Immune privilege of the eye involves the globe and its contents. Thus, only a passing reference will be made to the orbit and eyelids. The orbit is the bony, concave cavity in the skull that houses the globe, extraocular muscles, blood vessels and nerves of the eye. There is a very thin orbital floor (consisting of the maxillary, palatine, and zygomatic bones), a medial wall (consisting of the frontal process of the maxilla, lacrimal bone, orbital plate of the frontal bone, and lesser wing of the sphenoid), an orbital roof (consisting of the frontal bone), and a lateral wall (consisting of the zygomatic and greater wing of the sphenoid) (fig. 2) .
The globe is protected by the eyelids and lubrication of the ocular surface. The upper and lower eyelids are comprised of skin, subcutaneous connective tissue, and muscle. In addition, the tarsal plates in each lid consist of dense connective tissue and cartilage. They contain the meibomian glands - modified holocrine sebaceous glands - that are oriented vertically in two parallel rows through the tarsus. Movement of the eyelids assists lubrication of the surface of the globe, as well as protection from inadvertent trauma .
The surface of the cornea is protected by the tear film. It is a trilaminar layer consisting of an anterior lipid layer, a middle aqueous phase, and a posterior mucin layer . The anterior layer of the tear film contains polar and non-polar lipids secreted primarily by the meibomian (tarsal) glands. The sebaceous glands in the lid margin are in close relation to the eyelashes and also secret lipids (fig. 3). The middle aqueous layer is secreted by the main and accessory lacrimal glands. The main lacrimal gland is located in a shallow depression within the orbital plate of the frontal bone. The accessory lacrimal glands of Krause and Wolfring are located in the conjunctival fornices. The mucin layer of the tear film coats the superficial corneal epithelial cells and conjunctival surface. Tear mucins are secreted normally by the conjunctival goblet cells.
The thick outer coat of the eye, the sclera, is white and opaque. The transparent front window of the eye, which serves as the major refractive surface, is
the cornea (fig. 1). Light rays pass through the cornea into the anterior chamber which is filled with aqueous humor. The rays then continue as they pass through the lens, the major internal refractive structure of the eye. The rays then enter the clear vitreous cavity and impinge upon the neurosensory (neural) retina. The pigmented epithelium of the retina (retinal pigment epithelium) provides nourishment and support to the outer layers of the neurosensory retina - the photoreceptors (rods and cones). Nourishment for the photoreceptors is derived from the vessels within the choroid. The choroid extends anteriorly and with the ciliary body and iris comprises the uveal tract of the eye.
As light passes through the inner retina it is absorbed by the photorecep-tors in the neuorsensory retina. Light is either absorbed by rhodopsin, which is concentrated in the outer segment membrane of rods, or by opsin, which is located in cones. Phototransduction is the process by which light is captured by the photoreceptors and the small amount of energy is converted into a neural response. Through a series of biochemical reactions in the neural retina, the ganglion cells are depolarized and transmit a visual signal to the lateral geniculate nucleus of the central nervous system .
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