Anatomy - Anus and Rectum
INTRODUCTION
THE ANUS
The Anorectal Line
The Anal Valves
Fascia
THE RECTUM
The Columns Of Morgagni (Rectal Columns)
Measurements
The Mucosa
The Submucosa
The Musculature of the Rectum
The Serous Coat
The Rectal Valves
The Third Sphincter of O’Beirne
The Sigmoid
THE EXTERNAL SPHINCTER
The Subcutaneous Muscle
The Superficialis Muscle
The Profundus Muscle
THE CORRUGATOR CUTIS ANI MUSCLE
THE INTERNAL SPHINCTER
THE LONGITUDINAL MUSCLE
MILLIGAN'S SEPTUM
THE ANORECTAL MUSCLE RING
THE ANOCOCCYGEAL LIGAMENT OR BODY
THE LEVATOR ANI MUSCLE
The Pubococcygeus Muscle
The Puborectalis Muscle
The Iliococcygeus Muscle
The Ischiococcygeus Muscle
THE PELVIC TRIANGLES
THE PERINEOPELVIC SPACES
The Perianal Space
The Submucous Space
The Ischiorectal Fossae
The Supralevator Spaces (pararectal)
The Retrorectal Space (Pre-sacral)
The Rectovaginal Space
The Anococcygeal Triangle
THE BLOOD SUPPLY TO THE ANORECTAL REGION
THE VASCULAR SUPPLY TO THE RECTUM
THE VENOUS SUPPLY OF THE RECTUM
THE LYMPHATICS
Anorectal Lymphatics
The Perineal Group
The Anorectal Group
The Extrarectal Group
THE NERVE SUPPLY TO THE ANORECTAL REGION
NEUROPHYSIOLOGY (Defecation Reflex)
BIBLIOGRAPHY
INTRODUCTION
Perhaps the most influential osteopathic proctologists of
the twentieth century were Carlton M. Noll and Frank D. Stanton.
Presented here, as a tribute to these great men is the written
text of Dr. Noll, and the line drawings of Dr. Stanton.
Figure 1: The rectum and anal canal.
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THE
ANUS
The anus is the outlet of the gastro-intestinal tract. (Fig.
1) It is a slit in its resting period. It is expandable, conforming
to the size and shape of the fecal content, and it normally
varies from 1.2 to 3.5 cm in diameter in the act of defecation.
The anus is surrounded by the subcutaneous muscle of the external
sphincter (Fig. 2).
Figure 2: The anus, showing the subcutaneous muscle.
Fibers from the conjoined longitudinal muscle sphincter (Fig.
3) and its fibroelastic extensions pass into, through and
around the subpecten with its glands, lymphatics and capillary
network that is of great importance in anorectal suppurative
processes. This is the zone of anastamoses between the superior
hemorrhoidal and inferior hemorrhoidal plexuses, and to a
lesser extent, the middle hemorrhoidals. The pecten's vascular
anastamosis drains freely to either the portal or caval systems.
Changes in the nerve supply and lymphatic drainage, also takes
place at this zone. The pecten marks the area of greatest
narrowing of this region.
Figure 3: Conjoined longitudinal and corrugator cutis ani
muscles.
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The
Anorectal Line
The anorectal line (dentate, pectinate, valvular, papillary
etc.) marks the upper irregular margin of the pecten (Fig.
3). Anal papillae are more often absent than present, but
when present, they do not usually arise from the free edges
of the anal valves or crypts as some suppose. They correspond
usually to the rectal columns of Morgagni (Fig. 4). The tips
of the papillae frequently project above the lower margins
of the rectal columns and are referred to as anal papillae.
Figure 4: Papillae, crypts and Morgagnian columns.
According to Gorsch, the anal crypts (Fig. 4), a.k.a. anal
pockets, sinuses, Saccules of Harner etc.; are tiny recesses
projected between adjacent anal columns and behind the anal
valves. They vary in number, depth, and shape. The more constant
and larger crypts are usually just lateral to the posterior
commissure and are regularly described as an etiological factor
in anal fissure and fistulae. The blind ends of the crypts
extend into the pecten and the proximal open ends are directed
toward the rectum.
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The
Anal Valves
The so-called anal valves are folds of squamous epithelium
bridging adjacent anal columns from the free inner wall of
anal crypts (Fig. 4). Histologically, the valves are thickened
or cornified epidermis, which becomes continuous with the
rectal columnar epithelium projected distally between the
anal columns into the blind end of the crypts.
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Fascia
Gorsch describes the relations of the anal canal in detail.
Anteriorly in the male the anal canal is in relation with
the perineal body and the accumulated concentrations of fascia
at this point. From below and upward, these include the superficial
and deep layers of the superficial fascia, Colle’s Fascia,
at its attachment to the posterior margin of the triangular
ligament, in which lie the adjoining superficial transverse
perineal muscles of the bulb of the urethra, the superficial
and deep layers of the triangular ligament, and finally, the
rectourethralis muscle, forming the floor of the prerectal
space.
Figure 5: Muscles and fasciae.
In the female, the anal canal lies posteriorly in relation
to the sphincter vaginae (Bulbocavernosis) muscle and the
ill-defined posterior margin of the triangular ligament, the
rectovaginal muscle, which forms the floor of the rectovaginal
space (septum), or the prerectal space which is located just
above the deep portion of the external sphincter.
Laterally, the anal canal, covered by anal fascia, comes into
relation with the wider ischiorectal fossae (Fig. 5) with
its fibro-cellular matrix. Posteriorly, the anal canal is
in relation with the anococcygeal body, which through its
muscular and fascial reflections, contributes materially to
structural support.
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THE
RECTUM
Gorsch states that for practical purposes, the rectum may
be described as extending from the level of the third sacral
vertebral body to the anorectal line. The third sacral corresponds
to the termination of a definite mesentery; it marks the point
at which there is a change in the blood supply; the level
at which the tinea of the sigmoid spread out to reinforce
the longitudinal muscle coat; it corresponds to the site of
the rectal narrowing to join the sigmoid; it marks the change
in color, the capillary pattern the rugosity of the rectal
mucosa. From the functional and endoscopic points of view,
it may be divided into the sphincteric and ampullary portions.
The sphincteric portion corresponds to the annulus hermorrhoidalis,
surrounded by the levator ani and the fascial collar from
the supra-anal fascia. The ampullary portion extends from
the third sacral to the pelvic diaphragm at the insertion
of the levator ani.
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The
Columns Of Morgagni (Rectal Columns)
These are mucosal longitudinal folds formed in the bulbus
analis of the primitive rectum and persist as the rectal columns
of Morgagni (Fig. 4). These columns act as accommodations
for contractions and dilations of the anal canal and the sphincteric
portions of the rectum. Histologically, these consist of a
somewhat denser muscularis mucosa, with richer lymphatics,
vascular, and nerve supply than those of the adjacent intervening
rectal wall. Between the columns are the so-called sinuses
of Morgagni, which are directly continuous with the crypts.
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Measurements
The rectum varies from 10-15 cm in length, while the circumference
varies from 15 cm at the rectosigmoid junction, to 35 cm or
more at its widest ampullary portion.
Conformation
The ampulla of the rectum may be roughly pear shaped or balloon
shape. Longitudinally, the rectum conforms to the sacral curve.
It presents lateral curvatures, which may be quite prominent,
and correspond to the indentures opposite the rectal valves.
The adult rectum has four well-defined coats: Mucous, Submucous,
Muscularis (circular and longitudinal), and Serous (Fig. 6).
Figure 6: Coats of the rectum.
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The
Mucosa
The mucosa of the rectum (Fig. 6) is thick, darker and somewhat
more highly vascularized than any other mucosa in the gastro-intestinal
tract. It is more mobile and has a particularly well-developed
glandular apparatus, consisting of the mucigenous tubular
glands (Lieberkuhn). The epithelial surface of the rectal
mucosa is covered by a layer of stratified columnar cells,
which assumes a cuboid shape as they go proximally. The mucosa
contains abundant lymph follicles situated between the glands
of Lieberkuhn. The syncytial or intrafollicular tissue between
the glands conveys the lymphatics, nerve plexuses and vascular
supply. The deep anal intramuscular glands are found at the
anorectal junction.
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The
Submucosa (Muscularis mucosa)
In this region (Fig. 6), the layer underlying the columns
of Morgagni, the bulbus terminalis of the superior hemorrhoidal
arteries and veins ramify in a supportive syncytial network
of elastic and connective tissue, forming the internal or
superior hemorrhoidal plexus. This arrangement permits considerable
mobility of the area, and at the same time predisposes it
to arteriovenous dilation and redundancy. It contains so me
longitudinal muscle fibers and is particularly rich in lymphatic
and terminal nerve fibers and plexuses.
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The
Musculature of the Rectum
The rectum like the colon has inner circular and outer longitudinal
layers of muscle (Fig. 6). The inner circular layer has a
wing-like arrangement in which successive muscular bundles
sweep fan-like from the indentations, corresponding to the
valves over the lateral bulb-like expansions of the rectum.
At its lower extremity, about 3-4 cm from the anal margin,
the internal muscle becomes thicker and finally terminates
in the well defined internal anal sphincter, which is partially
encircled by the deeper layers of the external sphincter.
The outer longitudinal tunic of the rectum is formed by an
expansion of the colonic tinea at the termination of the sigmoid
colon. The expansion of the tinea forms a more or less diffuse
fibromuscular coat, which continues down on the rectum as
definite anterior and posterior banks. (Fig. 5) These become
more muscular at the anorectal junction and fuse with the
rectococcygeus, rectourethralis, and rectovaginalis. At the
levator-rectal junction, the longitudinal muscle joins with,
and is reinforced by the fibroelastic extensions from the
levator, forming the conjoined longitudinal muscle. The rectal
valves appear to be points of anchorage for muscle coordination.
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The
Serous Coat
The peritoneal coat (Fig. 6) is contained in this coat and
continues from the sigmoid, is reflected over the anterior
surface of the rectum, and into the interval between the bladder
and uterus, forming the rectovesical or uterine pouches. Laterally
the peritoneal folds are reflected diagonally upward and backward
to form the pararectal fossae and the leaves of the mesorectum
and sigmoid. Anteriorly, they form the paravesicular fossae.
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The
Rectal Valves
The rectal valves are crescentic plications, which have a
definite structure, including the circular muscle coat of
the bowel wall. In the strict sense they are not valves, and
the degree to which they are able to function as such, is
still debatable. Occasionally, the valves are absent.
The valves are quite variable in number, location and degree
of development. Usually there are three: an inferior, middle
and superior but occasionally there are five. The inferior
valve is usually located in the left posterior quadrant from
2.5 to 3.5 cm above the anal margin. The middle valve, usually
more prominent and more constant in location, is situated
over the base of the bladder or a little to the right, about
5-9 cm from the anal margin. It is referred to as the plica
transversalis of Kohlrousch. The superior valve lies 3-4 cm
above the middle valve.
Figure 7: Houston’s valves
According to Stanton, Houston described the rectal valves
which bear his name. These valves are reflections of the rectal
mucosa, which contain some fibers of the circular muscle coat
of the rectal wall. They lie obliquely and transverse to the
length of the rectum and project into the length of the rectal
lumen (Fig. 7). Their purpose seems to be, to serve as steps
or spiral supports to modify the flow of the feces as they
descend into the lower rectum.
There are usually three, sometimes four, Houston’s
valves. The lowest valve is located to the left of the midline
and extends somewhat anteriorly. It is a convenient landmark
in that it marks the usual limit of the downward reflection
of the peritoneum anteriorly. The next valve is on the right
side. This valve is said by some authorities to be on the
level of the pouch of Douglas in the female, and the rectovesical
pouch in the male.
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The
Third Sphincter (Sphincter of O’Beirne)
At a distance varying from 2-3 cm above the superior valve,
the lumen of the rectum decreases in caliber to conform to
that of the sigmoid (recto-sigmoid junction). At the site
of this narrowing, a more or less definite increase in the
circular muscular coat of the bowel is regularly described
as the third sphincter, or the Sphincter of O'Beirne, having
a special function in the act of defecation. It is not a true
sphincter, but similar in action.
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The
Sigmoid
The sigmoid is that portion of the large bowel that extends
approximately from the upper border of the left psoas muscle
to the proximal end of the rectum (Gorsch). It is of proctologic
interest because it is accessible to endoscopic examination.
The sigmoid may vary from 12-84 cm. Adults average 40 cm while
children average 18 cm. The mucosa and Submucosa differ only
from the rectum in that both are not so richly endowed with
vascular and glandular structure. The muscle layers consist
of an inner circular and an outer longitudinal tunic, which
follow the general arrangement of the colonic musculature.
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THE
EXTERNAL SPHINCTER
The external sphincter is formed by three striated muscles
(Fig. 8):
1. The Subcutaneous
2. The Superficialis
3. The Profundus
Figure 8: Coronal section showing anorectal muscles.
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The
Subcutaneous Muscle
This portion of the external sphincter (Fig. 9) is situated
immediately below the transitional anal skin (transiderm).
The bulk of the muscle is usually annular and disposed somewhat
to or on the same longitudinal plane with the internal sphincter.
It forms the lower wall of the anal canal. Occasionally it
presents small posterior extensions, continuous with the strong
converging legs of the superficialis muscle. Anteriorly, it
may decussate with the bulbocavernosis and the retractor scroti.
In the female anteriorly, it is continuous with the sphincter
vaginae.
The upper and inner margin is separated from the lower edges
of the internal sphincter by prominent insertions of the fibroelastic
extensions of the conjoined longitudinal muscle. This forms
the intersphincteric line. The subcutaneous muscle lies in
a septal network formed by the fibro-elastic muscle, and interweaves
with the subcutaneous, presenting support. These terminal
extensions into the skin form the corrugator cuis ani.
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The
Superficialis Muscle
Figure 9: Schematic drawing showing the posterior pull of
the superficialis muscle and the anterior pull of the puborectalis
muscle, in the conscious control of the rectal outlet.
This is an elliptical band of muscle fibers (Fig. 9), which
embraces the anal canal at the level of the internal sphincter.
It is the largest, longest and strongest portion. Arising
from the sides of the coccyx and forming the important muscular
component of the anococcygeal body, its diverging halves surround
the mid-portion of the anal canal. In the male anteriorly,
they converge and insert into the central tendinous raphe.
In the female, they diverge and fuse with the sphincter vaginae.
Anteriorly also, crossing fibers extend laterally into the
fascial shelf and attach to the ischial tuberosity and adjacent
fascia.
In both sexes, the anterior and posterior communicating spaces
extend directly above the superficialis fibers, and below
the profundus muscle.
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The
Profundus Muscle
This portion of the external sphincter (Fig. 9) is situated
immediately above the superficialis muscle. The fibers are
usually annular. Occasionally, uncrossed fibers extend posteriorly
to reach the anococcygeal ligament.
Anteriorly, the profundus forms the upper margin of the anorectal
muscle ring, but posteriorly the puborectalis muscle forms
the upper margin of this ring. The profundus lies in close
relation to the legs of the levator and a common crossed arrangement
of the entire muscle, extends to the opposite ischial tuberosity
on either side.
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THE
CORRUGATOR CUTIS ANI MUSCLE
This muscle (Fig. 8) represents the terminal insertions of
the fibro-elastic extensions of the longitudinal muscle into
the anal canal and perianal skin. The extensions seem to penetrate
the substance of the subcutaneous muscle as well as passing
on either side of this muscle.
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THE
INTERNAL SPHINCTER
The terminal portion of the circular muscle coat of the rectum
gradually thickens to become the component of the internal
sphincter. This muscle is surrounded by the superficialis
portion of the external sphincter and forms the en tire inner
muscular layer of the wall of the anal canal (Fig. 8). Immediately
below its lower margin, the internal sphincter is separated
from the upper border of the subcutaneous by the prominent
insertions of the longitudinal muscle, forming the intermuscular
septum of the intersphincteric line. Overlying the internal
sphincter is the pecten, covered by squamous epithelium with
subjacent areolar tissue containing lymphatics, crypts, preformed
anal glands, capillaries and nerves.
The anal canal measures from 1.5-2.5 cm anteriorly in length,
2-3 cm in length laterally, and 3-4 cm in length posteriorly.
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THE
LONGITUDINAL MUSCLE
An attenuation of the anterior and posterior longitudinal
bands, and tinea of the sigmoid form the longitudinal muscle
coat of the rectum, which spread out to surround the inner
circular muscle coat. At the anorectal junction, the longitudinal
coat becomes fibro-elastic in character, fuses with the levator
and fascial extensions and becomes the longitudinal muscle
(Fig. 10). This fascial arrangement fixes and protects the
anal canal and acts as a tendon sheathe for the divisions
of the anal musculature.
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MILLIGAN'S
SEPTUM
Milligan describes a septum of fascia, Milligan's Septum (Fig.
10), which extends from the lower border of the internal sphincter
muscle and turns outward below the superficialis and above
the subcutaneous muscles, to be inserted into the ischial
tuberosity and the skin. Posteriorly, the septum is incomplete.
Figure 10: The intermuscular sulcus.
According to Stanton, The intermuscular sulcus (Fig. 10)
readily palpated in the anal canal is located at the level
of Milligan’s septum, in the space between the subcutaneous
and internal sphincter muscles. It is a depression formed
by the retraction of the skin of the canal by the pull of
the insertions of the conjoined longitudinal muscle. It encircles
the canal and serves as a landmark in diagnosis and treatment.
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THE
ANORECTAL MUSCLE RING
The levator ani muscle, in conjunction with the profundus
portion of the external sphincter, forms a combined musculo-fascial
ring (Fig. 11), which completely surrounds the anorectal junction.
Figure. 11: This drawing shows the muscles, superficialis,
and subcutaneous
pulled laterally to show the profundus above and the levator
plate.
Posteriorly, the puborectalis division of the levator ani
reinforces the anal canal, and this forms the posterior and
upper margin of the anorectal ring.
Anteriorly, it decreases until there remains only a thin
sheath of reflector levator fibers called the Junction of
Luschka. In the anterior quadrant, only the profundus portion
of the external sphincter forms the anorectal muscle ring.
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THE
ANOCOCCYGEAL LIGAMENT OR BODY
This is a firm composite musculo-fascial structure extending
from the posterior aspect of the anal canal to the tip and
sides of the coccyx (Fig. 12), on the lower sacrum. Into it
fuses the strong insertions of the glutei muscles, the ischiococcygeus,
the pubococcygeus, and the puborectalis muscles, (all at different
levels) the superficialis fibers, and finally, the terminal
posterior extensions of the combined longitudinal muscle and
superficial fascia.
Inferiorly it is bounded by the skin. Superiorly it fascial
stratum is the supra anal fascia, which supports the rectal
ampulla.
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THE
LEVATOR ANI MUSCLE
The levator ani is comprised of many muscular coalescing parts
(Fig. 12). Four principal paired musc1es forming the levator
are described as the pubococcygeus, the puborectalis, the
iliococcygeus and ischiococcygeus.
Figure 12: The vertical dotted line illustrates the plane
relationship of the
anterior limit of the pubis with the anterior, superior spine
of the ileum.
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The
Pubococcygeus Muscle
The pubococcygeus (Fig. 9) has as its origin, a common origin
with the puborectalis, the posterior surface of the pubic
arch and the arcus tendineus. The main portion of the pubococcygeus
continues posteriorly along with the puborectalis and is interlaced
to a point of being inseparable, until the pubococcygeus passes
around the rectum and continues to its insertion into the
anococcygeal body, the coccyx, and the lower sacrum. Fibers
of pubococcygeus intermingle with the fibroelastic extensions
of the conjoined longitudinal muscle, to become part of the
corrugator cutis ani.
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The
Puborectalis Muscle
This muscle (Fig. 9) arises practically in common with the
pubococcygeus, but on a slightly lower plane. Owing to the
fact that the puborectalis and pubococcygeus, as they pass
posteriorly, the fibers intermingle and continue to do so
until the puborectalis begins to swing on the posterior side
of the rectum, to encircle the rectum and become part of the
anorectal muscle ring. Damage to the puborectalis, more than
any other of the anorectal muscle ring, may result in fecal
incontinence.
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The
Iliococcygeus Muscle
The Iliac portion of the levator (Fig. 11) arises from the
fascial covering of the obturator internus muscle and is directed
posteriorly and medially, converging somewhat with the pubococcygeus
as they conjointly insert into the coccyx and lower sacrum.
This muscle supports the anorectal shelf in the act of defecation.
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The
Ischiococcygeus Muscle
This muscle (Fig. 11) is covered with the same fascial planes
from the pubococcygeus and iliococcygeus. It originates from
the ischial spine and adjacent sacro-iliac fascia. It attaches
to the coccyx, the lower sacrum and the median portion of
the sacro-tuberous ligament. The combined levators fix the
pelvic structures and present a fulcrum against which increased
abdominal pressure may be exerted in the acts of lifting,
coughing, defecation, urination, coitus, and various other
activities.
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THE
PELVIC TRIANGLES
The pelvic outlet is commonly divided into the anterior and
posterior triangles by an interischial line. The anterior
triangles are the urogenital and urethral or genital. The
posterior triangles are the anal, ischiorectal or ischioanal.
The anal and urethral triangles embrace all the structures
of the pelvic floor, situated between the peritoneum and the
skin.
The urogenital triangles, according to Gorsch, contain the
following three superimposed musculofascial planes: 1) The
superficial group, containing the bulbocavernosis, (accelerator
urinae, sphincter vaginae and bulbospongiosus), the ischiocavernosis
muscle and the superficial transverse perineal muscle. 2)
The middle group, containing the deep transverse perineal
muscle and the sphincter urethrae membraneous muscle. 3) The
deep group, containing the pelvic portions of the puborectalis
and pubococcygeus muscles.
The anal triangles contain the following musculature: The
external and internal sphincters, the levator ani with the
ischiococcygeus muscle, making up the pelvic diaphragm, and
the pyriformis muscle. It should be emphasized here that the
deep muscular strata are closely interrelated with the sphincters.
By supporting and fixing the pelvic diaphragm, the levators
coordinate the activity of the sphincters.
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THE
PERINEOPELVIC SPACES
The perineopelvic spaces with their contained structures are
directly concerned in the surgical therapy of hemorrhoids,
fissure and fistulae. In several instances, the spaces are
merely planes of cleavage between closely opposed fascial
ensheathments of adjacent organs, as the prerectal, posterior
prostatic, retrorectal, or rectovaginal spaces. These are
all significant in that they are routes of infectious extensions.
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The
Perianal Space
According to Gorsch, this space surrounds the anus and the
lower third of the anal canal. Its conformation depends upon
the distribution of the fibro-elastic extensions of the conjoined
longitudinal muscle. Below the lower margin of the internal
sphincter, a prominent group of these extensions insert into
the anal canal as the intermuscular septum. From this septum
the extensions continue downward below the lining of the anal
canal into the perianal skin. Here they fuse with the lateral
extensions passing external to the subcutaneous muscle. The
inner and outer extensions roughly bound a space, which contains
the subcutaneous muscle and the external hemorrhoidal plexus
of veins with their supporting areolar network. This is the
perianal space. Laterally, this space is continuous with the
ischiorectal fossa. Posteriorly, it is designated as the post-anal
space.
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The
Submucous Space
Directly above the anorectal line is the internal hemorrhoidal
plexus of veins, which occupies the submucous space, extending
proximally to the upper part of the columns of Morgagni. This
space contains the veinous radicals, a well-marked muscularis
with loose areolar tissue, lymphatics, and arterial and venous
capillaries. Th: pecten lies between the submucous and perianal
spaces. This space is particularly important in hemorrhoidal
formation.
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The
Ischiorectal Fossae
The conformation of the ischiorectal fossae (Fig. 13) depends
upon the disposition of the levator ani muscle, which forms
the inner wall and roof of the ischiorectal fossae. Anteriorly,
the fossae are bounded by Colle's fascia and the extensions
of the fascial shelf along the posterior aspect of the superficial
perineal pouch and the triangular ligament posteriorly, the
boundary of these fossae is formed by the gluteus maximus
muscle and the sacrotuberous ligament.
In its posteriomedial angle, the fourth sacral nerve traverses
the fossae for a short distance. The perforating branches
of the second and third sacral nerves leave the fossa below
the gluteus, about midway between the coccyx and the ischium.
Posteriorly, a thin layer of fascia separates the two fossae.
Communication between them usually passes directly behind
the profundus. The deepest portion of the fossae is in the
area of the ischial spines.
Inferiorly, the skin, reinforced by Milligan’s Septum
forms the roof of the fossae. It is either incomplete posteriorly
or there are anomalous defects in the area which permit entrance
of infection into the fossae. Posteriolaterally is the vascular
pedicle containing the inferior hemorrhoidal artery, veins
and nerves.
The lateral walls of the fossae are formed by the fascia
of the obturator internus muscle. Behind the obturator fascia,
surrounding the pudendal artery, vein and nerve is the old
terminology, "Alcock's Canal" (Fig. 5).
The fossae are smaller, narrower and deeper in the male.
The average fossae are from 6-8 cm anteroposteriorly, 2-4
cm in width, and 6-8 cm in depth. The contents then are: inferior
hemorrhoidal veins and nerves crossing transversely; posteriorly,
the perineal and perforating branches (cutaneous) of the pudendal
plexus, and anteriorly, the posterior scrotal or labial vessels
and nerves.
Figure 13: Ischiorectal fossae and pararectal spaces.
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The
Supralevator Spaces (pararectal)
According to Stanton, the supralevator spaces (Fig. 13) lie
on either side of the rectum, above the levator and below
the peritoneal reflections of the abdominal cavity. These
spaces are protected from infections by formidable fascial
barriers. As the levator muscles join with the muscles of
the rectal wall, the fascia blends with the rectal fascia.
The supralevator and infralevator fasciae are thus continuous
with the fascial coverings of the rectum. The supralevator
spaces extend anteriorly to the prostate and seminal vesicles
in the male, and to the uterosacral ligaments in the female.
Infections in both sexes vary according to the conformation
of the spaces.
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The
Retrorectal Space (Pre-sacral)
This space lies posterior to the rectum and anterior to the
sacrum and coccyx (Fig. 14). It is lined anteriorly by rectal
fascia, posteriorly by the fascia of the sacrum and pyriformis
muscle, and inferiorly by the fascia of the structures of
the rectal shelf. The retrorectal space is separated from
the supralevator spaces by strong fascial concentrations,
the rectal stalks. It is a potential space and has the potential
of large capacity for infectious processes.
Figure 14: Retrorectal space.
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The
Rectovaginal Space
It is more or less a septum than an actual space, extending
from the vesicovaginal peritoneal fossa above, to the perineal
body be low. This space is filled with a thin reflection of
superficial fascia which forms a line of cleavage between
the rectum and the vagina.
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The
Anococcygeal Triangle
The anococcygeal triangle (Fig. 15), as described by Stanton,
lies in the sagittal plane posterior to the anal canal, anterior
to the coccyx below the levator shelf, and above the skin
of the gluteal cleft. This triangle is in the midline and
between the posterior extensions of the pubococcygeus and
the superficialis, and some posterior extensions of the puborectalis
muscles. The vertical triangle is outlined as follows: A line
drawn from the tip of the coccyx anteriorly to the midline
point at the posterior margin of the profundus muscle; from
this point, a second line extends downward in midline to a
point immediately posterior to the posterior encircling portion
of the subcutaneous muscle; the third side of the triangle
is on a line joining the latter point with the tip of the
coccyx. Gorsch describes the anococcygeal triangle as the
posterior triangular space, which is divided into a superficial
and a deep portion. The deep portion is situated above the
superficialis, below and behind the puborectalis and profundus,
and below the levator plate. It extends posteriorly to the
coccyx and it is at point at which the ischiorectal fossae
communicate through the posterior communicating space.
Figure 15: Anococcygeal triangle.
The anococcygeal triangle should not be confused with Minor’s
triangle (Fig. 16), which is a horizontal triangle of the
skin.
Figure 16: Minor’s triangle.
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THE
BLOOD SUPPLY TO THE ANORECTAL REGION
Gorsch describes the arterial supply to the anal canal and
the anal musculature as being somewhat distinct from that
of the rectum, and to avoid confusion, separate discussion
is necessary.
In general, the inferior hemorrhoidal artery supplies the
posterior and the lateral aspects of the anal musculature
and adjacent integument by superficial and deep branches,
while a separate transverse perineal branch, usually arising
independently from the pudendal, supplies the anterior aspect.
Terminal branches of the above vessels supply the musculature
at different levels, reaching the subepithelial areolar tissue
of the anal canal (the pecten) and the annulus hemorrhoidalis
of the rectum, anastomosing with the terminal radicals of
the superior hemorrhoidal vessels and to a lesser extent,
with the middle hemorrhoidal or inferior vesicle arteries.
The larger branches of the vessels follow the fascial planes
and define the separate divisions of the external sphincter.
The inferior hemorrhoidal artery, arising from the pudendal
(Fig. 17), crosses the posterolateral aspect of the ischiorectal
fossa, en-sheathed in a reflection of the superficial pelvic
fascia, which also includes the vena comites, a few lymphatics,
and sometimes nerves (hemorrhoidal stalks). There may be one
or more branches direct from the pudendal in addition to the
main branches described as the inferior hemorrhoidal.
The inferior hemorrhoidal commonly divides into three main
branches. The first, directed posteriorly and upward, to supply
the adjacent portions of the obturator internus, gluteus maximus
and levator ani muscles; the second, directed toward the posterior
aspect of the anal musculature, gives off three smaller branches,
one supplying the anococcygeal ligament and its musculature,
a second supplying the posterior aspect of the anorectal musculature,
and reaching the anal canal in the interval between the subcutaneous
and the superficialis muscles just lateral to the posterior
commissure; the third and largest branch is directed to the
mediolateral aspect of the anal sphincters, supplying them
at different levels by smaller branches, which finally reach
the subepithelial musculature and anastamose with the branches
of the superior and middle hemorrhoidals.
The anterior transverse perineal branch is usually a separate
vessel, arising from the pudendal, conveniently termed "the
anterior sphincterian artery". This vessel branches from
the pudendal just before the latter gives off the perineal
branch, which reaches the superficial perineal pouch, while
the main trunk of the pudendal continues between the layers
of the triangular ligament and deep perineal pouch, to supply
the penis. The perineal artery reaches the anterior aspect
of the anal canal, supplies the musculature and anastamoses
with the anterior terminals of the inferior hemorrhoidal artery.
The middle sacral artery, coursing down the medial aspect
of the sacrum behind the deep fascia, reaches the anococcygeal
body, and supplies it and the coccyx, finally ramifying in
the skin over the coccyx and the coccygeal ligament.
Figure 17: Blood supply of the anorectal area. Fascia propria,
not
shown here, encases the superior hemorrhoidal vessels (Fig.
5).
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THE
VASCULAR SUPPLY TO THE RECTUM
The vascular supply to the rectum is significant in the conception
of internal hemorrhoids. The inferior mesenteric artery, below
its last sigmoid branches, continues to the rectum as the
superior hemorrhoidal (Fig. 17). The sigmoid branches are
important in number and distribution. The superior hemorrhoidal
artery accompanied by the same vein, runs in the subserous
fascia of the mesorectum. Reaching the rectal wall at about
the level of the second sacral vertebra, it divides into right
and left main branches, which extend to the fascia propria
of the rectum, but finally pierce the rectal wall to reach
the submucosa.
After its main division, the right and left branches give
off several secondary lateral branches, which ramify around
the upper rectum, and piercing its musculature, reach the
submucosa where there it is free to anastamose between their
terminal capillaries and those of the corresponding veins.
The main branch of the right side continues downward and
penetrates the rectal musculature about two inches above the
anorectal line. Reaching the submucosa, it divides into an
anterior and a posterior branch. The anterior continues to
the annulus hemorrhoidalis undivided, while the posterior
branch, after reaching the annulus hemorrhoidalis of the posterior
quadrant, divides into smaller secondary branches, one of
which, reaches the mid-lateral aspect and the other, the posterior
midline of the annulus hemorrhoidalis.
The left main branch also pierces the rectal wall with its
accompanying veins, and reaching the submucosa, divides into
two fairly large terminals, an anterior branch reaching the
annulus hemorrhoidalis in its left anterior quadrant, and
one posterior, reaching the annulus hemorrhoidalis in the
left posterior quadrant.
The vena comites follow the arterial distribution closely,
and in the terminal rectum, their combined arteriovenous anastamotic
capillary network, together with the inosculation from the
inferior plexus, and to a slight degree from the middle hemorrhoidal
vein, form the important internal or superior hemorrhoidal
plexus.
Therefore, the primary internal hemorrhoids formed in the
plexus are fairly constant in relation to the terminations
of the larger arterial bundles. Well-developed internal hemorrhoids
are then situated usually in the right anterior, right posterior
and left lateral areas.
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THE
VENOUS SUPPLY OF THE RECTUM
Although the veins follow essentially the same course and
give off branches corresponding to the arteries, they require
additional description, because of their relation to the formation
of hemorrhoids and external or perianal hematomata. At the
anal verge, the inferior hemorrhoidal veins are prominent
and form the indefinite plexiform arrangement, which is commonly
referred to as the external hemorrhoidal plexus. Terminal
branches from this plexus, anastamose in the pecten of the
anal canal with the radicals of the venous plexuses of the
superior hemorrhoidals. This anastamosis seems to be of minor
consequence clinically.
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THE
LYMPHATICS
According to Gorsch, from the practical and clinical viewpoint,
the lymphatics of the perineum, anal canal and rectum should
be considered as a loose system in which free anastamosis
and overlapping occur.
Figure 18: The lymphatic vessels of the anorectal area.
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Anorectal
Lymphatics
These are divided as follows:
1. Perianal Group
2. Anorectal Group (anal and rectal portions)
3. Extrarectal Group
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Perineal
Group (Cutaneous System or Inferior)
This group drains the superficial and deep layers of the perineal
skin (No. 1, Fig. 18). The afferent collecting trucks follow
in general, the perineoscrotal or labial folds, and terminate
in the inferolateral group of inguinal nodes. There is much
overlapping with the lumbar and gluteal plexuses.
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The
Anorectal Group
The Anal Portion: Three plexuses of the anal canal and the
sphincteric portion of the rectum are demonstrable. They are
the mucosal, submucosal and intermuscular.
The mucosal and submucosal sets (No. 2, Fig. 18) are continuous
with those of the rectum proper and extend through the anal
canal to anastamose with the perineal plexus, draining into
the inguinal nodes. The submucosal ramifications are particularly
rich in the columns of Morgagni, and there are probably both
afferent and efferent channels from these zones.
The intermuscular plexus lies between the circular and the
longitudinal muscle coats and extends into the adjacent ischiorectal
fossae. Much overlapping is present at the level of the internal
sphincter, making the true anatomic course of a single plexus
very difficult to follow. Lymph flow may be reversible.
The Rectal Portion: The rectal portion of the anorectal group
is similar to that of the sigmoid with the same division of
plexuses and free anastamosis. These plexuses communicate
with the extrarectal group of lymphatics. The lymph sinus
of the rectum and the submucous plexus bears an important
relation to the pelvic superficial fascia in the supralevator
and retrorectal spaces. Afferent and efferent lymphatic channels
from these plexuses reach the extensive network of the extrarectal
group.
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The
Extrarectal Group
The downward zone includes afferent lymphatics, which arise
in the anal canal and rectal ampulla, and extending lateralward
and downward, may involve the anal sphincters or tissues of
the ischiorectal fossae. They spread along the ramifications
of the inferior hemorrhoidal vessels, to reach the obturator
fascia and the hypogastric nodes (No. 4, Fig. 18). The perianal
skin may al so be considered an important location in the
lower zone.
The lateral zone embraces the entire ramifications of the
subserous or superficial fascia between the pelvic wall, the
levators, and the peritoneum above.
The upward zone includes the lymphatics and tissues in the
retrorectal space with the important Nodes of Gerota (No.
3, Fig. 18), from which secondary lymphatics extend to those
of the mesorectum and sigmoid, and drain into the intercolated
nodes or those of the paracolic, iliac, and aortic groups.
The majority of the lymphatics from the rectal lymph sinus
either pass through or drain into nodes contained in this
zone, and is therefore, most important from the standpoint
of metastasis of cancer.
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THE
NERVE SUPPLY TO THE ANORECTAL REGION
In general, the nerve supply is divided into two systems;
the somatic or cerebrospinal system, which supplies nerve
innervation to skeletal muscle, and nerve sensory innervation
to joints, skin, and scalp, and the autonomic system. The
autonomic system is subdivided into three groups; the medullary
group (craniobulbar), the thoracolumbar, and the sacral group.
Gorsch states that the thoracolumbar is sympathetic, while
the medullary and the sacral subgroups are parasympathetic.
Figure 19: Schematic drawing showing the sympathetic
and parasympathetic nerve supply to the sigmoid.
From a practical viewpoint, the sympathetic portion stimulates
smooth muscle, causing vasoconstriction, contraction of the
sphincters, and inhibition to peristalsis. In the urinary
bladder and the rectum, it causes contraction of the sphincters,
and inhibition of the detrusor musculature.
The medullary (craniobulbar) and the sacral outflows are
secretomotor to the glands, motor to the musculature of the
gut, and inhibitory to the cardiac, pyloric, and ileocecal
sphincters, largely through the vagus. Afferent fibers convey
vesicle sensibility from the alimentary tract to the ganglion
nodosum of the vagus. The sacral outflow of the parasympathetics
supplies motor fibers to the musculature of the distal colon
and the rectum, the rectosigmoid junction and possible inhibitory
fibers to the internal anal sphincter. The sympathetic distribution
is similar to the cerebrospinal nerves.
In the pelvis, the essential sympathetic supply consists
of the hypogastric plexus, which is subdivided into the superior,
middle, and inferior plexuses.
The superior plexus is commonly referred to as the presacral
nerve. It is formed by the continuation of the intermesenteric
nerves, supplemented by branches from the lumbar sympathetic
chain. It extends from the bifurcation of the aorta to the
level of the promontory of the sacrum. Continuing into the
pelvis, the plexus divides into the hypogastric branches (the
middle plexus), which follows the hypogastric arteries, and
terminates in the large: secondary plexus (the inferior plexus).
This is situated at the level where the superior hemorrhoidal
artery leaves the hypogastric in the lateral aspect of the
pelvirectal space. Small secondary plexuses continue and join
with the ganglionated chain along the sacrum and coccyx, to
form the Ganglion Impar. Fibers from the inferior mesenteric
plexus mingle with the above.
Parasympathetic (Nervi erigentes) reach their pelvic destination
via the second, third, and fourth sacral spinal nerves. So
far as the actual termination of these fibers can be determined
in the visceral walls, it appears that the sympathetics terminate
in the glands, while the parasympathetics end in small ganglia
in the muscularis of the viscera.
Innervation of the anal canal and its musculature is derived
from the cerebrospinal sympathetics and parasympathetics (Fig.
19).
Cerebrospinal innervation, sensory and motor, is the second,
third, and fourth sacral and smaller coccygeal spinal filaments.
These are distributed by the peripheral nerves, which reach
the anal musculature and squamous lining of the anal canal
and surrounding tissues through the inferior hemorrhoidal
nerve, the anterior sphincterian nerve, perineal branches
of the fourth sacral nerve, and the coccygeal filaments.
Inferior hemorrhoidal nerves are branches of the pudendal
(Fig. 20) and follow the distribution of the arterial branches.
They supply the three divisions of the external sphincter
and the terminal filaments in the skin, or continue to the
anorectal junction.
Figure 20: Pudendal nerve and branches. Fourth sacral nerve.
The anterior sphincterian nerve arises independently from
the pudendal, goes transversely and supplies the anterolateral
aspect of the anal musculature and adjacent tissues.
The perineal branch from the fourth sacral nerve pierces
the coccygeus muscle and innervates the perianal skin (Fig.
21).
Figure 21: Diagram showing areas supplied by the sacral nerves.
The coccygeal spinal filaments are distributed anteriorly
and posteriorly. The posterior ramifies over the coccygeal
skin and inosculates with the dermal branches of the fourth
sacral. The anterior filaments ramify in the anococcygeal
body and skin.
Sympathetic innervation to the lower rectum and anal canal,
and its musculature is mainly from three sources:
1. From the superior hypogastric plexus (Presacral Nerve)
via the inferior hypogastric plexus, sympathetic fibers follow
the branches of the middle hermorrhoidal arteries to be distributed
around the anorectal junction. Visceral branches from the
spinal nerves, also reach the inferior and superior hypogastric
plexuses and have similar distribution (Viscerosacral nerves).
2. Sympathetic fibers from the inferior mesenteric plexus
are also distributed with the terminal branches of the superior
hemorrhoidal arteries, reaching particularly the annulus hemorrhoidalis
of the lower rectum, and continuing through the pecten to
reach the anal canal and perianal skin.
3. Sympathetic fibers, which are also carried by the inferior
hemorrhoidal nerves, are parietal sympathetics arising from
the sacral and coccygeal ganglionated cord, which supply the
glands and vessels of the skin surrounding the anal canal.
The terminal distribution of the sympathetics to the rectum
consists of the two plexuses of Auerbach and Meissner.
Figure 22: Schematic drawing showing Auerbach’s
and Meissner’s plexuses in the wall of the bowel.
The plexus of Auerbach (intermuscular plexus) ramifies between
the circular and longitudinal musculature of the rectum (Fig.
22), and continues in the conjoined longitudinal muscle to
reach the perianal skin (sympathetic).
Figure 23: Longitudinal section of the rectal wall.
The plexus of Meissner (submucous plexus) is distributed
through and innervates mainly the glandular apparatus of the
mucosa (Fig. 23). It ramifies in the submucosal layer of the
rectum and is disposed around and between the glands, continuing
down to reach the sweat, sebaceous, and apocrine glands of
the perianal skin (parasympathetic).
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NEUROPHYSIOLOGY
(Defecation Reflex)
Physiologically, Gorsch states that defecation is best explained
on the basis of a modified somatic autonomic reflex, normally
under cortical control, and in which the desire to defecate
may be conveniently distinguished from the act of defecation.
The so-called "trigger zones" at which the initial
sensory stimuli arise and produce the desire to defecate,
are probably in the rectal musculature as well as in the anorectal
line, which is the more important trigger zone. Threshold
stimuli arise normally from the anorectal junctional area
and are conveyed by the spinal sensory nerves, to initiate
the active phase of defecation. The distention of the rectal
wall also gives rise to some extent, to the desire to defecate
through the sympathetic afferent nerves. This results reflexly,
in a relaxation of the anal sphincters, particularity the
internal, and a contraction of the rectal musculature. The
act may be inhibited by the will.
On the other hand, voluntary relaxation of the anal sphincters
with voluntary contraction of the colon and its complimentary
muscles, with the expulsion of the rectal contents, is the
actual act of defecation.
In adult life, defecation is no longer a reflex, but normally
becomes a voluntary act, once the summation of sensory stimuli
is effected. It becomes a purely reflex act, however (sympathetico-parasympathetic),
in the autonomic innervated rectum, following destruction
of its cerebral connections.
The broad subject of constipation is directly related to
the sensorimotor response of the entire gastrointestinal tract
as well as those of the rectum.
"Trigger zones" may be entirely extrarectal and
in pathologic conditions, provoke a constant tenesmus leading
to rectal prolapse. Further, the sensory and motor dispersions,
before, after, and during the act of defecation are complex
and may be reflected throughout the entire nervous system,
e.g., fainting, abdominal cramping, orgasms, and neurocirculatory
phenomena, are common clinical observations.
Defecation may also be entirely a cortical response. Central
stimulation of the Vagus produces the defecation reflex, a
contraction of the rectum and a relaxation of the anal sphincters.
In this regard, it may be observed that the segmental movements
of the intestines are considered myogenic in origin, and the
intrinsic plexuses of Meissner and Auerbach control that peristalsis
(Fig. 22). The autonomic system (sympathetic and parasympathetic)
subserves a regulatory function. Diarrhea may be entirely
an intrinsic myogenic basis.
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BIBLIOGRAPHY
Noll, Carlton M. Procto-Basics. Medicus Publications, 1978,
pp. 07-44.
Stanton, Frank D. Newer Concepts in Clinical Proctology.
Clinton Massachusets: The Colonial Press Inc., 1958, pp. 05-45.
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