Now that we've looked at the extra-ocular muscles, we'll move on to look at the eyelids and the lacrimal apparatus. The eyelids form a movable protective covering for the eye.
The upper and lower lids are much alike: we'll look mainly at the upper one. In looking at the structures that form the eyelids, we'll switch between a dissection from in front, to one in which the structures have been divided in steps, a layer at a time, giving us this lateral view.
The eyelids are covered on the outside with skin that's extremely thin and mobile. They're covered on the inside by a thin, sensitive membrane, the conjunctiva. This is the conjunctiva. Conjunctiva also covers the front of the sclera,
The conjuctiva that covers the eye - the bulbar part - and the conjunctiva that lines the lid - the palpebral part - are continuous at this fold, the conjunctival fornix. The conjunctiva is continuous with the skin at the margin of the eyelid.
Directly beneath the skin of the eyelids, which we'll remove, is the orbicularis oculi muscle. As we saw in part 1 of this tape, the orbicularis extends beyond the margins of the orbit, onto the forehead and the cheek.
It's such a thin muscle, parts of it are almost colorless. Medially, many of its fibers arise from this structure, the medial palpebral ligament, which is attached to bone here at the anterior lacrimal crest. The more outlying fibers of orbicularis arise from bone here around the lacrimal groove.
Laterally, the innermost fibers of orbicularis insert into bone here. The more outlying fibers have no bony attachment. This is part of the orbicularis. It's been divided along this line.
Directly beneath it is this layer of fatty fibrous tissue, the orbital septum, that extends into the lid from the periosteum at the orbital margin. The orbital septum separates the contents of the orbit from the facial soft tissues. Here beneath the orbicularis is the structure that gives the eyelid its shape, the tarsus. It extends from here, to the lid margin.
To see the tarsus from in front we'll remove the orbicularis muscle, and then the orbital septum. We've also removed much of the orbital fat. The tarsus is here.
If we put something underneath it we can see its upper border. The tarsus is tethered at each end to the palpebral ligaments. The tarsus is quite flexible. It stiffens the eyelid, and gives it a curvature that varies so that the shape of the eyelid conforms to the changing curvature of the underlying eye.
Within the thickness of the tarsus are numerous tarsal glands. This is one of them. The tarsal glands open here, right along the margin of the lid. The tarsal glands produce an oily secretion which prevents the spillage of tears.
This structure coming down from behind is the levator muscle. This is its tendon. The levator tendon inserts mainly into the tarsus; its most lateral and medial fibers are attached to the palpebral ligaments.