Annals of the Royal College of Surgeons of England (I974) vol 54 Pelvic anatomy I]I. Anal canal and associated sphincters 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 anus. 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 rectum. described. In i874 Robin and Cadiat4, studying the mucous membrane, 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 perineum. The serial sections were cut 7-I 0 gm epithelium 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 pelvic floor 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 sphincter, 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 anal valves 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 mouths 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 urethra. 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- anal columns 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 muscularis mucosae is seen clearly in Above, the anal glands, extending from the the pars analis recti, but traced down to- crypts into the submucosa 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- internal anal sphincter 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 .s.0 '. .: ....:-X,:': f..: .:: , ;: '': '. ..: .:* :.:......' . ..:. .:.1.:, .........,.';, :2 :tx: ... : : .: :: .: :::: s *;,.:.;.:. .:. .... '.:*-P,.^...... ,..."t<'': o;.,t, A.... ;'' .&*. .#. :.*.:..Qy:. .: st.'n*...S $.-- r *. ..:...-....Xd *, ..1 *.... b 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 smooth muscle; (Io') main contribution from puboanalis; (ii) columnar epithelium; (12) internal sphincter. and 9). It can readily be distinguished from floor, where the lumen 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.