assisted by the configuration of the mucous membrane, hold it continually closed except for the temporary passage of flatus and faeces. The junction of rectum and anal canal is at the pelvic floor, i.e. at the level where the puborectalis part of levator ani clasps the gut and angles it forwards (Fig.
5.68). From this angled junction with the rectum, 2.5 cm in front of the tip of the coccyx, the anal canal passes downwards and somewhat backwards to the skin of the perineum.
The muscles of the anal canal can be regarded as forming ‘a tube within a funnel’ (Parks) (Fig. 5.69).
The sides of the upper part of the funnel are the levator ani muscles, and the stem of the funnel is the external sphincter which is continuous with levator ani. The tube inside the stem of the funnel is the internal sphincter which is a thickened continuation of the inner circular layer of rectal muscle.
Internally lies the submucosa and mucous membrane.
Figure 5.69 Coronal section of the anal canal and the right ischioanal (ischiorectal) fossa. For clarity only the inferior rectal nerve is shown leaving the pudendal nerve in the pudendal canal; the corresponding vessels pursue a similar course across the fossa.
External anal sphincter
The sphincter has been described in the past as having deep, superficial and subcutaneous parts, based largely on the attachments of the middle (superficial) part, but the parts blend with one another to form a continuous tube.
At its upper (rectal) end, circular skeletal muscle fibres of this sphincter blend with the puborectalis part of levator ani (Fig. 5.69), except of course in the midline at the front where there are no levator ani fibres; here the sphincter fibres alone complete the ring. The region where puborectalis fuses with the external sphincter (which is also the level of the upper end of the internal sphincter) is termed the anorectal ring, and is palpable on rectal examination (see p. 295). Fibromuscular strands from the middle part of the external sphincter pass backwards to the posterior surface of the coccyx, contributing to an anococcygeal ligament. A retrosphincteric space occupied by fibrofatty tissue lies between these fibres and the muscular raphe formed by the iliococcygeal part of levator ani (Fig.
5.68). The multilayered fibromuscular anococcygeal ligament, with its external sphincter, iliococcygeus and pubococcygeus components, together with the overlying superior fascia of the pelvic diaphragm is also termed the postanal plate, on which lies the rectum (see p. 291). Anteriorly there is some intermingling of external sphincter muscle fibres with the transverse perinei and
bulbospongiosus muscles at the perineal body; this is less evident in the male so that a surgical plane of cleavage can be established between the external sphincter and the perineal body. In the female, the external sphincter is shorter and the deep fibres are deficient anteriorly. The lowest part of the external sphincter curves inwards to lie below the lower end of the internal sphincter (Fig. 5.69).
This submucosal apposition of the two sphincters is at the site of the palpable intersphincteric groove in the lower part of the anal canal. When operating on the anaesthetized patient, however, the internal sphincter is often found to extend to the anal orifice.
Internal anal sphincter
This is the thickened downward continuation of the inner circular muscle of the rectum which, in cadaveric prosections, is usually found to extend along three-quarters of the length of the anal canal.
(Fig. 5.69). At the anorectal junction the outer longitudinal layer of rectal muscle becomes fibroelastic and, together with some striated muscle fibres of puborectalis, forms the conjoint longitudinal coat which runs down between the two sphincters. Strands from this sheet penetrate the internal sphincter and the lower part of the external sphincter; some reach the fat of the ischioanal fossa and the perianal skin; others pass through the internal sphincter to the mucosa of the anal canal, particularly at the pectinate line (see below), where strands tethering the mucous membrane were named the mucosal suspensory ligament by Parks. It is possible that the puckering of perianal skin is due to the attachment of these fibroelastic strands to perianal skin, but some investigators describe separate smooth muscle fibres in this region, forming the so-called corrugator cutis ani muscle.
Mucous membrane
In the upper third of the anal canal the mucous membrane shows 6 to 10 longitudinal ridges, the anal columns. They are prominent in children. At their lower ends adjacent columns are joined together by small horizontal folds, the anal valves; the pockets so formed above the valves are the anal sinuses, into which open mucus-secreting anal glands. About half the anal glands are submucosal and the rest penetrate through the internal sphincter. Infection in these glands results in anal abscesses and fistulae.
The level of the anal valves is the pectinate line (also called the dentate line) below which is a pale, smooth-surfaced area, the pecten, which extends down to the intersphincteric groove. Below the groove is a truly cutaneous area, continuous at the anus (anal margin) with the skin of the buttock.
Histologically the lining below the groove is typical skin with keratinized stratified squamous epithelium, hair follicles, sebaceous glands and sweat glands. The lining of the pecten is non- keratinized stratified squamous epithelium, with no hair follicles, sebaceous glands or sweat glands.
The anal column area, being continuous with rectal mucosa, has typical columnar intestinal cells and tubular glands. But immediately above and below the pectinate line there is a zone of variable, often mixed epithelial structure, so that there is no abrupt line of change from the single-layered gut type to multilayered pecten type. (This contrasts with the gastro-oesophageal junction where there is an abrupt change from stratified squamous to columnar epithelium.)
Small submucous masses, comprising fibroelastic connective tissue, smooth muscle, dilated venous spaces and arteriovenous anastomoses, form anal cushions at left lateral (3), right posterior (7) and right anterior (11 o'clock) positions in the upper anal canal. Smaller cushions may be located in between. By their apposition these anal cushions assist the sphincter in maintaining watertight closure of the canal. Excessive straining at stool may cause enlargement of these cushions and the formation of haemorrhoids (piles).
The lining of the upper part of the anal canal is embryologically derived from the cloaca, i.e. it is endodermal; the lower part is from the proctodeum or anal pit and is ectodermal (see p. 29). The dividing line between these territories is usually considered to be at the pectinate line.
Blood supply
Branches of the superior rectal artery supply the upper end of the canal, their terminations lying within the anal columns. A small part of the muscular wall is supplied by the median sacral arteries, while the lower end, including its mucous membrane, receives the ends of the inferior rectal vessels which have crossed the ischioanal fossae. Within the walls there is good anastomosis between the various vessels.
The veins correspond to the above arteries and are continuous with the rectal venous plexuses (see p.
294). The upper part of the canal and plexus drains via the superior rectal and inferior mesenteric veins to the portal system, whereas the lower end drains to the internal iliac veins through the inferior and middle rectal veins. The anal canal is thus a site of portal–systemic anastomosis (see p. 267), the union being in the region of the anal columns.
Lymph drainage
The lymph drainage shows a watershed corresponding to the vascular pattern. The upper canal drains upwards to join the lymphatics of the rectum (see p. 294) whereas lymph from the lower end passes to the (palpable) superficial inguinal group.
Nerve supply
The inferior rectal branches of the pudendal nerves supply the external sphincter; they also provide the sensory supply from 1–2 cm above the pectinate line downwards, where the lining of the anal canal is highly sensitive. The motor fibres originate from Onuf's nucleus, situated mainly in the anterior horn of S2 segment, which also innervates the sphincter urethrae. The puborectalis and the deep part of the external anal sphincter have a high proportion of slow twitch fibres and function as tonic muscles, showing constant electromyographic activity even in sleep and under light anaesthesia.
Autonomic nerves pass to the internal sphincter and the upper part of the canal. Sympathetic fibres from the pelvic plexus, with preganglionic cell bodies in the first two lumbar segments of the cord, cause contraction of the internal sphincter, and pelvic splanchnic (parasympathetic) nerves relax it.
Afferent fibres from the upper end of the canal are carried by both sympathetic and parasympathetic nerves.
The anal reflex is described on page 17.
Defecation
Several factors contribute to normal anal continence: contraction of puborectalis and the external sphincter, maintenance of the angle between rectum and anal canal with abdominal pressure flattening the lower anterior rectal wall over the upper end of the canal, and the presence of mucosal cushions in the canal. The internal sphincter, although assisting closure, can only maintain continence if there is no distension (which causes relaxation of the sphincter). The rectum can accommodate itself to receive a certain amount of colonic content without any significant increase in pressure. There are no specialized receptors in the rectal wall, but they are present in the anal canal where gas, fluid and
solid can be distinguished by the cerebral cortex, and there are also stretch receptors in levator ani and the perirectal tissues. When increasing rectal pressure causes faeces to enter the upper anal canal, the external sphincter contracts and forces the contents back into the rectum. If only gas enters, its presence can be tested by a slight conscious increase of abdominal pressure which will let it escape.
Defecation is allowed to occur by release of the cortical inhibition that developed during childhood training. Abdominal pressure is increased, puborectalis relaxes and the anorectal angle straightens with relaxation of the external sphincter and contraction of the lower colon and rectum (via their parasympathetic supply).
Incontinence may follow damage to the external sphincter or pudendal nerve (e.g. in obstetrics and perineal operations). In cerebral or spinal cord lesions there may be loss of cortical control.
Ischioanal (ischiorectal) fossa
The ischioanal (ischiorectal) fossa is a wedge-shaped space filled with fat lateral to the anal canal (Figs 5.69 and 5.71). The base of each fossa lies on the skin over the anal region of the perineum. The external sphincter of the anal canal and the sloping levator ani muscles form the medial wall of each fossa, while the lateral wall is formed by the ischial tuberosity below with obturator internus (covered by its fascia) above. The apex of the wedge is where the medial and lateral walls meet (where levator ani is attached to its tendinous origin over the obturator fascia). At the base the anterior boundary is the posterior border of the perineal body and muscles of the urogenital diaphragm (see p. 317), and the posterior boundary is the sacrotuberous ligament overlapped by the lower border of gluteus maximus. The fat in the lower part of the fossa adjacent to the skin is in small lobules, while the fat in the upper reaches of the fossa is in large lobules.
Figure 5.71 Muscles of the perineal region and the perineal pouches: A perineal muscles, female on the left and male on the right of the picture; B diagrammatic representation of the perineal pouches in the male.
Each fossa has an anterior recess that passes forwards above the perineal membrane, potentially as far as the posterior surface of the body of the pubis. The recesses of the two sides do not communicate across the midline. Posteriorly, however, the two fossae communicate with one another, low down through the fibrofatty tissue of the retrosphincteric space within the anococcygeal ligament (see p. 313), providing a horseshoe-shaped path for the spread of infection from one fossa to the other.
The pudendal canal (of Alcock) is a connective tissue tunnel in the lower lateral wall of the fossa, overlying obturator internus and the medial side of the ischial tuberosity. The canal is formed by a splitting of the obturator fascia above the falciform process of the sacro-tuberous ligament. It contains the pudendal nerve and internal pudendal vessels (Fig. 5.70), which it conducts from the lesser sciatic notch to the deep perineal pouch above the perineal membrane (see p. 317).
Figure 5.70 Lateral wall of the left ischioanal fossa from behind, with the connective tissue of the pudendal canal removed to show the pudendal nerve and vessels running forwards on the medial side of the ischial tuberosity. The middle part of the sacrotuberous ligament has been removed and the venae comitantes of the internal pudendal artery are not shown.
The pudendal nerve and internal pudendal vessels leave the pelvis through the greater sciatic foramen, passing beneath the lower border of piriformis to reach the buttock. Their course in the buttock is short. They turn and enter the lesser sciatic foramen, the vessels passing over the tip of the spine of the ischium, the nerve more medially over the sacrospinous ligament.
Running transversely across the ischioanal fossa from the pudendal canal towards the anal canal are the inferior rectal branches of the pudendal nerve and internal pudendal vessels. Their course is not straight across the base of the fossa, but arches convexly upwards through the fat towards the apex and then downwards to the anal canal. Incisions to drain ischioanal abscesses usually do not interfere with them. Accompanied by the vessels, the nerve breaks up into several branches which supply the external sphincter, mucous membrane of the lower anal canal and perianal skin.
At the front of the fossa the posterior scrotal (labial) nerves and vessels (from the pudendals) pass superficially into the urogenital region. At the back of the fossa the perineal branch of S4 nerve and the perforating cutaneous nerve traverse the fossa.
Perineal body
The perineal body, also called the central tendon of the perineum, is a midline fibromuscular mass to which a number of muscles gain attachment, and within which they decussate. It is attached to the posterior border of the perineal membrane (see below). It lies between the anal canal and the vagina (Fig. 5.64) or bulb of the penis (Fig. 5.71A). The rectovaginal septum blends into it above. The muscles running into it include the external anal sphincter, pubovaginalis (puboprostaticus) part of levator ani, bulbospongiosus, and the superficial and deep transverse perineal muscles. Its position and connections provide a stabilizing influence for pelvic and perineal structures. Injury to it during childbirth may weaken the pelvic floor and contribute to prolapse of the vagina and uterus.