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TOPOGRAPHIC ANATOMY OF THE PERINEUM.
THE SOME OPERATIONS
ON THE ORGANS OF THE PELVIC CAVITY AND
PERINEUM
The perineum is the region bounded by the pelvic outlet that lies below the pelvic
diaphragm. It follows from this that while the pelvic diaphragm forms the floor of the pelvic cavity,
it also forms the roof of the perineum. Also, because the floor of the pelvic cavity slopes downward
and medially, the roof of the perineum will do likewise. Thus, the perineum will be relaÂtively
deep laterally and shallow as the midline is reached. Furthermore, a number of structures must
either pass through the pelvic floor to reach the perineum (alimentary, urinary, and geÂnital
systems) or circumnavigate it (pudendal nerves and vesÂsels).
Most anteriorly is the arcuate pubic ligament lying below the symphysis pubis. This
boundary is continued laterally along the ischiopubic ramus to the ischial tuberosity and is
completed between the tuberosity and the coccyx by the sacrotuberous ligaÂment. It is
conventional to divide this diamond shape into an anteriorly situated urogenital triangle and a
posteriorly situaÂted anal triangle by drawing an imaginary line through the iscÂhial tuberosities.
Although this procedure simplifies descriptiÂon, it should not be forgotten that the two triangles
communicaÂte freely.
Next observe the boundaries of the fossa when seen from the lateral aspect. Note how the
diamond is folded about the ischial tuberosities and that the plane of the urogenital triangle forms
an angle with the plane of the anal triangle. The attachment of the roof of the perineum, i.e., the
levator ani, to the lateral wall of the pelvis is also indicated in the illustration. It can now be seen
that the depth of the perineum is minimal anteriorly and posteriorly and maximal in the
intermediate region. The peÂrineum is limited inferiorly by skin and fat-filled superficial fascia.
The anal triangle
The anal triangle contains the two ischiorectal fossae which are separated by
the anal canal and adjacent connective tissue (the fossae have been more accurately
named "ischioanal" in the most recent edition of Nomina Anatomica). The connective
tissue comprises the anococcygeal ligament posteriorly and the perineal body anteriorly.
The Anal Canal
The rectum ends at the floor of the pelvic cavity where it is partly encircled by the
puborectalis portion of the levator ani muscle. From this point the anal canal extends downward
and backward for about one and a half inches to terminate at the anus. In this way quite a sharp
angle is formed at the anorectal junction and this can be appreciated from the drawing in pictuÂre.
The canal is normally empty and flattened from side to side. On filling with feces it may
expand by displacing fat in the ischiorectal fossae which lie lateral to it. The upper part of the canal
is derived from the endoderm-lined cloaca and the loÂwer part from the ectoderm-lined
proctodeum, and this difference in origin is reflected in differences in the epithelial lining, blood
supply, lymphatic drainage, and nerve supply of each part.
The Lining of the Anal Canal
A number of features of the lining of the anal canal that are described can only be clearly
recognized in the living young person and are often difficult to demonstrate in the older laboÂratory cadaver. In its upper part the canal is lined by a columÂnar epithelium containing mucussecreting goblet cells and is similar to that found in the rectum. The mucous membrane in this
region is thrown into vertical folds called the anal columns and that the columns are joined at their
lower ends by horizontal folds which form the anal valves. The recesses which lie between the
columns and the valves are called the anal sinuses. Into the sinuses open a number of anal
glands which he in the submucosa or surrounding muscle. Obstruction and infection of these
glands may produce fistulae and painful abscesses.
The lower margins of the anal valves together form the pectinate line and below this there
is a zone of tranÂsitional stratified epithelium. This is limited below by the white or anocutaneous
line where the lining of the canal becomes true skin.
The Muscular Wall of the Anal Canal
The mucous membrane of the canal is surrounded by a muscuÂlar wall composed of the
sphincter ani internus and the sphincÂter ani externus. The arrangement of these two sphincters is
ilÂlustrated in picture.
The sphincter ani internus is a somewhat thickened continuÂation of the circular smooth
muscle of the rectum. It surrounds the upper three-quarters of the anal canal and terminates at
the level of the white line.
The sphincter ani externus is formed from three parts named subcutaneous, superficial, and
deep. Unlike the internal sphincÂters all are striated muscles and, therefore, under voluntary
control. The subcutaneous part is indeed subcutaneous and surroÂunds the anal opening. The
superficial and deep parts, which lie above it, are separated from the anal mucosa by the internal
sphincter and a layer of fibroelastic tissue continuous above with the outer longitudinal smooth
muscle coat of the rectum. Below, this coat breaks up into a number of septa which pass through
the subcutaneous part to become attached to the skin of the anal margin.
The superficial part is attached posteriorly to the coccyx by means of the anococcygeal
ligament and anteriorly to the peÂrineal body. The deep part is fused with the puborectalis portiÂon of levator ani. The external sphincter is supplied by the peÂrineal branch of the fourth sacral
spinal nerve and by the inferior rectal branch of the pudendal nerve (S2 and S3).
Voluntary contraction of the external sphincter is used to delay defecation initiated by the
filling of the rectum with feÂces. Damage to the important "anorectal ring" formed by the deep
part of the external sphincter, puborectalis, and the internal sphincter, either surgically or in
childbirth, may lead to inÂconti nence of feces.
The Blood Vessels of the Anal Canal
In keeping with its origin from the hind gut, the mucous membrane of the upper part of the
anal canal is supplied by the inferior mesenteric artery through its continuation the superior rectal
artery. Terminal branches of this vessel pass from the rectum to the anal canal between the
muscular and mucous coats to anastomose with branches of the middle rectal artery and supply
the muscular coat of the upper part of the canal.
The middle rectal artery is a branch of the internal iliac artery. The inferior rectal artery
arises from the internal puÂdendal artery on the lateral wall of the ischiorectal fossa, crosses the
fossa and, reaching the anal canal, supplies both muscle and the skin which lines the lower part of
the canal. It anastomoses with its fellow and with the middle and superior rectal arteries.
The venous drainage of the anal canal is to an internal veÂnous plexus, in the submucosa
and an external plexus outside the muscular coat. While the internal plexus drains mainly to the
superior rectal veins and thence to the inferior mesenteric veÂins, both plexuses are also drained
by the inferior rectal veins which are tributaries of the internal pudendal veins.
Middle rectal veins drain the muscular coat of the upper part of the anal canal. The internal
venous plexus is, therefoÂre, a site of a portocaval venous anastomosis and dilatation of the veins
of the plexus may ac company portal hypertension. HoÂwever, perhaps because of little support
from surrounding tissue
or the absence of valves in the inferior mesenteric veins, dilaÂtation of these veins, known
as hemorrhoids or piles, occurs much more frequently in the absence of any general rise in porÂtal
venous pressure .
The Lymphatic Drainage of the Anal Canal
The lymphatic vessels draining the upper part of the anal canal pass upward to nodes
alongside the rectum and sigmoid coÂlon, which in turn drain to preaortic nodes. Vessels from the
lower part, but above the mucocutaneous junction, drain to internal and common iliac nodes. The
skin lining the lower part draÂins to the most medial superficial inguinal nodes and an enlarÂged
or painful node here should always prompt a rectal examinaÂtion.
The innervation of the Anal Canal
As might be suspected, the lining of the upper part of the anal canal is supplied by the
autonomic system while the skin of the lower part is supplied by somatic spinal nerves in the form
of the inferior rectal nerves which are branches of the pudendal nerve.
The Examination of the Anal Canal
The anal canal cannot be left without mention of the imporÂtance of carrying out a digital
examination whenever it is indiÂcated. Not only will gross local lesions be revealed, but the
condition of the sphincters can be assessed and in the male the prostate, whether enlarged or not,
can be felt through the anteÂrior rectal wall. In the female this examination may be used as an
alternative to a vaginal examination. The cervix can be palÂpated through the anterior wall of the
rectum and other enlarÂged, abnormal, or painful pelvic structures detected. The examiÂnation
can be completed by direct observation through a proctosÂcope.
The Ischiorectal Fossae
The ischiorectal fossae lie on either side ofÂ
theÂ
midline of
the perineum. Each is shaped rather like a segment of an orange having a sharp linear
upper border and a curved lower surface which narrows as it approaches the upper border
anteriÂorly and posteriorly.
Note the following features:
l. The sharp upper border is formed where the levator ani muscle and the inferior fascia of
the pelvic diaphragm meet the obturaÂtor internus muscle and overlying obturator fascia.
2. The sloping roof and medial wall are formed by the levator ani muscle and the anal canal
which is surrounded by the interÂnal and external sphincters.
3.The lateral wall is formed largely by obturator internus and the obturator fascia as they
span the obturator foramen.
4. Below obturator internus, the lateral wall is completed by the medial surface of the
ischial tuberosity.
5. Between obturator internus and the ischial tuberosity the puÂdendal canal containing
the internal pudendal vessels and the pudendal nerve has been sectioned transversely.
6. The space is filled with coarsely loculated fatty tissue, but note that the fibroelastic septa
derived from the external lonÂgitudinal smooth muscle coat of the rectum separate this region
from the typical subcutaneous fat underlying the skin around the anus. This is the perianal space
which contains the subcutaneous portion of the external sphincter and the external anal venous
plexus. Posterior to the section illustrated the two fossae are separated by the anococcygeal
ligament and anterior to the secÂtion by the perineal body. Both posteriorly and anteriorly the
fossae narrow as the floor meets the upper border and an anteriÂor extension, which extends into
the urogenital triangle, will be encountered again when that region is described.
The Pudendal Canal
The internal pudendal vessels and the pudendal nerve leave the pelvis through the greater
sciatic foramen and enter the perineum through the lesser sciatic foramen below the level of the
levator ani. On leaving the lesser sciatic foramen these structurs come to lie on the medial surface
of obturator internus below the level of the attachment of levator ani, where they are enclosed by
a discrete sheath of fascia. This is the pudendal canal and it carries the vessels and nerves forward
toward the urogenital triangle at the posterior margin of the perineal membrane. The inferior rectal
artery and nerve leave the interÂnal pudendal artery and pudendal nerve as the pudendal canal
passes across the lateral wall of the ischiorectal fossa.
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THE UROGENITAL TRIANGLE IN THE MALE
Because of differences in the postnatal anatomy of the uriÂnary and genital tracts and the
external genitalia of the male and female, it is necessary to describe the urogenital triangle in the
two sexes separately. However, a study of the embryology of this region shows that the differences
are not as profound as appear at first sight. The urogenital triangle includes the uroÂgenital
diaphragm.
The Urogenital Diaphragm
The urogenital diaphragm lies below the anterior part of the pelvic floor and the genital
hiatus between the medial marÂgins of the levator ani muscle. The diaphragm consists of a laÂyer
of striated muscle sandwiched between two fascial layers. The deep layer or superior fascia of the
urogenital diaphragm is an insubstantial layer which blends posteriorly with the perineÂal body
and perineal membrane. The muscular layer consists of the deep transverse perineal muscles and
the sphincter urethrae. The more superficial fascial layer is known as the inferior fasÂcia of the
urogenital diaphragm or more commonly the perineal membrane. Through the diaphragm passes
the membranous part of the urethra.
The Perineal Membrane
The perineal membrane is a strong triangular fascial sheet spanning the space between
the two ischiopubic rarm. When the pelvis is correctly oriented the membrane
faces downward and slightly forward. The apex of the membrane is truncated and does
not reach the arcuate pubic ligaments a space is left for the passage of the deep dorsal vein
of the penis. The sides of the membrane are attached to the ischlopubic rarm and its base fuses
posteriorly with the perineal body and gives attachment to the superficial fascia of the
perineum .
Perforating the membrane in the midline is the urethra. This leaves the bladder and passes
into the prostate where it is joined by the ejaculatory ducts. On leaving the prostate it is
immediately surrounded by the sphincter urethrae muscle which lies deep to and above the perineal
membrane. It is over this short course (1.5-2.0 cm), between the prostate and perineal membranes
that it is called membranous. Lying superior to the perineal membrane and in the same plane as
the sphincter urethÂrae are the deep transverse perineal muscles.
The Deep Perineal Space
The region between the superior and inferior fascias of the urogenital diaphragm is
commonly known as the deep perineal spaÂce. As well as the deep perineal and sphincter urethrae
muscles, it contains the membranous urethras the bulbourethral glands, the pudendal vessels and
the dorsal nerves of the penis.
The Sphincter Urethrae
This striated muscle surrounds the membranous urethra in the male. Its deeper fibers
encircle the urethra while its suÂperficial fibers pass backward on either side of it from the anÂterior margin of the perineal membrane to the perineal body. ReÂlaxation of this sphincter is
necessary before micturition can occur. It also provides voluntary control over a reflexly initiÂated
desire to micturate. It is supplied by the perineal branch of the pudendal nerve.
The Deep Transverse Perineal Muscles
These striated muscles arise from fascia of the lateral wall of the perineum and passing
deep to the perineal membrane they fuse with each other at a tendinous raphe and with the perÂmeal body. Their attachment to the perineal body probably contÂributes to the support of the pelvic
floor. These muscles are supplied by the perineal branch of the pudendal nerve.
The Bulbourethral Glands
These two small glands lie on either side of the membranous urethra and are embedded in
the fibers of the sphincter urethrae and deep transverse perineal muscles. Their relatively long
ducts pierce the perineal membrane to join the spongy part of the urethra about 2.5 cm beyond the
membrane. The secretion of the glands is contributed to the seminal fluid. Superficial to and below
the perineal membrane lies the superficial perineal space.
The Superficial Perineal Space
The superficial fascia of the perineum consists of relatiÂvely thick areolar tissue containing
a variable amount of fat. Over the urogenital triangle there is a deeper but thin and strong
membranous layer.
The extent and attachments of the membranous layer are of clinical importance because
they determine the direction of the spread of urine which may leak from a rupture of the spongy
urethra. Note the posterior attachment of the fascia to the posÂterior border of the perineal
membrane. On each side its attachÂment is continued forward and upward along the ischiopubic
rami and then laterally over the thigh where it fuses with the fascia lata. Extending from these
attachments, the fascia surrounds the scrotum and the penis and then becomes continuous above
with the membranous layer of the superficial fascia of the anterior abdoÂminal wall.
In this way the superficial perineal pouch is formed. It can now be understood that urine
leaking from the urethra will not pass backward into the anal triangle or laterally into the thigh,
but after dis tending the scrotum and penis, will pass up onto the anterior abdominal wall.
Contained within the superfiÂcial perineal space are the male external genitalia which are
described next.
The Male External Genitalia
The male external genitalia are the penis and scrotum.
The Penis
The penis is described in two parts, namely the root which lies in the perineum and the free
body which is enveloped in skin. The root of the penis is formed by three masses of erectiÂle
tissue which are surrounded by fibrous tissue and lie below the perineal membrane. These are the
two crura and the bulb of the penis. The crura are attached to the everted margins of the ischiopubic
rami and the bulb is attached to the perineal membÂrane. It is here that the urethra passes into the
erectile tisÂsue of the bulb and continues in it to the external urethral orifice.
These three structures merge to form the body of the penis and in picture a transverse
section of this is shown. The crura now called the corpora cavernosa are each surrounded by a
fibroÂus sheath and the medial surfaces of these sheaths fuse in the midline to form the septum
penis.
The bulb now called the corpus spongiosum narrows and comes to lie in a groove on the
ventral surface of the fused corpora cavernosa. The distal end of the corpus spongiosum becomes
exÂpanded to form the glans penis whose hallowed proximal surface accepts the blunt
terminations of the corpora cavernosa. The slightly everted margin of the glans is called the corona
glanÂdis and narrowing of the body proximal to this is called the neck of the penis. It is necessary
here to point out that the surfaces of the penis are described as if it were suspended as in
quadupeds from the ventral surface of the body or in the poÂsition it assumes when erect. As a
results the anterior surface of the flaccid dependent penis is in fact its dorsal surface.
The bulb and the two crura of the penis are partially surÂrounded by the bulbospongiosus
and ischiocavernosus muscle. A further pair of muscles, called the superficial transverse periÂneal
muscles are closely associated with these. The bulbospongiÂosus muscle arises from the perineal
body and a rnldhne raphe lying over the ventral surface of the bulb. Its fibers fan out laterally to
be attached in sequence to the perineal membrane on either side of the bulb, to encircle the bulb,
and finally enÂcircle all three erectile masses to be attached to an aponeuroÂsis on the dorsum of
the penis.
The ischiocavernosus muscle arises from the ischial ramus on each side of the crus and
initially covers it. Its fibers extend forward to end in an aponeurosis on the deep surface of the
crus. Both muscles assist in producing an erection of the penis and the bulbospongiosus helps to
empty the urethra at the end of micturition.
The superficial transverse perineal muscle is a small slip that arises from the medial aspect
of the ischial tuberosity and crosses the posterior margin of the perineal membrane to meet its
fellow at the perineal body. Each of these superficial periÂneal muscles is supplied from the
perineal branch of the pudenÂdal nerve.
The body of the penis is surrounded by thin loose skin. At the neck of the penis the skin is
reflected upon itself to form the prepuce or fore skin which covers the glans. The prepuce is at
tached to the ventral surface of the glans by a fold ot skin called the frenulum of the prepuce.
Beneath the skin the superficial fascia contains no fat, but its deepest layer is...
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