At the ankle, the transverse septum is continuous with the flexor retinaculum.

The transverse septum is thin up here, but toward the ankle it becomes thicker. To see the transverse septum better, we’ll remove the rest of soleus.

Three muscles that we haven’t seen yet lie between the transverse septum and the bones. It runs from here on the tibia, to here on the fibula. In front of soleus, this transverse intermuscular septum crosses the back of the leg. Here’s soleus, divided, here’s the investing deep fascia, divided at a lower level. We’ll remove gastrocnemius and soleus, down to here. Together with the interosseous membrane, they divide the muscles of the leg into four compartments, two on the front of the leg, and two on the back. Now we’ll look at the fibrous septa, the singular of which is septum. Distally the investing deep fascia is continuous with the superficial part of the flexor retinaculum, with the peroneal retinaculum, and with the two parts of the extensor retinaculum. The investing deep fascia wraps around the back of the calcaneal tendon, like a sling. It’s attached to the fibula not directly, but indirectly by two fibrous septa here, and here, that we’ll see in a minute. The investing deep fascia is attached to the tibia here, and here. This outer layer is the investing deep fasica. Before we move on to see the muscles that produce inversion and eversion, we need to digress for two minutes, to look at the layer of deep fascia that surrounds all the muscles of the leg, and the three fibrous partitions, or septa that divide the leg muscles into somewhat distinct compartments.

Compartments of leg