The orbit is a cone-shaped bony socket that surrounds the eyeball and optic nerve. Its medial walls run nearly parallel, while the lateral walls meet at close to right angles.
Boundary | Constituents |
|---|---|
Roof | Frontal bone and lesser wing of sphenoid |
Floor | Maxilla, palatine and zygomatic bones |
Medial wall | Maxilla, lacrimal, ethmoid and sphenoid bones |
Lateral wall | Zygoma and greater wing of sphenoid |
Apex | Opening to optic canal |
Base | Bounded by eyelids (known as orbital rim) |
Muscles
The muscles of the eye can be divided into intraocular and extraocular.
Intraocular:
Ciliary muscles
Sphincter pupillae
Dilator pupillae
Extraocular (discussed below)
Extraocular muscles
The rectus muscles arise from a common tendinous ring at the back of the orbit and attach to the eyeball just in front of its equator, near the cornea–sclera junction. Their nerve supply follows the classic rule LR6 SO4, all others III: the lateral rectus is innervated by the abducens nerve (CN 6), the superior oblique by the trochlear nerve (CN 4), and the remaining recti plus the inferior oblique and the levator palpebrae superioris by the oculomotor nerve (CN 3).
As the orbit and the eyeball point along slightly different axes, each muscle tends to pair a vertical movement with either adduction or abduction. The medial rectus pulls the eye inward (adduction) and the lateral rectus pulls it outward (abduction). The superior rectus elevates and adducts, while the inferior rectus depresses and adducts. In contrast, the superior oblique depresses and abducts, and the inferior oblique elevates and abducts. In practice, any smooth, coordinated eye movement recruits at least three of these muscles working together.