The bowl-shaped pelvic cavity is continuous above with the abdominal cavity. The rim of the pelvic cavity (the pelvic inlet) is completely encircled by bone. The pelvic floor is a fibromuscular structure separating the pelvic cavity above from the perineum below.
The perineum is inferior to the pelvic floor and its margin is formed by the pelvic outlet. The perineum contains: the terminal openings of the gastrointestinal and urinary systems, the external opening of the reproductive tract, and the roots of the external genitalia.
The bones of the pelvis consist of the right and left pelvic (hip) bones, the sacrum, and the coccyx. The sacrum articulates superiorly with vertebra LV at the lumbosacral joint. The pelvic bones articulate posteriorly with the sacrum at the sacro-iliac joints and with each other anteriorly at the pubic symphysis.
The pelvic bone is irregular in shape and has two major parts separated by an oblique line on the medial surface of the bone (Fig. 5.19A):
The pelvic bone above this line represents the lateral wall of the false pelvis, which is part of the abdominal cavity.
The pelvic bone below this line represents the lateral wall of the true pelvis, which contains the pelvic cavity.
The linea terminalis is the lower two-thirds of this line and contributes to the margin of the pelvic inlet.
The lateral surface of the pelvic bone has a large articular socket, the acetabulum, which, together with the head of the femur, forms the hip joint (Fig. 5.19B).
Inferior to the acetabulum is the large obturator foramen, most of which is closed by a flat connective tissue membrane, the obturator membrane. A small obturator canal remains open superiorly between the membrane and adjacent bone, providing a route of communication between the lower limb and the pelvic cavity.
The posterior margin of the bone is marked by two notches separated by the ischial spine: the greater sciatic notch, and the lesser sciatic notch.
The posterior margin terminates inferiorly as the large ischial tuberosity.
The irregular anterior margin of the pelvic bone is marked by the anterior superior iliac spine, the anterior inferior iliac spine, and the pubic tubercle.
Components of the pelvic bone
Each pelvic bone is formed by three elements: the ilium, pubis, and ischium. At birth, these bones are connected by cartilage in the area of the acetabulum; later, at between 16 and 18 years of age, they fuse into a single bone (Fig. 5.20).
Of the three components of the pelvic bone, the ilium is the most superior in position.
The ilium is separated into upper and lower parts by a ridge on the medial surface (Fig. 5.21A).
Posteriorly, the ridge is sharp and lies immediately superior to the surface of the bone that articulates with the sacrum. This sacral surface has a large L-shaped facet for articulating with the sacrum and an expanded, posterior roughened area for the attachment of the strong ligaments that support the sacro-iliac joint (Fig. 5.21).
Anteriorly, the ridge separating the upper and lower parts of the ilium is rounded and termed the arcuate line.
The arcuate line forms part of the linea terminalis and the pelvic brim.
The portion of the ilium lying inferiorly to the arcuate line is the pelvic part of the ilium and contributes to the wall of the lesser or true pelvis.
The upper part of the ilium expands to form a flat, fan-shaped “wing,” which provides bony support for the lower abdomen, or false pelvis. This part of the ilium provides attachment for muscles functionally associated with the lower limb. The anteromedial surface of the wing is concave and forms the iliac fossa. The external (gluteal) surface of the wing is marked by lines and roughenings and is related to the gluteal region of the lower limb (Fig. 5.21B).