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:
•
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:
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.