The Extraocular Muscles

There are six extraocular muscles that help to move the eyeball in different directions: the superior, inferior, medial and lateral recti, and the superior and inferior obliques. All these muscles are supplied by the third cranial nerve except the lateral rectus (supplied by the sixth nerve) and superior oblique (fourth nerve).

All the extraocular muscles except the inferior oblique originate from a fibrous ring around the optic nerve (annulus of Zinn) at the orbital apex. The muscles fan out towards the eye to form a "muscle cone". All the recti muscles attach to the eyeball anterior to the equator while the oblique muscles attach behind the equator. The optic nerve, the ophthalmic blood vessels and the nerves to the extraocular muscles (except fourth nerve) are contained within the muscle cone (Figure 2.9).

The levator palpebrae superioris is associated with the superior rectus. It arises from just above the annulus of Zinn, runs along the roof of the orbit overlying the superior rectus and attaches to the upper lid skin and anterior surface of the tarsal plate of the upper lid. Tenon's capsule is a connective tissue covering plate of the eyelids. The tarsal plate gives stiff- that surrounds the eye and is continuous with ness to the eyelids and helps maintain its the fascial covering of the muscles. contour. The upper and lower tarsal plates are about 1mm thick. The lower tarsus measures about 5 mm in height, while the upper tarsus measures about 10-12mm.

The orbicularis oculi muscle lies between the skin and the tarsus and serves to close the eyelids. It is supplied by the facial nerve. The skin and subcutaneous tissue of the lids are thin. The inner surface of the eyelids is lined by the palpebral conjunctiva.

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