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.
This outer layer is the investing deep fasica. It surrounds all the muscles of the leg. The investing deep fascia is attached to the tibia here, and here. 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 wraps around the back of the calcaneal tendon, like a sling. 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.
Now we’ll look at the fibrous septa, the singular of which is septum. There are three of them. 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. We’ll look at the back first. We’ll remove gastrocnemius and soleus, down to here.
Here’s soleus, divided, here’s the investing deep fascia, divided at a lower level. In front of soleus, this transverse intermuscular septum crosses the back of the leg. It runs from here on the tibia, to here on the fibula.
Three muscles that we haven’t seen yet lie between the transverse septum and the bones. To see the transverse septum better, we’ll remove the rest of soleus. The transverse septum is thin up here, but toward the ankle it becomes thicker. At the ankle, the transverse septum is continuous with the flexor retinaculum.
The other two septa have cumbersome names: they’re the anterior and the posterior crural intermuscular septa. To see them, we’ll remove the investing deep fascia down to here, exposing several muscles that we haven’t met yet. We’ll be meeting them soon. This is the posterior crural septum, lying just in front of the soleus muscle. This is the anterior crural septum. These two septa are attached to the fibula here, and here.
The anterior crural septum divides the muscles in front of and lateral to the two bones into an anterior compartment, which contains four muscles including tibialis anterior, and a more laterally placed peroneal compartment, which contains two of the three peroneal muscles.