SURGICAL ANATOMY OF THE ANAL CANAL with special reference to the SURGICAL IMPORTANCE OF THE INTERNAL SPHINCTER AND CONJOINT LONGITUDINAL MUSCLE by C. Naunton Morgan, M.S., F.R.C.S. and Henry R. Thompson, F.R.C.S. Consultant Surgeons, St. Mark's Hospital "If terms be incorrect, then statements do not accord with facts; and when statements and facts do not accord, then business is not properly executed." Confucius THE DESCRIPTION BY Milligan and Morgan (1934) of the surgical anatomy of the anal canal with reference to fistula-in-ano and that of Milligan et al. (1937), on the treatment of haemorrhoids, is essentially correct for the normal anal canal when examination is carried out without anaesthesia. The fact, however, that the relationship of the anal musculature altered under anaesthesia was not realised and resulted in the misnaming of the internal sphincter. At operations for haemorrhoids and fissure-in-ano, this muscle in the original descriptions was confused with the subcutaneous external sphincter. Division of the internal sphincter muscle (sphincterotomy), described by Eisenhammer (1951), for " Chronic Internal Anal (Sphincteric) Contracture" re-focused our attention, at St. Mark's Hospital, on the exact position of the internal sphincter. The work of Fine and Wickham Lawes (1940), Wilde (1949) and, more recently, Parks (1954), together with the co-operative investigations over several years by the Surgical Staff at St. Mark's Hospital (Goligher et al., 1955), now enables a more accurate description of the anatomy of the anal canal to be presented. The object of this paper is: (1) To distinguish between the anatomical and surgical anal canal and to give a more accurate description of its anatomy and terminology. (2) To indicate how the terminal attachments of the conjoint longi- tudinal muscle may determine the path of perianorectal infection. (3) To describe variations and conditions in which the relationships of the external and internal sphincter are altered. (4) To correlate the internal sphincter and the pecten band. (5) To correct a point in previous anatomical descriptions from St. Mark's Hospital. (6) To describe the operation of sphincterotomy. (7) To draw attention to a common error in conception of the anorectal musculature. 88 SURGICAL ANATOMY OF THE ANAL CANAL THE ANATOMICAL AND SURGICAL ANAL CANALS The anatomical anal canal extends from the level of the valves of Morgagni (dentate line) to the anal margin. For surgical purposes, the anal canal may be regarded as that portion of the terminal intestine which extends from the level where the rectum passes through the pelvic visceral aperture- the anorectal ring-to the the anal margin. This concept of the anal canal is more apposite for surgical] purposes and as the anorectal ring is above the valves of Morgagni, the surgical anal canal is longer than its anatomical counter- part (Fig. 1). Anorectal rin (pelvic viuceri aperture) Surgical anal canal Anal marsin Fig. 1. The surgical and anatomical anal canals. THE LININGS OF THE SURGICAL ANAL CANAL (Fig. 2) The pink columnar mucous membrane of the rectum extends down- wards from the anorectal ring into the anal canal and overlies the upper half or two-thirds of the internal sphincter. It is loosely attached to the underlying structures and covers the internal haemorrhoidal plexus. As it approaches the level of the columns of Morgagni, the columnar mucous membrane becomes cuboidal, is darker in colour, and, just above the 89 C. NAUNTON MORGAN AND HENRY R. THOMPSON ./1.. Anorectal 4. , ring (pelvic visceral o i u Rectal aperture) 9X, mucosa ... I *.~~~v*~ (pink) . - .Ana Conjoint mucosa longitudinal (re. 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Thet zonte betweenth deTate lineandu eptheliadhernghabentclehanal canalskin.wsdsrbdbSTroud Owtilyingto the loose,attachmentaof thgenmucos coveringae thue sinterna hdnatemorrhoindalpeubvthe tetheredtanal canal skin esrbdan theStloose attachment of the skin over the external haemorrhoidal plexus below, two potential spaces are formed. They are the submucous space which is 90 SURGICAL ANATOMY OF THE ANAL CANAL situated above the level of the anal valves (dentate line) and the sub- cutaneous perianal space placed distal to the adherent anal canal skin. Both these spaces are capable of distension, by inflammatory exudate, venous engorgement, blood clot or injected fluid (Fig. 3). The fixity of the thin anal canal skin (pecten) to the underlying structures between these potential spaces prevents its detachment from the subjacent tissues at this site (Parks). The true skin of the anus and anal canal skin cover the external haemorrhoidal plexus. Short connecting vessels pass upwards under the smooth anal canal skin to the internal haemorrhoidal plexus. I . Submucous space -Adherent anal canal skin Subcutaneous perianal space Fig. 3. The submucous and the subcutaneous perianal spaces. 91 8 C. NAUNTON MORGAN AND HENRY R. THOMPSON The lining of the anal canal above the anal valves is insensitive except to stretching, such as by rapid injection of an internal haemorrhoid, whereas the anal canal skin and the true skin of anus are extremely sensitive to painful stimuli. To summarise-the surgical anal canal is lined from above downwards by pink rectal mucosa (columnar epithelium) covering the haemorrhoidal pedicle at the anorectal ring; by dark red anal mucosa (cuboidal and transitional epithelium) covering the main haemorrhoidal mass; by smooth, parchment coloured anal canal skin (thin squamous epithelium) covering the pecten zone and, finally, by the true skin of the anus (squamous epithelium with hair follicles and sweat glands) covering the external haemorrhoid. THE ANAL MUSCULATURE The internal sphincter (Fig. 2) The internal sphincter is the thickened terminal portion of the circular muscle coat of the rectum. It arises where the rectum passes through the pelvic diaphragm and its termination can be felt just within the anal orifice. Its position and relation to other structures of the anal canal and its variations will be described subsequently. The conjoint longitudinal muscle (Fig. 2) The conjoint longitudinal muscle of the anal canal is formed by the continuation downwards of the longitudinal muscle coat of the rectum together with fibres from the puborectalis muscles. As this thickened conjoint muscle passes downwards between the internal sphincter and the external sphincter, its fibres become more and more fibro-elastic. Whilst lying between these sphincter muscles, some fibres of the longi- tudinal muscle pass inwards through the internal sphincter, between its muscular fasciculi, to enter the submucous space. This is most clearly shown in foetal sections (Figs. 4a and b). Here the fibres appear to con- dense mainly in the region of the inner aspect of the enlarged distal portion of the internal sphincter. In this situation, the muscularis mucosae of the bowel is much thicker and the longitudinal " muscle " fibres apparently pass into it. Fine and Wickham Lawes (1940), have called this sub- mucous fibro-muscular layer the musculus submucosae ani. In this region (pecten) the anal canal skin is attached to the subjacent fibro-muscular layer and indirectly, therefore, to the lower portion of the internal sphincter (Fig. 5). This attachment of the longitudinal muscle to the lining of the anal canal separates the submucous from the subcutaneous perianal space. It should be noted that the perianal space extends upwards within the anal canal for a short distance and that the thickened lower end of the internal sphincter lies within this subcutaneous space. 92 SURGICAL ANATOMY OF THE ANAL CANAL _@.@;wg~~~~~~~~~~N AM r §it #t id (With acknowledgment to Prof. H. Butler.) Fig. 4a. Coronal section of ano-rectal region of foetus. Some fibres of the fibro-elastic termination of the longitudinal muscle loop around the lower border of the internal sphincter to join the inner- most fibres of the conjoint longitudinal muscle (Wilde). The main insertion of the fibro-elastic fibres of the conjoint longitudinal muscle is into the true anal skin overlying the subcutaneous external sphincter. This insertion is obtained by fanning out of the longitudinal fibres which then pass through the circular fibres of the subcutaneous 93 8-2 C. NAUNTON MORGAN AND HENRY R. THOMPSON external sphincter. Some of these fibres lie immediately under the skin of the anal canal and skin of the anus and constitute the corrugator