Definition of Muscle, piriformis

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Muscle, piriformis: A muscle that begins at the front surface of the sacrum (the V-shaped bone between the buttocks, at the base of the spine) and passes through the greater sciatic notch to attach to the top of the thighbone (femur) at its bony prominence (the greater trochanter). The gluteus maximus muscle covers the piriformis muscle in the buttocks.