Extrinsic Muscles
Within an orbit, the eye is anchored in place by the extrinsic muscles, whose contractions move the eyes. There are three pairs of antagonistic extrinsic muscles:
Superior rectus Rolls eye upward Inferior rectus Rolls eye downward
Lateral rectus Turns eye outward, away from midline Medial rectus Turns eye inward, toward midline
Superior oblique Rotates eye counterclockwise Inferior oblique Rotates eye clockwise
Three cranial nerves-the oculomotor, abducens, and trochlear-control these muscles. The oculomotor nerve innervates the superior, inferior, and medial rectus muscles, as well as the inferior oblique muscles; the abducens nerve innervates the lateral rectus muscle; and the trochlear nerve innervates the superior oblique muscle.
Eyelids and Eyelashes
The thin skin of an eyelid covers muscle and connective tissue. Lining the inner surface is a transparent mucous membrane, called the conjunctiva. The conjunctiva folds back to cover the anterior of the eye, except for the cornea, and prevents tears from entering the orbits. The eyelids have eyelashes that can trap a tiny particle of grit, causing the eyes to close immediately. Sebaceous glands associated with each eyelash produce an oily secretion that lubricates the eye. Inflammation of one of the glands is called a sty.
The eyelids are operated by the orbicularis oculi, which closes the lid, and the levator palpebrae superioris, which raises the lid. A person with myasthenia gravis has muscle weakness due to an inability to respond to acetylcholine, and the eyelids often have to be taped open.
Lacrimal Apparatus
A lacrimal apparatus consists of the lacrimal gland and the lacrimal sac with its ducts. The lacrimal gland, which lies in the orbit above the eye, produces tears that flow over the eye when the eyelids are blinked. The tears, collected by two small ducts, pass into the lacrimal sac before draining into the nose by way of the nasolacrimal duct.
Accessory structures of the eye include the extrinsic muscles. vvhich move the eye; the eyelids and eyelashes; and the lacrimal apparatus. vvhich produces tears.
