Sense of Vision
The photoreceptors for sight are in the eyes. The eyes are located in orbits formed by seven of the skull’s bones (frontal, lacrimal, ethmoid, zygomatic, maxilla, sphenoid, and palatine). The bony ridge superior to the orbits, called the supraorbital ridge, protects the eye from blows, and serves as a location for the eyebrows. The eye has certain accessory organs.
Accessory Organs of the Eye
Accessory organs of the eye include: (1) the eyebrows, eyelids, and eyelashes; (2) the lacrimal apparatus, which produces tears; and (3) the extrinsic muscles that move the eye.
Eyebrows, Eyelids, and Eyelashes
Eyebrows have short, thick hairs positioned transversely above the eye along the supraorbital ridge (Fig. 9.5a). Eyebrows shade the eyes from the sun and prevent perspiration or debris from falling into the eye. Eyelids are a continuation of the skin. The eyelashes of the eye can trap debris and keep it from entering the eyes. Sebaceous glands associated with each eyelash produce an oily secretion that lubricates the eye. Inflammation of one of the glands is called a sty. Blinking of eyelids keeps the eye lubricated and free of debris. The eyelids are operated by the orbicularis oculi muscle which closes the lid, and by the levator palpebrae superioris muscle which raises the lid.
A person with myasthenia gravis has weakness in these muscles due to an inability to respond to acetylcholine, and the eyelids often have to be taped open. The inner surface of an eyelid is lined by a transparent mucous membrane, called the conjunctiva. The conjunctiva folds back to cover the anterior of the eye, except for the cornea which is covered by a delicate epithelium.
Figure 9.5 Accessory structures of the orbit. a. Sagittal section of the eye and orbit. b. The lacrimal apparatus.
A lacrimal apparatus consists of the lacrimal gland and the lacrimal sac with its ducts (Fig. 9.5b). 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.
Within an orbit, the eye is anchored in place by the extrinsic muscles, whose contractions move the eyes. Each of these muscles originates from the bony orbit and inserts by tendons to the outer layer of the eyeball. There are three pairs of antagonistic extrinsic muscles (Fig. 9.6):
Superior rectus Rolls eye upward
Inferior rectus Rolls eye downward
Lateral rectus Turns eye outward, away from mid-line
Medial rectus Turns eye inward, toward midline
Superior oblique Rotates eye counterclockwise
Inferior oblique Rotates eye clockwise
Although stimulation of each muscle causes a precise movement of the eyeball, most movements of the eyeball involve the combined contraction of two or more muscles. For example, if your left eye is directed upward toward your nose, which muscles are required? The answer is the superior and medial rectus muscles. Three cranial nerves-the oculomotor, abducens, and trochlear nerves-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. The motor units of these muscles are the smallest in the body. A single motor axon serves only about 10 muscle fibers, allowing eyeball movements to be very precise.
Figure 9.6 Extrinsic muscles of the eye, along with the anatomy of the eyelids and eyelashes.