Stratificational relationship among the main nerves from the dorsal division of the sacral plexus and the innervation of the piriformis.

In order to comprehend more completely the morphology of the nerves to the piriformis, it is necessary to obtain a detailed understanding of the relationship of the origin and the course of these nerves from the dorsal division of the sacral plexus, with reference to the superior and inferior gluteal nerves. Twelve of seven human pelvic halves were carefully dissected in order to examine the origins of the nerves from the dorsal division of the sacral plexus. Six of these pelvic halves were further dissected under a stereomicroscope to examine the nerves to the piriformis. 1. The origin of the superior gluteal nerve was more proximal and dorsal in the sacral plexus than that of the inferior gluteal nerve. 2. The superior gluteal nerve consisted of a thick cranial part and a thin caudal part; the former continued as the inferior branch of the nerve, and the latter, the superior branch. The cranial and caudal parts crossed before reaching the glutei medius and minimus. 3. The nerves to the piriformis arose from three main nerves from the dorsal division of the sacral plexus: 1) the caudalmost root of the superior gluteal nerve, 2) the caudal roots of the inferior gluteal nerve and 3) the common peroneal nerve. Considering the stratificational relationship among the main nerves from the dorsal division of the sacral plexus, the piriformis appears to be composed of parts from different muscle layers.