(4.09)

Here's the superior oblique muscle. Like the rectus muscles it arises from the anulus. The superior oblique narrows into a tendon, which passes around this sling of fibrous tissue, the trochlea.

The trochlea is attached to bone here, behind the orbital margin. The trochlea acts as a pulley. The superior oblique tendon fans out, passing deep to the superior rectus to insert into the sclera on the top of the eyeball.

We'll go round to our view of the eye from in front, and add the oblique muscles to that picture. Here's the trochlea, here's the superior oblique tendon. The superior oblique muscle, acting alone, would rotate the top of the eye medially.

The inferior oblique muscle is down here. It's the only extra-ocular muscle that doesn't arise from the anulus. It arises here, behind the inferior orbital margin. It inserts into the sclera quite far back. To see that we'll go round to our view from above and behind.

Here's the inferior oblique, inserting into the sclera between the lateral rectus and the inferior rectus muscles. The inferior oblique, acting alone, would rotate the top of the eye laterally.

The oblique muscles act in conjunction with the rectus muscles, the inferior oblique with the superior rectus and vice versa. This prevents our eyes from rotating about their long axes, as we look up and down.

The last of the seven extra-ocular muscles to look at is the levator of the upper eyelid, levator palpebrae superioris.

The levator lies just above the superior rectus muscle: here it is. Arising from the anulus fibrosus back here, the levator fans out to become a broad tendon which inserts, as we'll see later, mainly into the tarsus of the upper lid.

Incorporated in the underside of the levator is a strip of smooth muscle, the superior tarsal muscle, that's innervated by sympathetic fibers. Changes in the tone of this smooth muscle cause our upper lids to droop when we're tired, and open wide when we're excited.

As we saw in the section on the cranial nerves, the extra-ocular muscles get their nerve supply from the third, fourth and sixth cranial nerves. The superior oblique is supplied by the fourth nerve, the trochlear; the lateral rectus is supplied by the sixth nerve, the abducent. The other five muscles are supplied by the third nerve, the oculomotor.

In the images that images that we've seen so far, the muscles have been dissected bare to show them clearly, but in reality each of the muscles as it passes forwards becomes surrounded by a sliding sheath of fibrous tissue. The fibrous sheaths of adjoining muscles blend together to form a hood above and below the eye. This hood is firmly attached to periosteum at two points: here laterally and here medially.

These fibers of attachment form the posterior parts of the lateral and medial palpebral ligaments. Indirectly, they hold the eye in position in the orbit. The tendon sheaths of the muscles are also continuous with this sheath of fibrous tissue, the capsule of the globe, that surrounds the posterior two thirds of the eyeball.