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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. 1 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. 2 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 3 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. 4 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. 5 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 6 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 7 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. 8 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. 9 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. 10 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, 11 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. 12