(3.12)

Now we'll move on to look at the extra-ocular muscles. There are seven of them. One, as we'll see, raises the upper eyelid. The other six, the four rectus muscles and the two oblique muscles, move the eye. The best way to look at these muscles is from above.

Here we're looking down into the orbit, in a dissection in which the orbital roof has been removed

We'll start by looking at the rectus muscles. To see them, we'll take these two muscles, the levator and the superior oblique, out of the picture. We'll also remove some of the fat that largely fills the posterior part of the orbit.

Here are three of the rectus muscles: superior, lateral and medial. To see the inferior rectus we'll remove the superior rectus, the optic nerve, and the rest of the orbital fat. Here's the inferior rectus muscle. Between the origins of the rectus muscles the optic nerve emerge.

The rectus muscles arise together from inside a ring of fibrous tissue, the anulus tendineus, of which this is the upper part. The anulus is attached to the periosteum of the apex of the orbit. Its ring of attachment, represented in blue here, encircles the optic canal, and this part of the superior orbital fissure. The optic nerve, the ophthalmic artery, and several of the nerves to the orbit pass through the anulus.

The rectus muscles thin out into four flat tendons as they pass forwards around the eye. They insert into the sclera quite far forward. To see where they insert, we'll go round to a view from in front. Here's the insertion of the superior rectus, here's the lateral, here's the inferior, here's the medial.

The main actions of the rectus muscles are obvious. The superior and inferior rectus muscles turn the eye upwards, and downwards, the lateral and medial ones turn it outwards and inwards.

The inferior and superior rectus muscles have another action too. They don't only pull the top or bottom of the eye straight backwards, they also tend to rotate it a little, about its long axis.

The superior rectus muscle acting alone would rotate the top of the eye inward as we look up, the inferior rectus would rotate it outward as we look down.

In fact the rectus muscles don't act alone. They act in conjunction with the two oblique muscles, the inferior oblique, and the superior oblique. We'll add those to our picture, starting with the superior one.