30 dissections have confirmed that the skin of the gluteal area is innervated by the posterior branches of T12 and L1, 19 times out of 30 (64 p. cent) or T12, L1 and L2, in 8 instances (26 p. cent) or T12, L1 and L2 receiving an anastomosis from L3, in 3 instances. This explains certain lumbo-gluteal pain coming from the dorso-lumbar area ("low lumbalgias of high origin"). In addition, the posterior branch, the most medial (most of the time L1, sometimes L2) goes through an osteo-aponeurotic channel formed by the iliac crest below and the dorso-lumbar fascia above. This channel, located at 7 or 8 cm of the spinal process, may be at the origin of a neuropathy. It projects at the same level in relation to the mid-line as the iliac insertion of the ilio-lumbar ligament. The latter seems impossible to feel as it is too deep and hidden by the superficial aspect of the iliac crest. Lumbalgias, sometimes related to a strain of this ligament, seem to be, most of the time, the result of an irritation of the posterior branch of L1 or L2.