The primary organ responsible for vision is the eye. The eyeball is located within a bony orbital encasement, which protects it. The lacrimal apparatus keeps the eye moist and free of dust and other irritating particles through the production and drainage of tears. Eyelids protect the eye from external stimuli such as dust, wind, and excessive light.
The bony orbit is the region of the skull that surrounds the eye and is composed of the following structures (Figure 18-1A):
- Superior wall. Formed by the frontal bone and the lesser wing of the sphenoid bone.
- Supraorbital foramen. Transmits the supraorbital nerve [cranial nerve (CN) V-1] and vessels to the scalp.
- Lateral wall. Formed by the zygomatic bone and the greater wing of the sphenoid bone.
- Inferior wall. Formed by the maxillary, zygomatic, and palatine bones.
- Infraorbital foramen. Transmits the infraorbital nerve (CN V-2) and vessels to the maxillary region of the face.
- Medial wall. Formed by the ethmoid, frontal, lacrimal, and sphenoid bones.
- Anterior and posterior ethmoidal foramina. Transmits the anterior and posterior ethmoidal nerves and vessels, to the nasal cavity and the sphenoid and ethmoid sinuses.
- Nasolacrimal canal. Formed by the maxillary, lacrimal, and inferior nasal concha bones. Drains tears from the eye to the inferior meatus in the nasal cavity.
- Optic canal. Transmits the optic nerve (CN II) and the ophthalmic artery.
- Superior orbital fissure. An opening between the greater and lesser wings of the sphenoid bone that transmits the oculomotor, trochlear, ophthalmic, and abducens nerves (CNN III, IV, V-1, and VI, respectively), and the ophthalmic veins.
- Inferior orbital fissure. Communicates with the infratemporal and pterygopalatine fossae. The inferior orbital fissure transmits CN V-2 and the infraorbital artery and vein.
A. Bony orbit. B. Lacrimal apparatus. C. Sagittal section of the eyelid. D. A normal right eye in contrast to an eye with Horner's syndrome.
The lacrimal gland lies in the superolateral corner of the orbit (Figure 18-1B).
- Tears. The lacrimal gland secretes tears that spread evenly over the eyeball through blinking and cleanse the eye of dust and foreign particles.
- Drainage. Tears drain from the eyeball, via the nasolacrimal duct, into the inferior nasal meatus of the nasal cavity.
- Innervation. The lacrimal gland is innervated by visceral motor parasympathetic neurons from CN VII (Figure 18-4C).
Because the mucosa of the nasal cavity is continuous with the mucosa of the nasolacrimal duct system, a cold or “stuffy” nose often causes the lacrimal mucosa to become inflamed and swollen. Swelling constricts the ducts and prevents tears from draining from the eye surface, causing “watery” eyes.