The Orbit of the Eye Overview

Overview of the Orbit

• The orbit is a pyramid-shaped bony recess in the anterior part of the skull, lined by the periosteum called the periorbital fascia.
• The orbit contains the eye, extrinsic muscles, optic nerve, oculomotor nerve, ciliary ganglion, trochlear nerve, CN V1, abducens nerve, ophthalmic artery, superior and inferior ophthalmic veins, lacrimal apparatus, and fatty tissue.

Openings in the Orbit

• Optic foramen: located in the lesser wing of the sphenoid, passes through the optic nerve and ophthalmic artery.
• Superior orbital fissure: located in the greater and lesser wing of the sphenoid, passes through CN III, IV, VI, and superior and inferior ophthalmic veins.
• Inferior orbital fissure: located in the greater wing of the sphenoid and maxilla, passes through the infraorbital nerve and vessels, and zygomatic nerve.
• Supraorbital foramen/notch: located in the frontal bone, passes through the supraorbital nerve and vessels, and supratrochlear nerve and vessels.
• Infraorbital groove and canal: located in the maxilla, passes through the infraorbital nerve and vessels.
• Zygomatic foramen: located in the zygomatic bone, passes through branches of the zygomatic nerve.
• Nasolacrimal canal: located in the lacrimal bone, passes through the nasolacrimal duct.

The Eye

• The eye is a spherical globe with a diameter of approximately 2.5 cm, located in the anterior portion of the orbit.
• The eye is surrounded by a thin capsule, fascia bulbi (Tenon's capsule), and has three coats: fibrous, vascular pigmented, and nervous.

Coats of the Eye

• Fibrous coat:
  • Sclera: outermost layer, very fibrous, white, opaque, and pierced by the optic nerve.
  • Cornea: transparent, avascular, and nourished by diffusion from the aqueous humor and capillaries.
• Vascular pigmented coat:
  • Uveal tract: composed of the choroid layer, ciliary body, and iris.
  • Choroid: pigmented (outer) and vascular (inner) layer between the sclera and retina.
  • Ciliary body: located between the choroid and iris, ring-shaped, and has a series of transparent fibers that form the suspensory ligament of the lens.
  • Ciliary muscle: changes the shape of the lens, supplied by the parasympathetic fibers from CN III (oculomotor).
• Nervous coat:
  • Retina: innermost coat, thin, highly vascular, and consists of an outer pigmented layer and an inner nervous layer.
  • Three areas: optic disc, macula lutea, and fovea centralis.

Contents of the Eyeball

• Refractive media:
  • Aqueous humor: clear fluid that fills the anterior and posterior chambers, believed to be a secretion from the ciliary process.
  • Vitreous body: transparent gel that fills the eyeball behind the lens, contributes to the magnifying power of the eye, and supports the posterior surface of the lens.
  • Lens: biconvex structure enclosed in a transparent capsule, responsible for focusing.

Clinical Correlate

• Accommodation of the eye:
  • Refers to the ability of the eye to change its focus from distant to near objects.
  • Involves the contraction of the ciliary muscle, relaxation of the suspensory ligament, and constriction of the pupil.
  • Convergence of the eyes during accommodation of the lens.
• Movements of the eyeball:
  • Terms used to describe eye movements: elevation, depression, abduction, adduction, intorsion, and extorsion.
  • Six voluntary extrinsic muscles: superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, and inferior oblique.
  • Innervation of the eye: sensory (optic nerve, trigeminal nerve), motor (oculomotor, trochlear, abducens nerves), and autonomics (parasympathetic, sympathetic).