Pelvis
The pelvis creates the basin of the lower abdominal cavity. It
is formed by three separate bones which become fused: the ilium, the ischium,
and the pubis. The ilium is the broad, wing-like segment which features
the wide, slightly concave surfaces of the back and sides of the pelvic
girdle. The ischium forms the smaller, lower, portion which bears
the weight of the body while sitting. The pubis creates an archway
in the front of the basin which allows the urethra, blood vessels, and
nerves to pass through the pelvic girdle to the external genitalia and
lower body. The pelvis articulates with the sacrum in the back (and
thereby connects to the rest of the vertebral column) and to the legs through
the ball-and-socket joint formed by the two
acetabula of the pelvis and the head of each femur.
The acetabulum is the large, circular cavity in each side of the pelvis which accepts the head of the femur to form the ball-and-socket hip joint. It receives its name from the Latin term for the vinegar cup, which it resembles.
The femur is the longest bone in the body, and composes the upper leg, or thigh. Articulating with the acetabulum of the pelvis at its head, and the tibia, fibula, and patella to form the knee joint at its lower end, each femur directly bears the weight of the upper body.
The upper end of the femur features two prominences which serve as attachment sites for muscles and ligaments. The larger prominence is called the greater trochanter and projects from the upper side of the femur. The smaller, or lesser, trochanter, projects medially from the side of the femur, just at the neck of the femur.
The head of the femur is a rounded protrusion, covered with articular cartilage, which fits into the acetabulum of the pelvis. This articulation creates a ball-and-socket joint which is both strong while allowing a wide degree of articulation.
The ilium is one of the three pelvic bones fused together to form the pelvic girdle. It is the broad, flared portion which constitutes the upper and lateral sections of the pelvis. The ilium is distinct for its ala, or wings, which flare out on either side of the spinal column, resembling an airplane propeller when seen in the side view.
The iliac crest marks the upper ridge of the broad ala, or wings, of the ilium. The iliac crest features an internal and external lip, with an intermediate line between the two.
The lower margin of each ilial wing features a sharp concavity where the ilium and the sacrum join. This is called the greater sciatic notch, and it allows the sciatic nerve to pass to the legs below.
The iliac fossa is the broad, slightly concave inner surface of each ala, or win, or the ilium. The fossa, along with the pubis and ischium, create a basin in which the lower abdominal organs rest. Hence, the term pelvis, which is derived from the Latin term for basin.
The iliopubic eminence is a slightly broader projection in each side of the front of the pelvis. Each iliopubic eminence marks the dividing point between the superior pubic ramus and the ilium bone.
The pubis is one of the three pelvic bones which fuse together to form the pelvic girdle. The pubis is located just to the front and below the bladder. In the center of the pubis is the symphysis, which marks a point of fusing of the two sides of the pubis.
The pubis features two segments on each side of the symphysis pubis. These two segments are divided at the pubic tubercle. The upper segment is called the superior pubic ramus, and it joins with the ala, or wings, or the ilium. The lower of the two segments is called the inferior pubic ramus and it joins with the ischium of the pelvis.
The pubic tubercle is a bony projection from the front of each half of the pubis. It is situated midway between the superior pubic ramus and the inferior pubic ramus. The two pubic tubercles flank the symphysis pubis.
The symphysis pubis (or pubic symphysis) is the connection between the two halves of the pubis. It is marked by a line of hardened, calcified cartilage and rests in a slight concavity in the center of the pubis, situated between the two pubic tubercles. In the female, the symphysis pubis is covered by a thick pad of fatty tissue, called the mons pubis.
The ischium is one of the three sections of bone which are fused in the adult to form the pelvis. The ischium bears the weight when sitting, and is attached to the pubis in front and to the ala, or wings, of the ilium laterally and to the back. The large openings in the ischium on either side of the pelvis, just below the pubis, are called the obturator foramina.
The obturator foramen is the large opening in each ischium. These openings admit blood vessels and nerves from the abdominal cavity to the inside of the upper legs.
The ischial tuberosity is a broadening, or eminence, in the frontal portion of the ischium. It marks the point of fusion of the ischium and the pubis.
The sacrum is the portion of the vertebral column between the lumbar
vertebrae and the structures of the coccyx. It is composed of five
vertebrae which are fused together to form a single bone structure.
The sacrum features a median crest (running down the back, or posterior
of the sacrum) which is made of the fused spinous processes of its component
vertebrae. Beneath this crest is the sacral canal, a tunnel which
runs lengthwise from the top of the sacrum to a hiatus (opening) near the
bottom. Four pairs of holes (sacral foramina) pierce the sacrum,
flanking the medial (center) line, where the intermediate
sacral crests are formed by the fused articular processes of the component
vertebrae. To the outside of the intermediate sacral crests are the
lateral crests, formed by the fused transverse processes of the component
vertebrae. In the sacrum, therefore, unlike the upper vertebrae in
the spine, the intertransverse ligaments have been replaced by fusion of
these processes together. The crests are not represented on the front
(pelvic) surface of the sacrum, though the sacral foramina are evident.
The coccyx (or "tailbone") is composed of three to five rudimentary
vertebrae. Often, the first of these coccygeal vertebrae is separate,
while the remainder are fused together. The articulation between
the coccygeal vertebrae and the sacrum allow some flexibility in the coccyx,
which is particularly beneficial in taking the stresses of sitting and
falling. The coccyx is extremely
susceptible to shock fracture, as might be induced from a fall.
Furthermore, since a number of nerve pathways pass near this area, damage
to the coccyx threatens damage to the nerves of the lower body. The
juncture of the first coccygeal vertebra with the sacrum occurs at the
lower facet of the sacrum.