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OF the ANO-RECTAL REGION by A Postgrad Med J: first published as 10.1136/pgmj.34.393.360 on 1 July 1958. Downloaded from 360 MODERN CONCEPTS OF THE ANATOMY OF THE ANO-RECTAL REGION By A. G. PARKS, M.D., M.Ch., F.R.C.S. Senior Surgical Registrar, Guy's Hospital Introduction downwards and intra-abdominal pressure con- It is remarkable that the anatomy of the anal stantly exerts a force tending to drive the pelvic canal should be still the subject of discussion. organs out through it. The muscles of the pelvic There are several reasons for this. The region is floor are arranged to prevent herniation of inaccessible in the cadaver because of the sur- abdominal contents, but at the same time they must rounding bony structure. Distinction between allow the egress of the alimentary and genito- muscle fibres and connective tissue is often difficult urinary tracts. Most of the hiatus is closed by the in fixed specimens and it is easy to create artefacts two fan-shaped levator ani muscles; the only weak by blunt dissection. Many of the complicated point is in the midline, where the viscera pass arrangements described in anatomical and surgical through to the exterior. The muscles adjacent to journals may well be due to excessive zeal in this the viscera are well developed in order to protect respect. this source of weakness and are composed of The academic anatomists of the I9th century striated voluntary muscle of somatic origin. By gave excellent but rather complicated descriptions; their action voluntary control of the outflow of the in recent years much has been written by surgeons visceral tract is established. copyright. specializing in ano-rectal disease. The articles of The anal canal is the termination of the ali- Milligan and Morgan (I934), Milligan, Morgan, mentary viscus and possesses its own intrinsic Jones and Officer (1937) and more recently Morgan muscle layers of smooth, involuntary muscle fibres. and Thompson (1956) are of special importance, The inner circular layer is well developed to form as they summarize the views of surgical anatomists. the internal sphincter ani and is ensheathed by the There have been several excellent American con- longitudinal muscle coat. tributions to this subject, those of Courtney (I950), There is thus a division of the fairly sharp regionhttp://pmj.bmj.com/ Uhlenhuth (I953) and Gorsch (I955) being into visceral and somatic components which will especially noteworthy. form a useful basis for classification in the Three methods are available for the study of anal anatomy: dissection of the cadaver, dissection ensuing description (Fig. i). during pelvic and perineal operations, and histo- logical examination. Each has limitations and a Embryology true picture can be obtained only by integrating The lower rectum and upper half of the anal the obtained from all three canal are formed from the cloaca the knowledge techniques. primitive by on September 25, 2021 by guest. Protected It is still not possible to give a final account growth of a longitudinal septum which separates because of the complex nature of the region. the genito-urinary tract in front from the alimentary I have investigated the structure of the pelvic tract behind. The muscle surrounding the cloaca floor, using the thick celloidin section technique is modified in the course of development into the previously described (Parks, I956), and have also complex perineal musculature of the adult. It studied the anatomy during the perineal dissection seems likely that the most superficial part dif- for excision of the rectum. A detailed account of ferentiates to form the subcutaneous and super- this work will be published in the near future. The ficial parts of the external sphincter ani, the bulbo- following description, which is simplified for the cavernosus, ischiocavernosus and superficial trans- sake of clarity, is an attempt to synthesize the verse perinei muscles. The sphincter urethrae and opinions of previous authors with my own deep external sphincter muscles develop from the observations. deep part, most of which, however, remains as a sling from the pubic arch, passing around the anal General Description canal and genito-urinary tracts, which it still treats The pelvic outlet is directed almost vertically as one. This becomes the pubo-rectalis muscle of July 1958 PARKS: Modern Concepts of the Anatomy of the Ano-Rectal Region 361 Postgrad Med J: first published as 10.1136/pgmj.34.393.360 on 1 July 1958. Downloaded from r UPPER LAMELLA OF ---- 'LEVATOR ANI a) c) - -COCCYGEUS 1 ILI . RECTUM.'--- RECTU " ANAL CANAL ANA ANAPUBO-COCCYGEUS -LONGITUDNAL 1 MUSCLE L OF ANAL . -CANAL b) --PUBO-RECTALS-IN.- SPHINCTER J ~- -GENITO-URINARY TRACT --EXT. SPHINCTER ANI---- FIG. I.-To illustrate the muscular arrangements of the pelvic floor and anal canal. (a) Shows the visceral component with attached upper lamella of the levator ani; (b) demonstrates the somatic component. Fusion of the two produces the final form as seen in (c). The genito-urinary tract is represented by the simple tube labelled ' urethra.' adult anatomy, the most important element of the aspects of the longitudinal muscle coat of the pelvic floor. rectum. The largest components are inserted close The proctodaeum and external sphincter ani are to the midline; laterally the lamella becomes formed in response to the presence of the develop- tenuous. A few fibres cross in front of the rectum ing rectum; if the latter is absent, the external to join those of the opposite side (the decussating sphincter does not appear. The deep cloacal fibres of Lushka). Thejunction of the upper leaf of sphincter then encircles the vagina in the female the levator ani with the longitudinal muscle forms copyright. and passes as a sling under the urethra in the male. what has been called the conjoined longitudinal These aberrations of development are important muscle of the anal canal (Morgan and Thompson, for the surgeon when he is attempting to create a I956). The levator prevents prolapse of the new anal opening in cases of imperforate anus. anterior wall of the upper half of the anal canal The deep part of the cloacal sphincter is usually and may also act as a sphincter of the urethra n the the only muscle capable of controlling the new male and of the vagina in the female. During the hence the rectum must be down of resection of the anus; brought operation abdomino-perineal http://pmj.bmj.com/ through its arc. This entails dissecting between rectum it is the highest structure which must be the muscle and the vagina in the female and divided anteriorly to free the rectum and allow between the muscle and the urethra in the male. dissection in the plane of Denonvillier's fascia. Attempts to bring the rectum through the levator It seems likely that muscles given such titles as ani behind the pubic muscle sling merely result in a levator prostatae ' and ' deep transverse perinei' perineal colostomy. by various authors are, in fact, portions of the upper lamella of the levator. The structure desig- THE VOLUNTARY SPHINCTERS nated the' membranous diaphragm ' by anatomists on September 25, 2021 by guest. Protected The Somatic Component is probably the connective tissue covering the lower This group of muscles encircles the viscera as surface of this muscle. At least part of the external they pass through the pelvic hiatus; they form a sphincter urethrae in the male is formed by fibres continuous layer (Goligher et al., I956), but may passing under the membranous urethra. The com- be divided into three parts for convenience of plex arrangements of the upper layer of the levator description. and its associated fascial connections require further elucidation and a fresh nomenclature. i. The Levator Ani (b) The lower leaf. The muscle takes origin The twin levatores ani muscles compose most of from a wide arc on the pelvic wall from the pubis the pelvic diaphragm. Innervation is derived from to the spine of the ischium and is inserted into the branches of the lower sacral nerves (S4 and 5). coccyx. It is a continuous sheet, but for con- Each muscle has an upper and lower lamella. venience may be divided into two parts, the pubo- (a) The upper leaf. Striated muscle fibres arising coccygeus and ileo-coccygeus. The pelvic floor from the pubis are inserted into the antero-lateral is completed by the coccygeus muscle, which is 36a POSTGRADUATE MEDICAL JOURNAL July 1958Postgrad Med J: first published as 10.1136/pgmj.34.393.360 on 1 July 1958. Downloaded from continuous with the levator ani. These muscles stitute the subcutaneous part of the external counteract the force exerted on the pelvic floor by sphincter; they are intersected by strands of the the pressure of the abdominal contents; the pubo- longitudinal coat of the anal canal which pass coccygeus, the part nearest the midline, is the most through to be inserted into the peri-anal skin. powerful. THE ANAL CANAL 2. The Pubo-rectalis The Visceral Component This is continuous with the pubo-coccygeus The alimentary tract terminates by passing and is, strictly speaking, the distal and most through the somatic pelvic muscles to form the powerful part of the levator ani; it is sufficiently anal canal. It retains the three layers common to different functionally, however, to justify separate all alimentary viscera, namely, the longitudinal description. It takes origin from the inferior sur- muscle, circular muscle and mucosa. The muscle face of each side of the pubic arch, close to the coats are composed of smooth, unstriped fibres midline in the male and further laterally in the innervated by the autonomic nervous system. female; passing backwards and downwards, it Their pale coloration contrasts markedly with the forms a sling behind the anal canal.
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