The soft tissues of the orbit are divided by a tough, fibrous septum that arises from the periosteum of the bony structure of the orbit circumferentially.
The septum becomes thickened to form the tarsal plates of the upper and lower eyelids. This divides the soft tissues of the orbit into the preseptal areas, and the more constrained post-septal space.
The structures of the orbital septum are outlined in blue on the image.