Pelvic cavity

Question 1

Describe the bony pelvis.

Answer 1

• The individual features of the hip bone, sacrum and coccyx are considered separately.
• When articulated the bones enclose a cavity; from the brim of the cavity the ala of each ilium projects up to form the iliac fossa, part of the posterior abdominal wall. * The pelvic brim is formed in continuity by the pubic crest, pectineal line of the pubis, arcuate line of the ilium, and the ala and promontory of the sacrum.
• The plane of the brim is oblique, lying at 60˚ with the horizontal; the vagina lies in the same plane. From the brim the pelvic cavity projects back to the buttocks.
• Sex differences are due to the two facts that the female pelvis is broader than that of the male for easier passage of the fetal head and that the female bones, including the head of the femur, are more slender than those of the male.
• As viewed from the front, in the male pelvis the sturdy bones make an acute subpubic angle, pointed like a Gothic arch, while in the female the slender bones make a wide subpubic angle, rounded like a Roman arch.
• The outline of the pelvic brim differs. In the male the sacral promontory indents the outline, and the brim is widest towards the back (a ‘heart-shaped’ outline) while in the female there is less indentation of the outline by the sacral promontory and the brim is widest further forwards (a ‘transversely oval’ outline).

Question 2

Describe the position of the pelvis.

Answer 2

• In the erect individual the anterior superior iliac spines and the upper margin of the symphysis pubic lie in the same vertical plane.
• The upper border of the symphysis pubis, the spine of the ischium, the tip of the coccyx, the head of the femur and the apex of the greater trochanter lie in the same horizontal plane.
• This plane passes through the pelvic cavity at a level with the tip of the finger of the clinician during rectal or vaginal examination. The lower poles of the ovaries in the female and the seminal vesicles in the male lie in this plane.

Question 3

Describe the pelvic walls.

Answer 3

• The word pelvis is Latin for a basin and, when tilted forwards into the anatomical position, the bony pelvis does bear some resemblance to a pudding basin but with much of the front wall missing.
• The deficiency is made good by the lower part of the anterior abdominal wall where the aponeuroses of all three anterolateral muscles lie in front of rectus abdominis.
• The pelvic brim divides the ‘false pelvis’ (above the brim, and part of the general abdominal cavity) from the ‘true pelvis’ or pelvic cavity (below the brim).
• The muscles of the true pelvis are obturator internus and piriformis (which are also classified as lower limb muscles), and levator ani and coccygeus, (which, with their fellows of the opposite side, constitute the pelvic floor or pelvic diaphragm).
• The side wall of the pelvis is formed by the hip bone, clad with obturator internus and its fascia.
• The curved posterior wall is formed by the sacrum with piriformis passing laterally into the greater sciatic foramen.

Question 4

Describe the pelvic floor.

Answer 4

• The pelvic floor consists of a gutter-shaped sheet of muscle, the pelvic diaphragm, slung around the midline viscera (urethra and anal canal and, in the female, the vagina).
• The muscles of the pelvic floor are the levator ani and the coccygeus.
• They arise in continuity from the body of the pubis, from the tendinous arch over the obturator fascia, and from the spine of the ischium, and are inserted into the coccyx and the postanal plate.
• From their origin the muscle fibres slope downwards and backwards to the midline; the pelvic floor so produced is a gutter that slopes downwards and faces forwards.

Question 5

Outline levator ani.

Answer 5

• Levator ani consists of two main parts, pubococcygeus and iliococcygeus.
• Their fibres arise in continuity from the body of the pubis to the ischial spine across the obturator fascia, along a condensation of the fascia, the tendinous arch.
• The levator ani originally arose from the pelvic brim (its present origin in most mammals) and in man has migrated down the side wall of the pelvis, bringing the tendinous arch with it.
• Residual aponeurotic fibres of levator ani contribute to the strength of the obturator fascia above the tendinous arch.
• The pubococcygeus part is that part of levator ani which arises from the anterior half of the tendinous arch and from the posterior surface of the body of the pubis.
• The pubococcygeus fibres are in different functional sets.
• The bulk of its posterior fibres sweep backwards in a flat sheet on the pelvic surface of the iliococcygeus and form a tendinous plate in the midline, attached posteriorly to the front of the coccyx. These constitute the pubococcygeus muscle proper.
• Fibres arising more anteriorly, from the body of the pubis, swing more medially and more inferiorly around the anorectal junction and join with fibres of the opposite side and the top of the external anal sphincter. This part of the muscle is called puborectalis and forms a U-shaped sling which holds the anorectal junction angled forwards.
• Some of the fibres blend with the longitudinal muscle of the rectum and the conjoint longitudinal coat of the anal canal and are termed puboanalis.
• The most medial fibres of pubococcygeus pass backwards alongside the prostate and the sphincter urethrae in the male and decussate across the midline behind the urethra; they are referred to as puboprostaticus or pubourethralis.
• In the female, these fibres sling around the posterior wall of the vagina and are referred to as pubovaginalis.
• In both sexes, fibres also attach to the perineal body.
• The iliococcygeus part arises from the posterior half of the tendinous arch and the pelvic surface of the ischial spine and, overlapping the pelvic surface of coccygeus, its fibres are inserted into the side of the coccyx and the anococcygeal raphe, which extends from the tip of the coccyx to the junction of rectum and anal canal.
• Although the iliococcygeus does not arise from the ilium, its name derives from its former origin on the iliac bone at the pelvic brim.
• The postanal plate, also referred to as the anococcygeal ligament, is a layered musculotendinous structure between the anal canal and the caudal part of the vertebral column, on which the terminal rectum sits.
• From above downwards it consists of the superior fascia of the pelvic diaphragm, the tendinous plate of pubococcygeus, the muscular raphe of iliococcygeus, and the posterior parts of puborectalis and the external anal sphincter.

Question 6

Outline coccygeus.

Answer 6

• The coccygeus is best thought of as ischiococcygeus.
• It arises from the tip of the ischial spine and its fibres fan out to be inserted into the side of the coccyx and the lowest piece of the sacrum.
• It lies edge to edge with the lower border of piriformis and is overlapped anteriorly by iliococcygeus.
• Its gluteal surface is fibrous tissue, and is indeed the sacrospinous ligament.

Question 7

Describe the nerve supply of the pelvic floor.

Answer 7

• Levator ani is mainly supplied from the sacral plexus by branches of S3 and S4 which enter the upper (pelvic) surface of the muscle.
• Some of these somatic fibres may travel in or very close to the pelvic splanchnic nerves.
• Puborectalis, pubourethralis and pubo- vaginalis are supplied from below by the perineal branch of S4 and the inferior rectal branch of the pudendal nerve, in common with the external anal sphincter.
• Levator ani, like the external anal and urethral sphincter muscles, has a high proportion of slow twitch fibres.
• Coccygeus is supplied by branches of S3 and S4.

Question 8

Describe the actions of the pelvic floor.

Answer 8

• The pelvic floor helps to support the pelvic viscera and retain them in their normal positions.
• The floor contracts to counteract increased intra-abdominal pressure, which may be momentary, as in coughing and sneezing, or more prolonged as in muscular efforts like lifting.
• If an expulsive effort is required, the floor relaxes. Thus in defecation when the abdominal wall and diaphragm contract, puborectalis relaxes to straighten out the anorectal junction and the floor descends to become more funnel-shaped, rising again as the process comes to an end.
• The pubovaginalis fibres of levator ani may be important in assisting the urethral sphincter at the end of micturition in the female.
• In parturition the floor initially directs the fetal head to the pelvic outlet, but the degree of stretching to which the muscular and fibrous parts of the floor are subjected may render it liable to damage by tearing.