Ah hree bones make the pelvis; two innominate (without a name) bones and one sacrum (sacrificial) bone.
The sacrum is a wedge about the size of the hand but more perfectly shaped, like a half-bent hand, and carrying a very small tip (coccyx) about as big as the last joint of the thumb. It forms the central piece in the back, curving first back and down and then down and in.
The two innominate bones are formed like two propellers, with triangular blades twisted in opposite directions. The rear corners of the top blades meet the sacrum in the back, and the front corners of the lower blades meet in front to form the symphysis pubis. The hip socket itself forms the central point for the shaft. The two blades stand at right angles to each other.
The upper blade is called the ilium, the lower is called the pubis in front and the ischium behind, with an opening between. The only superficial parts are the top of the upper blade (iliac crest) and the front tip of the lower (symphysis pubis).
Masses and Markings The size of the pelvis is due to its position as the mechanical axis of the body; it is the fulcrum for the muscles of the trunk and legs, and is large in proportion. Its mass inclines a little forward, and is somewhat square as compared with the trunk above. At the sides the ridge is called the iliac crest. It is the fulcrum for the lateral muscles and flares out widely for that purpose, rather more widely in front than behind.
Above the rim is a roll of muscle belonging to the abdominal wall; immediately below it a groove or depression, made by the sag of the hip muscles, obliterated when these are contracted in action.
The greater part of the movement of a figure is based on the pelvis. Its bony basin in front supports the fleshy mass of the abdomen. Behind, a circle of bones forms the extreme lateral part, of which the sacrum is the keystone.
The muscles that are visible are all situated at the back to form the gluteal region. Only two of these are prominent; they are, the gluteus maximus and the gluteus medius. With the pelvis as a base, these two act on the femur, which acts as a crank shaft. The upper end of the femur is in the shape of a bent lever on which the whole body rests.
Gluteus Maximus: From crest of ilium, rear portion, sacrum and coccyx to femur. Action: Extends, rotates and turns out thigh.
The Pelvis and Hip
4 Gluteus medius
5 Gluteus maximus
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