Posterior crural intermuscular septum

Overview
The posterior crural intermuscular septum is a band of fascia which separates the lateral compartment of leg.

The fascia cruris gives off from its deep surface, on the lateral side of the leg, two strong intermuscular septa, the anterior and posterior peroneal septa, which enclose the Peronæi longus and brevis, and separate them from the muscles of the anterior and posterior crural regions, and several more slender processes which enclose the individual muscles in each region.