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Home , Anal canal, Anal columns, Anal valves, Anus, Internal anal sphincter

Annals of the Royal College of Surgeons of England (I974) vol 54

Pelvic I]I. and associated

J 0 N Lawson F-RCS Consultant Paediatric Surgeon, St Thomas's Hospital and Westminster Children's Hospital

Summary tions, the third part represented by the ter- Further observations are made on the an- minal narrow portion ending at the . This atomy of the anal canal and the arrangements description was widely accepted until i888, of the muscles acting as sphincters and slings when Symington2 referred to this third part to the canal. The mode of insertion of those as the anal canal, and I900, when Merkel' muscles of the pubovisceral group which tend called it the pars analis recti or the anal part to elevate and open the anal canal is also of the . described. In i874 Robin and Cadiat4, studying the , noted the absence of Technical note glands in the lower portion of the anal canal. This account of the anatomy of the anal canal and The junction with the upper glandular part its associated sphincters is based on a study of neo- they described as the anorectal line. Herr- natal and infant pelves. The study was carried out mann and Desfosses' in i88o noted an inter- by gross dissection and serial section of 13 pelves mediate zone and described the change from isolated by subcutaneous dissection to include the the intestinal type of . The serial sections were cut 7-I 0 gm above, with thick and stained with either haematoxylin and its columnar cells, to a stratified cuboidal eosin (HE) or phosphotungstic acid and haematoxy- cell epithelium before the appearance of the lin (PTAH) stratified squamous epithelium of the lower The paucity of fibrous tissue in the neonate and anal canal. infant makes identification of muscle groups and individual muscles easier. In this age group also the The 'surgical' anal canal' corresponds specimen size was more suitable for serial section closely to the third part of the rectum with and mounting on microscope slides. its upper limit at the and lower at the anus. Also the 'surgical' anal canal Introduction corresponds well with the upper limit of the Descriptions of the anal canal are bedevilled internal anal , both anatomically and by differences in nomenclature. In the igth when judged by rhythmical pressure changes century the alimentary tract ended in the and inhibitorv response to rectal contraction7. rectum and the rectum at the anus. Sanson' For the purpose of this discussion I will in i8I 7 subdivided the rectum into 3 por- refer to the anal canal as that part of the Pelvic anatomy '289 alimentary tract lying between the junction tate line the stratified cuboidal gives way to of the columnar and stratified cuboidal cell columnar epithelium. In the infant sections epithelium (dentate line) and the anal verge. the stratified cuboidal cell epithelium lines For that part of the rectum lying below the both sides of the anal valve and extends up pelvic floor and above the dentate line I have the lateral wall of the crypts to a variable ex- retained Merkel's3 term, pars analis recti or tent. Between the valves the stratified cuboidal anal part of the rectum. cell epithelium extends well up the anal col- unis, representing the dens of the dentate Lining of the anal canal line (Figs. i and 2). These projections rep- In our sections we have confirmed the find- resent nearly half the total depth of the ings of Herrmann and Desfosses5. The squa- stratified cuboidal cell zone. In the neonate mous epithelium in the lower anal canal sections the are prominent and the changes abruptly to stratified cuboidal cell changes from stratified squamous to stratified epithelium before an equally sudden transition cuboidal and from stratified cuboidal to to the columnar epithelium of the dentate columnar epithelium are abrupt, no islands of line. In our sections, as in the detailed study differing cells being identified except at the carried out by Walls', the hairy skin of the of anal glands, where the stratified anal verge gives way to hairless skin (Figs. cuboidal cells give way to a single layer. I and 2). This skin is very elastic and in sec- The mucosa is thrown into folds, usually tions of an anal canal where the anus is held in the region of 4-6 major folds, with occa- open during fixation this skin is thin and sional intervening minor folds (Fig. 3). Major attenuated. Above this the stratified cuboidal valves are slung between the larger folds and epithelium is identical in appearance with smaller valves between large and minor folds. that in the posterior . It is readily When followed up from the anus, the skin differentiated from the hairless skin by the folds at the anal verge continue up into the latter's distinct basal cell laycr. At the den- and from the anal columns into

g / _ _ ~~~Puboanalis

nmucosa Straified cuboidal Anal crypt cell zone tTS ~~and gland

Ccnjoined longitudinal muscle Musculus submucosae ani

FIG. I Diagrammatic representation of the mucosal relationships of the anal canal in infancy. 290 J 0 N Lawson

3 '2

*a ~ P b FIG. 2 Coronal sections (PTAH X I8) showing: (a) extensions of stratified cuboidal cell epithelium up the anal columns to form the dens of the dentate line; (b) columnar cell epithelium extending down be- tween the folds to a level just above the anal crypts. (l) Anal skin; (2) stratified cuboidal cell epithe- lium; (3) columnar cell epithelium; (4) anal crypt; (5) submucosal muscle in anal column and fold. folds in the pars analis recti; these finallv fan must contribute to complete closure of the out in the ampulla of the rectum (Fig. I). anal canal by producing a 'mucosal choke'9. These folds are constant in configuration. The is seen clearly in Above, the anal glands, extending from the the pars analis recti, but traced down to- crypts into the and internal sphinc- wards the anal canal it comes to lie more ter, next the slips of the puboanalis, and below superficially and disappears completely at the this the attachment of the lower part of the junction of the columnar cell and the strati- inserting into the fied cuboidal cell epithelium; it is thus seen mucocutaneous junction, tether the mucosa to be quite distinct from the submucosal between the folds. On the other hand the muscle (Fig. 5). folds are rendered more prominent, particu- larly during contraction of the internal sphinc- Internal anal sphincter ter, by bundles of the submucosal muscle In sagittal or coronal section the internal passing down from their origin from the sphincter can readily be identified by the upper internal anal sphincter to their inser- characteristic arrangement of the muscle tion into the perianal skin. This arrangement bundles in cross-section'0 (Figs. 2, 4, 5, 7, Pelvic anatomy 291

FIG. 3 Transverse sections of anal canal (PTAH) (cut slightly obliquely) showing: (a) (X 12) hairy skin (i) anteriorly and non-hairy skin (2) posteriorly; (b) (X i6) stratified squamous epithelium (2) extending up the anal columns ('anal papillae'), with stratified cuboidal epithelium (3) in the folds; (c) (X i6) stratified cuboidal epithelium (3) extending up the anal columns at the den- tate line, with cuboidal cell epithelium ('rectal mucosa') in the folds (W). 292 J 0 N Lawson

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*, ..1 *....

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FIG. 4 Coronal sections of anal canal (Schofield stain X 7) to show internal anal sphincter and its re- lationships to covering mucosa and voluntary sphincters: (a) anterior section, through anal fold; (b) pos- terior section, between anal fold. (Ix) Longitudina! muscle of rectum; (2) puboanalis insertion; (3) 'puborectalis' (puboanal sphincteric sling); (4) deep anal sphincter; (5) superficirzl anal sphincter; (6) hairy skin; (7) most medial band of superficial sphincter; (8) non-hairy anal skin; (9) stratified cuboidal cell epithelium to include anal valve and crypt (9') covering fold containing musculus sub- mucosae ani; (io) conjoined longitudinal ; (Io') main contribution from puboanalis; (ii) columnar epithelium; (12) internal sphincter. and 9). It can readily be distinguished from floor, where the narrows from the the circular muscle of the rectum above (Fig. ampulla, circular fibres of the upper internal 5). The rectal smooth muscle, in cross-section, sphincter are arranged in oval bundles which is aggregated into quadrilateral bundles which in this part lies obliquely with their axes run- receive only small contributions from the ning medially and downwards"0. Thus the longitudinal coat. At the level of the pelvic junction between the internal sphincter and Pelvic anatomy 293

FIG. 5 Close-up of junction between rectal circular muscle and circular muscle in upper part of internal anal sphincter (sagittal section stained PTAH). (i) rectal circular muscle; (2) internal anal sphincter; (3) lon- gitudinal smooth muscle coat extending into conjoined longitudinal muscle; (4) longitudinal muscle contributing to internal anal sphincter; (5) striated muscle of puboperineus; (6) perineal body; (7) fibres of submucosal anal muscle arising from internal anal sphincter; (8) mus- cularis mucosae disappearing at junction of columnar and stratified cuboidal cell epithelium; (g) lumen of rectum; (Io) paraurethral gland. the rectal smooth muscle can be readily dis- external sphincter. tinguished. Followed down, the internal anal The internal sphincter is made up of two sphincter becomes more prominent until at distinct parts"' separated from each other by its lower border it presents a rounded edge, the fibres of the puboanalis and anterior sacro- which in the closed anal canal of the living coccygeal ligament, forming the mucosal sus- subject lies within the anal verge. This level pensory ligament (Fig. 6). In the upper part is subject to some variation in the normal sub- above the pelvic floor insertion, as we have ject', particularly in the female, where the seen, the oval bundles slope down and in- internal sphincter may project through the wards. These receive large contributions 294 J 0 N Lawson

Superficial sphincter Superficial sphinctr FIG. 6 Diagrammatic representation of itnternal anal sphincter showing the two parts and their relationship to surrounding structures. S.C.Z. = stratified cuboidal epithelium; A.C.S. = anal canal skin. (Reproduced by permission from the Pro- ceedings of the Royal Society of Medicinie). from the longitudinal coat which enter the can be seen to arise from the perineal body as periphery (Fig. 5). From the luminal aspect a series of loops passing back round the anal fibromuscular bundles arise, giving origin to canal (Fig. 8). In some sections fibromuscular the submucosal muscle (Figs. 2, 4, 5, and 7), extensions can be seen to pass through the which courses down in the anal folds to reach perineal body to blend with the internal uri- the skin of the lower anal canal and anal nary sphincter in the region of the membran- verge. Between the two parts the fibro- ous urethra or to blend with the lowermost muscular slips of puboanalis and the anterior circular smooth muscle coat of the , sacrococcygeal ligament (see Part I) pass to suggesting an origin from a more primitive insert into the dermis of the anal skin just cloacal sphincter. below the mucocutaneous junction (the junc- From the medial aspect of the bundles of tion of non-hairy skin to stratified cuboidal the lower part of the internal sphincter cell epithelium). Below this attachment, in musculotendinous fibres converge on a con- the lower part of the sphincter, the bundles densation of fibroelastic tissue just below the of smooth muscle come to lie less and less mucocutaneous junction, the fibres running obliquely until, near the lower border, they between the bundles of the submucosal lie first transversely and then with their muscle. This attachment appears to act as axes lying medially and upwards towards a point of fixation round which this part of the lumen. These bundles receive no sig- the muscle acts. Anteriorly, in sagittal sec- nificant contribution from the longitudinal tion, the muscle fibres arising from the muscle coat. perineal body and passing downwards and In transverse section this part of the muscle backwards into the sphincter were formerly Pelvic anatomy 295 pelvic floor these muscle bundles are separated by the musculotendinous slips of the pubo- anals, the upper layer passing between the bundles to reach their insertion and the lower interdigitating with them to reach the peri- : 4 *> D anal skin as the conjoint longitudinal muscle (Figs. I, 2, 4, and 8). Behind, the anterior sacrococcygeal ligament inserts, with this layer, into the back of the anal canal. The

~~~~~~~

FIG. 7 Details of coronal section (stained PTAH) _ to demonstrate the submucosal muscle, its originX from the upper part of the internal anal sphincter,E and its insertion into the anal and perianal skin. (ri Submucosal muscle; (i') its origin from the internal anal sphincter; (2) upper internal anal sphincter; (3) lower internal anal sphincter; (4) conjoined longitudinal muscle; (5) superficial anal sphincter; (6) deep anal sphincter; (7) 'pubo- rectalis' or puboanal sphincteric sling; (8) dia- 3 phragmatic group of pelvic floor muscles; (g) non- hairy anal canal skin; (io) stratified cuboidal cell 7 epithelium; (iI) hairy skin; (I2) ischial tuberosity; .C .4' (X3) columnar epithelium. FIG. 8 Detail from transverse section of female pel- vis (stained PTAH) to show region of perineal body known as the rectourethralis inferioris or the and origin of the lower part of the internal anal rectourethral muscle of Roux"2 (Figs. 8 sphincter from it. (I) 'Rectourethralis' (rectoperi- and 9). neus; (2) perineal body; (3) origin of lower part of internal anal sphincter from perineal body; * (4) submucosal muscle; (5) conjoined longitudinal Conjoine longitu l muscle muscle, bundles of smooth muscle interspersed with The longitudinal muscle of the rectum con- fibromuscular bundles derived from pevisc floor; tinuestinusdowndoninointo ththe anaanal canlcanal asdistnctas distinct (6) transverse perineal muscles; (7) stratified cub- bundles. At the level of the insertion of the oidal epithelium. 296 1 0 N Lawson

FIG. 9 Sagittal section (a) with de- tail (b) of male pelvis (stained HE) to show origin of the lower part of the internal anal sphincter from the perineal body, the inferior recto- urethral muscle (of Roux). (i) Pubis; (2) cocCyx; (3) ; (4) rec- tum; (5) membranous urethra; (6) bulbous urethra; (7) deep anal sphincter; (8) internal anal sphincter; (9) superficial anal sphincter; (so) puboanal sphincteric slin1g; (iI) pubo- analis sling; (I2) pelvic floor muscles; (I3) transverse perineal muscle; (I4) perineal body; (I5) inferior rectourethral muscle (of Roux); (16) superior rectourethral muscle; (I7) anal canal (stratified cuboidal epithelium); (i8) anal canal (non- hairy skin). Pelvic anatomy 297

conjoined muscle completes its course by portion. In coronal section, in more anterior breaking up into fibromuscular septa that cuts, the pubosphincteric sling (puborectalis) insert into the perianal skin between the peri- can be seen as a separate muscle and may anal apocrine glands, giving the dermis a have been mistaken for a third striated muscle honeycomb appearance on horizontal section. sphincter, but in more posterior cuts this In front two more distinct bundles, the muscle can be seen to blend with the deep rectourethralis (Figs. 6, 8, and 9) (recto- sphincter. urethralis superioris'3), inserts into the peri- neal body. This muscle is better termed 'recto- Deep anal sphincter perineus' as in the male it fails to reach the The deep anal sphincter consists of an annu- urethra and in the female it lies behind the lus of striated muscle. The upper fibres in- vagina. Above the pelvic floor the pars analis sert into either side of the perineal body, the recti is invested in the strong investing lower fibres forming a complete annulus. of the rectum, the layer seen between the There is no direct attachment to the coccyx longitudinal muscle bundles of the rectum behind, from which it is separated by the after the internal anal sphincter has been postsphincteric space. It is probable that the divided during sphincterotomy. muscle is a composite one. As we have seen, the pubosphincteric sling (Figs. 4, 7, 9, blends with the deep sphincter. Courtney22 and io) believes that the fibres from the sling decus- Most authors up to the end of the igth cen- sate behind the anal canal to reach the tury described two parts to the external anal perineal body on the opposite side, thus sphincter, though Santorini"4, 5 appears to contributing to that part inserting into the have subdivided the muscle into three por- perineal body. Gorsch9'20 describes them as tions, a superficial, a subcutaneous, and a being adherent or attached but not contribut- deep portion. Later authors"', I -2 have agreed ing to the deep anal sphincter. I have been to three subdivisions, though Fowler21 ques- unable to resolve this argument. I have seen tions the presence of a third part. Apart from the fibres of the pubosphincteric sling decus- individual variation, close association of the sating on the upper surface of the deep sphincters with the pelvic floor and the inter- sphincter and continuing back and down to spersing fibres of the conjoined longitudinal blend with the upper surface of the anococcy- muscle makes dissection difficult. On gross geal raphe, but also sending fibres up to blend dissection there are two distinct groups of with the undersurface of the anterior ano- muscles. The deep sphincter above forms a coccygeal raphe. In the male dissection the distinct annulus. The superficial sphincter be- fibres did appear to contribute to the deep lov lies in two planes; a medial one lies in the sphincter and to reach the perineal body on same vertical plane as the internal anal the opposite side. sphincter and a lateral one forms a horizon- Fibres from the puboperineus also pass back tal sheet of muscle bundles in the subcuta- into the sphincter (prerectal fibres'6'23) (Fig. neous tissue of the perianal skin. These muscle I o). Some fibres in the female deep anal bundles are separated and anchored by the sphincter pass forward on either side of the tails of the conjoined longitudinal muscle. perineal body to blend with the sphincter They cannot be readily divided on gross dis- vaginae2". section into a superficial and a subcutaneous No fibrous septa traverse the deep anal 298 J 0 N Lawson

Superficial sphincter t bundle of superficial sphincter FIG. IO Diagrammatic representation of voluntary anal sphincters and slings. Puboanalis and part of coccygeus cut away. sphincter, the muscle being separated from trated by Morgan and Thompson' a distinct the conjoined longitudinal muscle by a loose bundle is seen cupped in the lower margin areolar tissue plane. of the internal anal sphincter. On studying serial sections cut transversely and sagitally Superficial anal sphincter (superficial this bundle can be seen as a loop of striated sphincter and subcutaneous sphincter) fibres arising from the perineal body below As we have seen, the bundles of the super- the internal sphincter and representing the ficial sphincter lie in a subcutaneous plane, most medial bundle of the superficial anal the more medial lying in the anal canal at sphincter (Fig. IO). right angles to the more lateral. There are Finally, the fibromuscular tails from the three main subdivisions of these fibres (Fig. conjoined longitudinal muscle traverse the Io). The most lateral bundles, lying in the superficial muscle bundles serving to attach horizontal plane, arise from the back of the them to the perianal skin. sacrum and the raphe passing forwards in the bottom of the natal cleft. These fibres arch supply widely round the anus to pass into the ipsi- The nerve supply to the external sphincter lateral or decussate below the complex is derived from two main branches perineal body to reach the contralateral labia on either side (Fig. i i). The posterior inferior or, in the male, to pass forwards in the peri- haemorrhoidal nerve arises from the puden- neal raphe to the scrotum. dal nerve soon after it enters the pudendal The medial bundles arise from the raphe canal at the apex of the ischiorectal fossa. posteriorly to lie just below the anal skin It crosses the apex of the fossa to course in the anal canal. These fibres are nearly down its medial wall, deep to a fascial cover- circumferential and lie in the vertical plane. ing, to penetrate the superficial sphincter and They decussate in the anterior wall of the to reach the anal and perianal skin. It sup- anal canal below the perineal body. plies branches to the deep sphincter and In our sagittal sections and those illus- puboanal sphincteric sling as it crosses them. Pelvic anatomy 299

Puboperineus PubourethraU/ (prostaticus),

Pubosphincteric (puborectalis) sling Pdna

DorsalpenisPerineanervenerve of IleococcYgeus T.V. Perineii Subcutaneous and Superficial Ext. sph. superficial Ext. sph. Subcutaneous Ext. sph. Deep Ext. sph.\ HaemorrhodidLl FIG. I i Diagrammatic representation of nerve supply of the volunt.ary anal sphincter from the pudendal and perineal nerves. A second nerve, the anterior haemorrhoidal stitute of Child Health. Thcse grants have been nerve, crosses from the perineal nerve an- made to Mr H H Nixon for the work on the pelvic teriorly, passing down over the puboanal floor and anal canal and to Mr D Innes Williams for the work on the urogenital system. sphincteric sling (puborectalis) and transverse I should like to express my gratitude to the De- perineal muscles to reach the deep sphincter, partmellt of Morbid Anatomy, Hospital for Sick where it is distributed to the anterior quad- Children, and in particular to Mr J I Stiff and rant of the muscle, puboanal sphincteric Miss I Skinner for their careful preparation of sling, and anal and perianal skin. our serial sections. Also I should like to thank Mr R J Lunnon, Director of the Department of Medical The course of these nerves, across the apex Illustration, Hospital for Sick Children and Institute of the ischiorectal fossa and down its medial of Child Health, for his preparation of photomicro- wall, explains why extensive excisions can be graphs and Mlr G V Lyth (medical artist) for his made in the ischiorectal fossa without de- preparation of drawings and diagrams. nervating the sphincter or anal or perianal skin. Further, these nerves have a higher root References value (S2, 3, 4) than the pelvic floor nerves I Sanson, (I817) quoted by Merkel (Ref 3). (S3, 4, 5), which may explain some of the 2 Symington J (i888) Journal of Anatomy and findings in neurogenic bladder and rectum re- Physiology, 23, I o6. suilting from sacral agenesis involving S4. It is significant that the anal sphincter and slings, 3 Merkel, F (I900) Ergebnisse der Anatomie, io. the perianal skin, and the skin lining the 524. lower anal canal are all supplied below the 4 Robin, C, and Cadiat (1874) Journal de l'anato- mie et de la physiologie normales et patholo- pelvic floor, whereas the upper anal canal and giques de l'homme et des animaux, I0, 589. muscles elevating and opening the anal canal are supplied from the nerves above the pel- 5 Hermainn, G, and Desfosses, L (i88o) Comptes rendus hebdomadaires des se'ances de l'Acade6mie vic floor. des sciencees, go, 130I. These studies have been carried out with grants 6 Morgan, C N, and Thompson, H R (1956) An- fronm the Joinit Research Board of the Hospital for nals of the Royal College of Surgeons of Sick Childrcn, Great Ormond Street, and the In- England, I9, 88. 300 J 0 N Lawson

7 Lawson, J 0 N, and Nixon, H H (I967) Journal i6 Holl, M (1897) in Handbuch der Anatomie, ed of Pediatric Surgery, 2, 544. Bardelben, Jena, Fischer. 8 Walls, E W (I957) British Journal of Surgery, 17 Thompson, P (I899) The Myology of the Pelvic 45, 504- Floor. Newton, McCorquodale. 9 Chrispin, A R, Friedland, G W, and Wright, i8 Milligan, E T C, and Morgan, C N (I 934) D E (I967) Thorax, 22, i88. Lancet, 2, II50 and I213. Io Goligher, J C, Laycock, A G, and Brossy, J J 19 Gorsch, R V (I94I) Perineopelvic Anatomy (I955) British Journal of Surgery, 43, 51. from the Proctologist's Viewpoint. New York, Tilgman. ii Lawson, J 0 N (1970) Proceedings of the Royal Society of Medicine, 63 (Suppl.), 84. 20 Gorsch, R V (I960) CIBA Clinical Symposia, 12, No. 2. 12 Elliot-Smith, G (I908) Journal of Anatomy, 42, 198. 2I Fowler, R (I963) in Congenital Malforma- 13 Elliot-Smith, G (I908) Journal of Anatomy, 42, tions of the Rectum, Anus and Genito-urinary Tract, ed. Stephens, J D. Edinburgh and Lon- 252. don, Livingstone. I4 Santorini, G D (17I5) Anatomici summi septem decim tabulae (VI and VII). I775 edn. Parmac, 22 Courtncy, H (I950) American Journal of Surgery, ex Regiat Topographia. 79, 155. I5 Santorini, G D (1724) Observatione anatomi- 23 Uhlenhuth, E (i953) Problems in the Anatomy cae. VeIiice, Rccurti. of the Pelvis. Philadelphia, Lippincott.