Anatomy and physiology

Surgical anatomy

The anal canal commences at the level where the rectum passes through the pelvic diaphragm and ends at the anal verge (the external or distal boundary of the anal canal). The muscular junction between the rectum and anal canal can be felt with the finger as a thickened ridge the anorectal bundle or ‘ring’.

Anal canal musculature (Fig. 61.1)

The internal sphincter is a thickened continuation of the circular muscle coat of the rectum. This involuntary muscle commences where the rectum passes through the pelvic diaphragm and ends at the anal orifice, where its lower border can be felt. The internal anal sphincter is 2.5 cm long and 2—5 mm thick. When exposed during life, it is pearly white in colour, and its individual, transversely placed fibres can be seen clearly. Spasm and contracture of this muscle play a major part in fissure and other anal affections.

The longitudinal muscle is a continuation of the longitudinal muscle coat of the rectum intermingled with fibres from the puborectalis. Its fibres fan out through the lowest part of the external sphincter, to be inserted into the true anal and perianal skin. The longitudinal muscle fibres that are attached to the epithelium provide pathways for the spread of perianal infections, and mark out tight ‘compartments that are responsible for the intense pressure and pain that accompany many localised perianal lesions.

Beneath the anal skin lie the scanty fibres of the corrugator cutis ani muscle.

The external sphincter, formerly subdivided into a deep, superficial and subcutaneous portion is now considered to be one muscle (Goligher). Some of its fibres are attached posteriorly to the coccyx, while anteriorly they are inserted into the midperineal point in the male, whereas in the female they fuse with the sphincter vaginae. In life the external sphincter is pink in colour and homogeneous. Unlike the pale internal sphincter muscle, which is involuntary, the red external sphincter is composed of voluntary (somatic) muscle.

Between the internal (involuntary) sphincter and the external (voluntary) sphincter muscle mass is found a potential space, the intersphincteric plane. This plane is important as it contains the basal parts of eight to 12 apocrine glands, which can cause infections, and it is also a route for the spread of pus. It can also be opened up by a surgeon to provide access for operations on the sphincter muscles.

The puborectalis plays a key role in maintaining the angle between the anal canal and rectum and, hence, is essential for the preservation of continence (Fig. 61.2). There is a close association between the puborectalis portion of the levator ani and the external sphincter muscle.

The mucous membrane. The pink columnar epithelium lining the rectum extends through the anorectal ring into the surgical anal canal. The mucosa of the upper anal canal is attached loosely to the underlying structures, and covers the internal rectal plexus. Passing downwards where it clothes the series of eight to 12 longitudinal folds known as the columns of Morgagni, the mucous membrane becomes cubical, and red in colour (Fig. 61.3); above the anal valves the mucous membrane becomes plum coloured. Just below the level of the anal valves there is an abrupt, albeit wavy, transition to squamous epithelium, which is parchment colour. This wavy junction constitutes the dentate line. The squamous epithelium lining the lower anal canal is thin and shiny, and is known as the anoderm. This squamous epithelium differs from the true skin in that it has no epidermal appendages, i.e. hair and sweat glands. The anoderm passes imperceptibly into the pigmented skin of the anus. At the dentate line the anoderm is attached very firmly indeed to deeper structures.

The dentate line is a most important landmark both morphologically and surgically. It represents the site of fusion of the proctodaeum and post-allantoic gut, and the position of the anal membrane, remnants of which may frequently be seen as anal papillae situated on the free margin of the anal valves. The dentate line separates:

  above

cubical epithelium;

autonomic nerves (insensitive);

portal venous system;

below

from squamous epithelium;

from spinal nerves (very sensitive);

from systemic venous system.

The anal valves of Ball are a series of transversely placed semilunar folds linking the columns of Morgagni. They lie along and actually con­stitute the waviness of the dentate line. They are functionless remnants of the fusion of the post-allantoic gut with the proctodaeum.

The crypts of Morgagni (syn. anal crypts) are small pockets between the inferior extremities of the columns of Morgagni. Into several of these crypts, mostly those situated posteriorly, opens one anal gland by a nar­row duct. This duct bifurcates, and the branches pass outward to enter the internal sphincter muscle, in 60 per cent of people (Fig. 61.4). Issuing from this ampula there are three to six tubular sub-branches that extend into the intermuscular connective tissue, where they end blindly. In some lower animals, these glands secrete an odoriferous substance during the rutting season; in humans, their function, if any, is obscure. Some of their cells have been shown to give a positive staining reaction for mucin, but as the lining epithelium is mainly cubical, the mucus secreting propensity of the anal glands must be extremely small. Infection of an anal gland can give rise to an abscess, and in the opinion of a number of surgeons, infection of an anal gland is the most common cause of anorectal abscesses and fistulae.

The anorectal ring marks the junction between the rectum and the anal canal. It is formed by the joining of the puborectalis muscle (Fig. 61.2), the deep external sphincter, conjoined longitudinal muscle and the highest part of the internal sphincter. The anorectal ring can be clearly felt digitally, especially on its posterior and lateral aspects. Divi­sion of the anorectal ring results in permanent incontinence of faeces. The position and length of the anal canal, as well as the angle of the anorectal junction, depend to a major extent on the integrity and strength of the puborectalis muscle sling. 

Arterial supply 

The anal canal is supplied by branches from the superior, middle and inferior haemorrhoidal arteries. The most important is the superior haemorrhoidal, whose left branch sup­plies the left half of the canal by a single terminal branch, while its right has two terminal branches. All of the arteries contribute to a rich submucous and intramural plexus, so that interruption of the arterial supply from above by division of the superior and middle rectal arteries does not deprive the anus of its blood supply. 

Venous drainage 

The anal veins are distributed in similar fashion to the arterial supply. The superior and middle haemorrhoidal veins drain via the inferior mesenteric vein into the portal system, having become the superior rectal vein en route. The superior haemorrhoidal vein drains the upper half of the anal canal. The inferior haemorrhoidal veins drain the lower half of the anal canal and the subcutaneous perianal plexus of veins: they eventually join the external iliac vein on each side.

Lymphatic drainage

Lymph from the upper half of the anal canal flows upwards to drain into the post-rectal lymph nodes and from there goes to the para-aortic nodes via the inferior mesenteric chain. Lymph from the lower half of the anal canal drains on each side first into the superficial and then into the deep inguinal group of lymph glands. However, if the normal flow is blocked, e.g. by tumour, the lymph can be diverted into the alternative route.

Surgical physiology of the anal muscles and pelvic floor

The function of the anal canal and pelvic floor muscles is not only to contain the contents of the rectum, but also to allow effortless, unimpeded voiding at defecation. Interference with the integrity of the anatomy or physiology of the mus­cles of the anus and pelvic floor can lead to the extremes of intractable constipation or incontinence. If the muscles of the pelvic floor become too floppy, the entire anorectal mechanism can drop down (‘perineal descent’), or alternatively can gape open, so allowing intussusception and prolapse of the rectum.

If the puborectalis and anorectal ring of muscles fail to relax appropriately (so-called ‘inappropriate function’ or anismus) to allow the rectum to empty at defecation, obstructed defecation ensues: this can usually be overcome by excessive voluntary straining efforts, but frequently ends in intractable constipation. Excessive straining can cause both partial and complete rectal prolapse. When a patient presents with incontinence caused by weak or damaged anorectal musculature, or if bizarre or extreme complaints of constipation are elicited, it is now possible to investigate these symptoms to obtain objective data on which to base a management protocol (Swash and Henry). The length, resting tone and the power to relax and contract the anal sphincter muscles can be assessed by manometry and electromyography (Figs 61.5, 61.6 and 61.7): these studies can be combined with delineation of rectal sensibility and function by balloon distension and radiology (‘defecatory proctography’) and the abnormalities identified. In addition to the intrinsic defects, mechanical deviations can be mapped: the level and angle of the anorectal junction can be established by clinical observation and by an appliance (‘perineometer’). Further­more, it is possible to take radiographs of the acts of straining and evacuation while simultaneously recording electromyographs of the sphincter muscles and intrarectal pressure (Williams) (Fig. 61.8). This technique is known as dynamic integrated protography. Endoluminal ultrasound can also be used to visualise the integrity of both the internal and external anal sphincters (Fig. 61.9). The above techniques provide information which enables many patients with incontinence and constipation to be treated effectively.