Download Pelvic cavity and diaphragm 2013

Pelvic Cavity and Diaphragm
The bones of the pelvic girdle and their associated ligaments, muscles, and fascia
form the pelvic wall. The bony component comprises the right and left hip bones
anterolaterally and the sacrum and coccyx posteriorly. Inside this girdle is the pelvic
cavity. The pelvic cavity is usually wider and shallower in females because of the
differences in the shapes of the surrounding bones.
Hip Bone
Only the medial or pelvic surface of the hip bone is considered with the pelvic cavity.
Each hip bone is formed by the fusion of three components - ilium, ischium and
pubis. The anterosuperior part of the ilium contributes to the abdominal wall and
gives attachments to the iliacus muscle. The lower part of the ilium extends below
the pelvic inlet (linea terminalis) and contributes to the lateral wall of the pelvis. On
the posterior part of the bone is the articular surface, which articulates with the
corresponding surface of the sacrum at the sacroiliac joint.
The ischium has a rounded tuberosity inferiorly, which bears body weight in the
sitting position. Posteriorly is the pointed spine, which separates the greater and
lesser sciatic notches, while anteriorly the ramus of the ischium ascends to fuse with
the inferior pubic ramus.
The pubic bone has a superior pubic ramus that merges with the ilium near the
iliopubic eminence, and an inferior ramus which is continuous below the obturator
foramen with the ramus of the ischium. The bodies of the right and left pubic bones
articulate at the pubic symphysis.
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The obturator foramen is a large aperature, which is almost completely occluded by
the obturator membrane. Superiorly the membrane leaves a small gap, the
obturator canal, which provides access between the pelvis and the medial
compartment of the thigh.
Joints
The pelvic girdle forms a stable ring because its constituent bones are bound
together at the two sacroiliac joints and the pubic symphysis.
The symphysis is a secondary cartilaginous joint containing a pad of fibrocartilage
that separates the bodies of the right and left pubic bones. The joint is stabilized by
ligaments attached around the articular margins.
The sacroiliac joints allow very little movement because the articulating surfaces of
their synovial cavities are irregular and behind each cavity is the thick posterior
interosseous ligament. Each joint is further supported by the anterior and posterior
sacroiliac ligaments and iliolumbar, sacrospinous, and sacrotuberous ligaments.
Body weight acting downwards through the lumbosacral disc tends to rotate the
sacrum, tipping its lower part backwards, a movement prevented by the
sacrospinous and sacrotuberous ligaments.
The iliolumbar ligament attaches medially to the transverse process of the fifth
lumbar vertebra and laterally to the iliac crest and front of the sacroiliac joint. The
sacrospinous ligament passes from the lateral margins of the sacrum and coccyx to
the ischial spine. The larger sacrotuberous ligament passes from the side and
dorsum of the sacrum and posterior surface of the ilium to the ischial tuberousity.
These two ligaments convert the greater and lesser sciatic notches into the greater
and lesser sciatic foramina.
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Muscles
Two muscles cover the pelvic walls. The piriformis is a flat muscle attached to the
pelvic surface of the second, third and fourth pieces of the sacrum. Running laterally
through the greater sciatic foramen, it enters the buttocks and attaches to the upper
part of the greater trochanter of the femur. Piriformis rotates the hip joint laterally
and is innervated by the first and second sacral nerves. Numerous vessels and
nerves accompany the muscle through the greater sciatic foramen.
The obturator internus is a fan shaped muscle with an extensive attachment to the
margins of the pelvic surface of the obturator foramen and the obturator membrane.
The muscle fibers converge on the lesser sciatic foramen to form a tendon, which
turns laterally to enter the gluteal region. The tendon is attached to the medial
aspect of the greater trochanter. The muscle laterally rotates the hip joint. The nerve
to the obturator internus enters the muscle within the perineum, having traversed
the greater and lesser sciatic foramen.
Pelvic floor
The pelvic floor or diaphragm is a muscular partition separating the cavity of the
pelvis above from the perineum below. It slopes downward toward the mid line,
forming a trough that inclines downwards and backwards. In the midline anteriorly,
a narrow triangular gap (urogenital hiatus) between the muscle fibers transmits the
urethra in both sexes and the vagina in the female. Posteriorly, the pelvic floor is
pieced by the anal canal.
The pelvic floor is formed principally by the right and left levator ani muscles, which
are supplemented posteriorly by the coccygeus muscles. The coccygeus muscle is
applied to the medial surface of the sacrospinous ligament. Medially it attaches to
the lateral border of the sacrum and coccyx and laterally to the ischial spine. Each
levator ani muscle has a linear attachment to the pelvic wall. The attachment begins
anteriorly on the pelvic surface of the body of the pubis and continues backwards as
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the tendinous arch along the obturator fascia as far as the ischial spine. The levator
ani muscle has three parts; the anterior part comprises the puborectalis, the middle
part is the pubococcygeus and the posterior part, the iliococcygeus.
The most anterior fibers, the puborectalis, reach the anal canal and either attach to
its wall or loop behind the anorectal junction. The pubococcygeus runs backward
and downward near the midline and passes close to the urethra. Muscular slips in
the male support the prostate; in the female they attach to the vagina. The bulk of
the pubococcygeus attaches to the coccyx or fuses in the midline with fibers from
the other side forming part of the anococcygeal body (ligament).
The fibers of the iliococcygeus muscles pass downward and medially below those of
the pubococcygeus and attach to the coccyx and to the anococcygeal body.
The levator ani muscles support the pelvic contents, actively maintaining the
positions of the pelvic viscera. In particular, the pubococcygeus muscles compress
the urethra and vagina and provide support for the bladder and uterus. The levator
ani fibers that loop behind the anal canal help to maintain the angulation of the
anorectal junction and play an important role in the continence of feces. During
defecation, the fibers attaching to the wall of the anal canal pull the organ upwards.
Levator ani and coccygeus are innervated by the fourth sacral nerve. Weaking of
these muscles, a common gynecological problem, may result in the decent
(prolapse) of the pelvic organs.
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Pelvic fascia
This term includes the fascial lining of the pelvic walls and the extraperitoneal
connective tissue surrounding the pelvic viscera. The pelvic surfaces of the
obturator internus, coccygeus, piriformis and levator ani are covered by fascia that
is continuous superior with the transversalis and iliac fascia. Between the pelvic
organs, the pelvic fascia mostly comprises a loose meshwork of connective tissue.
However, it is condensed anterior to the rectum to form the rectovesical septum.
Some of the arteries to the pelvic organs, such as the vaginal and uterine vessels, are
accompanied by thickened bands of fascia called “ligaments”. Radiating from the
uterine cervix to the pelvic walls are the transverse cervical and uterosacral
ligaments, which provide support to the uterus.
Pelvic nerves
The pelvic organs receive their autonomic innervation from the right and left pelvic
plexuses (inferior hypogastric plexus), which lie adjacent to the internal iliac
arteries and their branches. Nerves pass from the plexuses to the bladder,
reproductive organs and the rectum by accompanying the arteries to these organs.
The plexuses contain efferent fibers from both the parasympathetic and
sympathetic systems, which reach the pelvis from different parts of the spinal cord.
The parasympathetic component of the pelvic plexuses is provided by the pelvic
splanchnic nerves (nervi erigenti), which leave the spinal cord in the second, third
and fourth sacral nerves. These fibers control micturition, dilation of the erectile
tissues in both sexes, and defecation. The pelvic plexuses also provide the
parasympathetic innervation of the descending and sigmoid colons. These fibers
ascend into the abdomen in the hypogastric plexus and are distributed with the
branches of the inferior mesenteric artery.
The sympathetic fibers destined for the pelvic autonomic plexuses arise from the
lower thoracic and upper lumbar segments of the spinal cord and pass through the
lumbar portions of the sympathetic trunks on the posterior abdominal wall. From
here they pass in the hypogastric plexus to reach the pelvis. Sympathetic fibers
innervate the smooth muscles of the reproductive organs in both sexes and in the
male are responsible for coordinating ejaculation.
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The lower lumbar and upper sacral spinal nerves are predominately concerned with
the innervation of the lower limb. However, a few fibers derived from theses spinal
nerves are distributed to the pelvic walls and floor and perineum.
The obturator branch of the lumbar plexus emerges from the medial side of the
psoas major muscle and enters the pelvis by crossing in front of the ala of the
sacrum. It descends lateral to the common and internal iliac vessels and the ureter
and reaches the medial surface of the obturator internus. The nerve approaches the
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obturator vessels from above and continues with them through the obturator canal
into the medial compartment of the thigh.
The anterior rami of the first four sacral nerves emerge through the anterior sacral
foramina and mege to form the sacral plexus. The fifth sacral nerve and the
coccygeal nerves are small and do not contribute to the plexus. All the sacral and
coccygeal nerves receive grey rami communicates from the sympathetic trunk.
The sacral plexus lies on the posterior pelvic wall in front of the piriformis muscle,
covered anteriorly by pelvic fascia. The plexus is formed by the upper four sacral
nerves and is supplemented by the lumbosacral trunk, which carries fibers from the
fourth and fifth lumbar nerves. The branches of the sacral plexus are distributed to
the lower limb, pelvic walls and perineum. Those branches that leaves the pelvis
accompany the piriformis through the greater sciatic foramen to enter the buttock.
The nerve to the obturator internus going to the muscle and the pudendal nerve
then pass forward through the lesser sciatic foramen to supply the perineum as its
main motor and sensory nerve.
Pelvic vessels
The external iliac artey from the common iliac runs along the pelvic brim with its
companion vein, but the internal iliac artery runs down into the pelvis on the
posterior wall and gives off numerous branches to the pelvic structures.
Pelvic viscera
The rectum is continuous with the lower end of the sigmoid colon opposite the third
sacral vertebra. It is about 6 inches long and internally shows three prominent folds
of mucous membrane, jointly known as Houston’s (Hester) valves. The rectum
follows the forward curvature of the lower sacrum and coccyx and ends about one
inches below and in front of the coccyx, where it becomes continuous with the anal
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canal at the anorectal junction. There is an angle of about 120 degrees between the
rectum and the anal canal, and the forward bulge of the gut at this point is
maintained by the muscular sling formed by the puborectalis part of the levator ani
muscles. The sling is a higly important element in maintaining rectal continence.
In the pelvis, the peritoneum covers the front and the sides of the upper part of the
rectum, and the front of the middle portion. The lower part is below the point where
the peritoneum is reflected, in the male onto the back of the bladder (the
rectovesical pouch) and in the female, on the upper part of the vagina as the
rectouterine pouch (pouch of Douglas).
In both sexes the bladder is situated in the front of the pelvis, behind the pubic
symphysis. From the apex of the bladder, which is the uppermost anterior part, a
fibrous cord – the remains of the urachus, extends up behind the peritoneum of the
anterior abdominal wall to the umbilicus. The bladder posteriorly, the base, lies in
front of the rectum in the male and the vagina in the female. In the male, the lower
end of the ductus deferens and the seminal vesicle unite at the junction of the
bladder and prostate to form the ejaculatory duct which enters the prostate.
Internally, the lower part of the base is the trigone of the bladder, where the
openings of the two ureters and the internal urethral meatus – the upper end of the
urethra, are situated. These openings of the trigone form a distinct triangle. The
region of the internal urethral meatus at the lower part of the trigone is called the
neck of the bladder.
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The upper surface and the sides of the bladder are adjacent to coils of intestine, but
in the female the body of the uterus normally lies over the top of the bladder, rising
and falling with it as it fills and empties.The lowest part of the peritoneal cavity is
the rectovesical pouch in the male or the rectouterine pouch (pouch of Douglas) in
the female. Both are highly important because they are within reach during a rectal
and vaginal examination.
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The distinctive male internal genital organs in the pelvis are the vas deferens
(ductus deferens), seminal vesicles, ejaculatory ducts and prostate. The vas deferens
is the continuation of the epididymis and is a narrow, rather thick walled muscular
tube running from the lower end (tail) of the epididymis to the ejaculatory duct in
the prostate. With the testicular blood vessels it forms one of the main constituents
of the spermatic cord which runs through the inguinal canal into the abdomen.
Having reached the abdomen, the duct runs down the side wall of the pelvis and
then crosses the pelvic floor under cover of the peritoneum to reach the posterior
surface of the prostate where it is joined by the duct of the seminal vesicle to form
the ejaculatory duct.
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The seminal vesicles are a pair of coiled structures about 2 inches long with smooth
muscle in their walls. Each seminal vesicle lies below the ureter on its own side
against the back of the bladder and in front of the rectum. It is also lateral to the
lower end of the vas deferens, which has run down the side wall of the pelvis from
the inguinal canal and crossed over the ureter. The ejacuatory duct, the result of the
union of the duct of the seminal vesicle with the vas deferens, runs through the back
of the prostate to open into the prostatic part of the urethra.
The prostate is a glanduar organ situated in the lowest part of the pelvis below the
bladder and surrounding the first part or inch of the urethra. It is “chestnut” shaped,
resting on the levator ani muscles just in front of the rectum. Therefore, its posterior
surface can be felt on rectal examination. Consisting of small glands embedded in a
mixture of fibrous tissue and smooth muscle, it gets its blood supply from the
prostatic branch of the inferior vesical artery or sometimes the middle rectal artery.
The female genital organs contained in the pelvis consist of the paired ovaries and
uterine tubes, and the single uterus and vagina. The uterus is a muscular organ
composed of smooth muscles, which is shaped like a flattened pear. It lies usually
above the bladder with its lower end, the cervix, opening into the upper end of the
vagina. The main part of the uterus is the body and its broad upper end is the
fundus. On each side the uterine tubes join the uterus where the fundus and body
meet. The body is usually bent forward to make a single angle with the cervix (the
angle of anteflexion), and the cervix makes a similar angle with the vagina (angle of
anteversion). Thus, the uterus is normally said to be anteflexed and anteverted.
The uterus is suspended from each side of the pelvis by a double fold of peritoneum,
called the broad ligament. The upper edge of this fold encloses the uterine tube. The
uterine tube’s lateral end possesses a number of finger like projections – the
fimbriae that open near the side wall of the pelvis below the ovary, so that at
ovulation the ovum can migrate into the tube. Just below and in front of the
attachment of the tube to the uterus, a cord like solid band of tissue, the round
ligament, runs within its own fold of peritoneum to enter the inguinal canal,
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eventually merging with the tissue of the labia majora. Just below and behind the
tubal attachment is a much shorter and smaller band, the ligament of the ovary,
attaching one end of the ovary to the uterus.
The broad ligament and round ligaments are rather lax structures and while they
help to hold the uterus in its normal position, the most important structures are
some condensations of connective tissue under the peritoneum in the region of the
cervix of the uterus and the fornix of the vagina. These condensations pass laterally
beneath the broad ligament to the side wall of the pelvis as the transverse cervical
ligaments (also called cardinal or Mackendrodt’s ligaments). Another set passing
backwards on either side of the rectum to the front of the sacrum are the
uterosacral ligaments. Their undue stretching in childbirth or by other pelvic
conditions may lead to various kinds of uterine displacements.
Each ovary is an almond shaped structure lying near the side wall of the pelvis,
suspended from the back of the broad ligament in a fold of peritoneum called the
mesovarium. Its blood supply is from the ovarian artery arising from the abdominal
aorta high up near the renal artery.
The vagina is a smooth muscular tube lying anterior to the rectum and anal canal
and behind the pubic symphysis, urinary bladder and urethra. The female urethra is
embedded in the lower part of the anterior vaginal wall and its opening, the external
urethral meatus, is in front of the vestibule of the vagina. The upper end of the
vagina, into which the cervix of the uterus projects, is the fornix. The posterior, but
not the anterior part of the fornix is covered on the pelvic cavity side by peritoneum.
Thus, misguided instruments can perforate the posterior fornix, enter the peritoneal
cavity and cause serious risk of peritonitis.
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