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The pelvis is bounded posteriorly by the sacrum and coccyx and antero-
laterally by the innominate bones.
The pelvic brim (also termed the pelvic inlet) separates the pelvis into
the false pelvis (above) and the true pelvis (below). The brim is formed
Os innominatum (hip bone) (Figs 23.1 and 23.2) The pelvic cavity
by the sacral promontory behind, the iliopectineal lines laterally and the
This bone comprises three component parts, the: ilium, ischium and symphysis pubis anteriorly. The pelvic outlet is bounded by the coccyx
pubis. By adulthood the constituent bones have fused together at the behind, the ischial tuberosities laterally and the pubic arch anteriorly.
acetabulum. Posteriorly each hip bone articulates with the sacrum at the The true pelvis (pelvic cavity) lies between the inlet and outlet. The
sacro-iliac joint (a synovial joint). false pelvis is best considered as part of the abdominal cavity.
• Ilium: the iliac crest forms the upper border of the bone. It runs back-
wards from the anterior superior iliac spine to the posterior superior The ligaments of the pelvis (Fig. 23.4)
iliac spine. Below each of these bony landmarks are the corresponding These include the:
inferior spines. The outer surface of the ilium is termed the gluteal sur- • Sacrotuberous ligament: extends from the lateral part of the sacrum
face as it is where the gluteal muscles are attached. The inferior, anter- and coccyx to the ischial tuberosity.
ior and posterior gluteal lines demarcate the bony attachments of the • Sacrospinous ligament: extends from the lateral part of the sacrum
glutei. The inner surface of the ilium is smooth and hollowed out to and coccyx to the ischial spine.
form the iliac fossa. It gives attachment to the iliacus muscle. The The above ligaments, together with the sacro-iliac ligaments, bind
auricular surface of the ilium articulates with the sacrum at the sacro- the sacrum and coccyx to the os and prevent excessive movement at the
iliac joints (synovial joints). Posterior, interosseous and anterior sacro- sacro-iliac joints. In addition, these ligaments create the greater and
iliac ligaments strengthen the sacro-iliac joints. The iliopectineal line lesser sciatic foramina with the greater and lesser sciatic notches.
courses anteriorly on the inner surface of the ilium from the auricular
surface to the pubis. It forms the lateral margin of the pelvic brim (see The pelvic floor (Fig. 23.5)
below). The pelvic floor muscles: support the viscera; produce a sphincter
• Ischium: comprises a spine on its posterior part which demarcates action on the rectum and vagina and help to produce increases in intra-
the greater (above) and lesser sciatic (below) notches. The ischial abdominal pressure during straining. The rectum, urethra and vagina
tuberosity is a thickening on the lower part of the body of the ischium (in the female) traverse the pelvic floor to gain access to the exterior.
which bears weight in the sitting position. The ischial ramus projects The levator ani and coccygeus muscles form the pelvic floor, while piri-
forwards from the tuberosity to meet and fuse with the inferior pubic formis covers the front of the sacrum.
ramus. • Levator ani: arises from the posterior aspect of the pubis, the fascia
• Pubis: comprises a body and superior and inferior pubic rami. It overlying obturator internus on the side wall of the pelvis and the
articulates with the pubic bone of the other side at the symphysis pubis ischial spine. From this broad origin fibres sweep backwards towards
(a secondary cartilaginous joint). The superior surface of the body the midline as follows:
bears the pubic crest and the pubic tubercle (Fig. 23.1). • Anterior fibres (sphincter vaginae or levator prostatae)athese
The obturator foramen is a large opening bounded by the rami of the fibres surround the vagina in the female (prostate in the male) and
pubis and ischium. insert into the perineal body. The latter structure is a fibromuscular
node which lies anterior to the anal canal.
The sacrum and coccyx (Fig. 23.3) • Intermediate fibres (puborectalis)athese fibres surround the
• The sacrum comprises five fused vertebrae. The anterior and lateral anorectal junction and also insert into the deep part of the anal
aspects of the sacrum are termed the central and lateral masses, respect- sphincter. They provide an important voluntary sphincter action at
ively. The upper anterior part is termed the sacral promontory. Four the anorectal junction.
anterior sacral foramina on each side transmit the upper four sacral • Posterior fibres (iliococcygeus)athese fibres insert into the lateral
anterior primary rami. Posteriorly, the fused pedicles and laminae form aspect of the coccyx and a median fibrous raphe (the anococcygeal
the sacral canal representing a continuation of the vertebral canal. body).
Inferiorly, the canal terminates at the sacral hiatus. Sacral cornua • Coccygeus: arises from the ischial spine and inserts into the lower
bound the hiatus inferiorly on either side. The subarachnoid space ter- sacrum and coccyx.
minates at the level of S2. The sacrum is tilted anteriorly to form the
lumbosacral angle with the lumbar vertebra. Sex differences in the pelvis
• The coccyx articulates superiorly with the sacrum. It comprises The female pelvis differs from that of the male for the purpose of child-
between three and five fused rudimentary vertebrae. bearing. The major sex differences include:
1 The pelvic inlet is oval in the female. In the male the sacral promon-
The obturator membrane tory is prominent, producing a heart-shaped inlet.
The obturator membrane is a sheet of fibrous tissue which covers the 2 The pelvic outlet is wider in females as the ischial tuberosities are
obturator foramen with the exception of a small area for the passage of everted.
the obturator nerve and vessels which traverse the canal to pass from 3 The pelvic cavity is more spacious in the female than in the male.
the pelvis to gain access to the thigh. 4 The false pelvis is shallow in the female.
5 The pubic arch (the angle between the inferior pubic rami) is wider
and more rounded in the female when compared with that of the male.

The pelvis Ibthe bony and ligamentous pelvis 55