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Which of the following muscles originates from the ventral surface of the sacrum?
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Of the muscles listed, only the piriformis muscle originates from the ventral surface of the sacrum. Illustrations A and B show the origin and insertion of the piriformis muscle from the sacrum to the piriformis fossa on the proximal femur.
Illustration C is a T1 MRI showing the origin of the piriformis muscle from the ventral surface of the sacrum. (a) shows the right piriformis muscle overlying the S2 nerve (arrow). (p) shows the left piriformis muscle.
1&2: Obturator internus and externus both originate from the anterior pelvis inferior pubic ramus (Illustration B) and obturator membrane.
3: Quadratus femoris originates from the inferior pubic ramus near the ischial tuberosity (Illustration B)
5: Superior gemellus originates from anterior pubic ramus near the acetabulum (Illustration B)
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The inferior and superior gluteal nerves are designated as such based on their relationship to what structure?
The gluteus maximus
The gluteus minimus
The piriformis muscle
The sciatic nerve
The sacrospinous ligament
The superior gluteal nerve arises from the posterior roots of L4, L5 and S1 in the lumbosacral plexus. It exits the pelvis through the superior part of the greater sciatic notch, just superior to the piriformis tendon. It courses between the gluteus medius and minimus, supplying both muscles, as well as the tensor fascia lata. The inferior gluteal nerve arises from the posterior roots of L5, S1 and S2 in the lumbosacral plexus and exits the pelvis through the greater sciatic notch, under the piriformis. It courses on the deep surface of gluteus maximus and provides the sole motor innervation for this muscle.
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