base of the prostate (Symington), and is thus lower in relation to the bladder; but this may be partly accounted for by the high position of this organ in the child.
As a rule it will be found that 2 inches (50 cm.) of the front of the rectum, exclusive of the anal canal, are entirely free from peritoneum, and it is this and the adjacent portion of the bowel which, being free from the restraining influence of the peritoneum, is most distensible, and forms the rectal ampulla. Including the anal canal, 3 inches (8.7 cm.) of the rectum, measured along the front of the tube, have no serous covering. On the other hand, the back is free from peritoneum for 5 or 6 inches (12.5 to 150 cm.)-or sometimes much more-above the anus.
It is also of interest to notice that the connexion of the peritoneum to the rectum varies in its character at different parts:-Above and in front it is closely adherent, and can be removed only with the greatest difficulty; at the sides and inferiorly the connexion is much looser. As a result, the peritoneum can be stripped off the rectum in its inferior third or half without much difficulty, whilst in its superior portion this is not the case-an arrangement which admits of the free expansion of the rectal ampulla.
General Relations of the Rectum (Figs. 959 and 960).-Posteriorly, the rectur rests on the front of the sacrum and coccyx, and below them upon the posterior part of the pelvic floor-formed by the meeting of the two levatores ani in the anococcygeal raphe. When much distended it also comes into relation, on each side with the lower part of the piriformis and the sacral plexus, but is separated from them by a very considerable amount of connective tissue, arranged (apparently in several layers) around the tube. In this tissue the two chief branches of the superior hæmorrhoidal vessels lie behind the superior part of the bowel, but lower down they are placed in relation to its sides.
At its sides above are the pararectal fossæ and their contents (pelvic colon, or ileum); below the pararectal fossæ the rectum is in contact with the coccygei and levatores ani muscles, which run backwards to the coccyx on each side of the bowel. The branches of the superior hæmorrhoidal vessels are also found running down on its muscular coat, as far as the middle of the rectum, where they pierce the wall of the bowel.
Anteriorly, in the male the rectum is separated from the bladder, to within an inch of the prostate, by the recto-vesical pouch of peritoneum, which usually contains some coils of small intestine. Below the reflection of the peritoneum the front of the bowel is in contact with the posterior aspect of the bladder, the deferent ducts, vesiculæ seminales, and the posterior aspect of the prostate gland (Fig. 960), from all of which it is separated by the recto-vesical layer of the pelvic fascia
The lower portions of the rectum and bladder in the male are separated by the recto-vesical fascia only, over a narrow triangular area which measures about an inch (2.5 cm.) in vertical height. The base of the triangle corresponds to the reflection of the peritoneum from one organ to the other, and the apex to the union of the sides formed by the deferent ducts, which lie very close to one another except above, near the base of the triangle, where they diverge rather abruptly (Fig. 960). Through the triangle the operation of tapping the bladder from the rectum used to be performed.
The vesiculæ seminales, unless when of a small size, slope laterally and posteriorly round the front and sides of the distended rectum (Fig. 960), which they thus embrace. as it were, within their grasp.
The ureters, as they run medially towards the base of the bladder, lie close in front of the deferent ducts, and are not far separated from the distended rectum (see Fig. 960).
In the female the rectum is separated from the posterior surface of the uterus and the upper end of the vagina by the recto-uterine pouch and the intestine which it usually contains. Below the peritoneal reflection it is in direct contact with the posterior vaginal wall, to which it is connected loosely above, but more closely below.
The portion of the rectum below the level of the peritoneal reflection is surrounded by the rectal fascia, a layer of connective tissue which is derived from the visceral layer of the pelvic fascia.
In the child the rectum, or at least its superior part, is relatively larger, and it pursues a much straighter course than in the adult. As pointed out above, its peritoneal covering likewise descends lower at birth, and reaches as far as the base of the prostate.
PARS ANALIS RECTI-ANAL CANAL.
Pars Analis Recti-In order to reach the exterior, it is necessary for the lower end of the bowel to pierce the floor of the pelvis. This it does by passing through the narrow interval left between the medial borders of the levatores ani muscles (Fig. 963). As it passes between them, the two muscles pinch in the tube, and by the apposition of its side walls obliterate its cavity, reducing it to a mere slit-like passage. This passage, through which the rectum communicates with the exterior, is the "anal canal" (Symington).
The term pars analis recti refers, strictly, to the lower half only of this canal which is lined by squamous epithelium, and shows the columnæ rectales.
Formerly this terminal portion of the tube was described as the "third part of the rectum," and, like the rest of that bowel, it was supposed to form a reservoir for the retention of the fæces. It is probable that only when the rectum is distended is the superior part of the anal canal occupied by the wedge-shaped lower end of the contained fæcal mass.
The anal canal begins where the rectum proper terminates, namely, at the level of the levatores ani muscles, opposite the inferior part of the prostate. When the distended rectum is cut across near its inferior end, in a hardened body. and the cavity examined from the interior, a distinct projection, formed by the medial border of the levator ani (O.T. puborectalis, or sphincter recti portion), is seen on each side, indicating the superior limit of the canal. It is said that these ridges. can also be felt during life by the finger introduced into the rectum. Below, the anal canal ends at the anus, or anal orifice, by opening on the exterior.
Its length is usually from 1 to 1 inches (2-5 to 37 cm.), being greater when the bowel is empty, and less when it is distended. Its antero-posterior diameter when closed varies between and inch (12 to 19 mm.).
The direction of the anal canal, as already pointed out, is downwards and posteriorly, often forming an angle of nearly 45° with the horizontal, although it is usually somewhat nearer to the vertical.
Relations. It is surrounded by both the external and internal sphincters, and above also by the borders of the levatores ani, these muscles forming a muscular cylinder around it (Fig. 963). On each side is situated the ischiorectal fossa with its contained fat, which allows of the distension of the canal during the passage of fæces. Posteriorly is placed a mass of mixed connective and muscular tissue, known as the ano-coccygeal body (Symington), which intervenes between it and the coccyx. Finally, anteriorly, it lies close behind the bulb of the urethra and the base of the urogenital diaphragm in the male, and a sound in the urethra can be easily felt by the finger introduced into the anal canal, particularly in thin bodies. In the female it is separated from the vagina by the wedge-shaped mass of fatty and muscular tissue known as the "perineal body.'
Structure of the Rectum and Anal Canal.-The wall of the rectum is made up of four coats, viz. :-1. The outer coat, formed in part by peritoneum (already described), and, where the peritoneum is absent, of connective tissue which can be dissected off in several layers. In this connective tissue the hæmorrhoidal vessels run until they pierce the wall of the tube. In it also, at the back and sides of the rectum, are found embedded a number of rectal lymph glands.
2. The tunica muscularis, which is much thicker than in any other portion of the intestine, is composed of two stout layers of unstriped muscle-an outer longitudinal and an inner circular- like that of the intestine generally. The longitudinal fibres, although present all round, are accumulated chiefly on the front and back of the tube (see p. 1212), where they form two broad bands; at the sides they are reduced to a thin layer, the deepest fibres of which are folded in and take part in the formation of the rectal valves.
Where the rectum pierces the floor of the pelvis, the outer layer of longitudinal fibres is united to the deeper portion of the levator ani, partly by tendinous fibres and partly by an interchange of muscular fibres, between the levatores and the muscular coat of the rectum. Below, the longitudinal fibres pass between the external and internal sphincter muscles, or through the latter to join the skin around the anus.
In sagittal sections of the pelvis near the median plane there can generally be seen a distinct band of red, longitudinally arranged, muscular fibres, which descends on each side from the front of the coccyx to blend with the longitudinal fibres on the back of the rectum. This band is the rectococcygeus muscle. It is composed of striped fibres above, but becomes unstriped below.
Some unstriped muscular fibres which are found descending in the subcutaneous tissue of the lower part of the anal canal, to join the skin around the anus, have been described by Ellis as the corrugator cutis ani. According to Roux, they are some of the longitudinal fibres of the rectum which have passed through the internal sphincter to the submucous tissue, and then descended to the skin. The front of the rectum at the perineal flexure is, in the male, connected to the back of the membranous urethra by a band of muscle, termed the recto-urethralis.
The circular fibres form, along the whole length of the tube, a continuous layer, which is doubled inwards to assist in the formation of each rectal valve, and is thickened below to form the internal sphincter of the anus. The sphincter ani internus, as just pointed out, is formed by a great, and rather sudden, increase of the circular muscular fibres, which begins at the superior end of the anal canal. It surrounds the canal for about an inch (2·5 to 3·0 cm.), and terminates at its junction with the skin.