Keratocentesis

Keratoplasty

Keratotomy

Ocular Adnexa

Each of our paired eyes is encased in a protective, bony socket called the orbit or orbital cavity.

Within the orbit, the eyeball is protected by a cushion of fatty tissue. The eyebrows mark the supraorbital area and provide a modest amount of protection from perspiration and sun glare. Further protection is provided by the upper and lower eyelids and the eyelashes that line their edges (Fig. 13-1).

The corners of the eyes are referred to as the canthi (sing. canthus); the inner canthus is termed medial (toward the middle of the body), and the outer canthus is lateral (toward the side of the body). The area where the upper and lower eyelids meet is referred to as the palpebral fissure. This term is related to the function of blinking, called palpebration through the combining form palpebr/o, meaning “eyelid.” Note the Be Careful box for another potentially confusing combining form that is also used for the dense connective tissue within the eyelids, the tarsal plates.

The eyelids are lined with a protective, thin mucous membrane called the conjunctiva, (pl. conjunctivae) that spreads to coat the anterior surface of the eyeball as well. The conjunctival sacs (also referred to as the upper and lower fornix of the eye) are the folded extensions of this membrane that provide the looseness necessary for movement of the eye.

Also surrounding the eye are two types of glands. Sebaceous glands in the eyelids called meibomian glands, or tarsal glands, secrete oil to lubricate the eyelashes, and lacrimal glands above the eyes produce tears to keep the eyes moist. These glands can become blocked or infected. The lacrimal gland, or tear gland, provides a constant source of cleansing and lubrication for the eye. The process of producing tears is termed lacrimation. The lacrimal glands are located in the upper outer corners of the orbit. The constant blinking of the eyelids spreads the tears across the eyeball. The tears then drain into two small holes called the lacrimal puncta in the upper and lower eyelids in the medial canthus, then into the lacrimal ducts (also called the lacrimal canals or canaliculi [tiny canals]), next into the lacrimal sacs, and finally into the nasolacrimal ducts, which carry the tears to the nasal cavity. Normally, there are few tears that need draining, but when an individual cries, the excess tears exit down the cheeks and through the nose.

The extraocular muscles attach the eyeball to the orbit and, on impulse from the cranial nerves, move the eyes (Fig. 13-2). These six voluntary (skeletal) muscles are made up of four rectus (straight) and two oblique (diagonal) muscles. The origin of five of these muscles is in a ringlike structure surrounding the optic nerve behind the eyeball called the annulus of Zinn (also referred to as the annular tendon). This is mentioned only because later, when the lens of the eye is described, another structure in the lens called a zonule of Zinn will be named. Note that the muscle to raise the eyelids, the levator palpebrae superior muscle, is also labeled. “Levator” is used for any muscles whose function it is to elevate a structure. When this muscle is dysfunctional, it can result in an eyelid that droops (ptosis). The orbicularis oculi are the sphincter (ringlike) muscles that close the eye.

The Eyeball

The anatomy of the eyeball itself is traditionally explained in three layers or tunics. The outer layer, or fibrous tunic, consists of the sclera and cornea. The middle layer, or vascular tunic, is composed of the choroid, ciliary body, and iris. The inner layer, or nervous tunic, consists of the retina (Fig. 13-3).

The Inner/Nervous Layer (Retina)

The inner layer of the eye, called the retina, is composed of several parts. The pars optica retinae contain the sensory receptors (rods and cones), the optic disk, the ora serrata, the macula lutea, and the fovea centralis. This layer is nourished by the retinal vessels that radiate from the optic nerve (Fig. 13-4).

retina = retin/o

The sensory receptors for the images carried by the light rays are named for their appearance. They are the rods, which appear throughout the retina and are responsible for vision in dim light and the cones, which are concentrated in the central area of the retina and are responsible for color vision (Fig. 13-5). Three types of cones, termed L, M, and S (for long, medium, and short) cones, are endowed with photopigments that react to different wavelengths of light that produce the perception of red, green, and blue vision. Those individuals who have difficulty with their color vision (through inheritance or trauma) have deficiencies in one or more of these cones.

The optic disk is the small area in the retina where the optic nerve enters the eye. Also called the optic papilla for its nipplelike appearance, it is referred to as the “blind spot” of the eye because of its lack of light receptors. The ora serrata (ora is the plural of os, meaning “an opening,” whereas serrata refers to its “notched” appearance) is the jagged border between the retina and the ciliary body of the choroid. The macula lutea (literally meaning a “yellow spot”) is the area of central vision in the retina, whereas the fovea centralis, or simply fovea, is the depression in the middle of the macula that is the area of sharpest vision because of its high density of cones (color receptors). The term fovea means a “small pit,” so the fovea centralis is literally a small pit in the middle of a yellow spot.

optic disk = papill/o

macula lutea = macul/o

Vision

The ocular adnexa and the fibrous, vascular, and nervous layers, or tunics, are essential to vision. All parts work together with impressive harmony. The eye muscles coordinate their movements with one another; the cornea and pupil control the amount of light that enters the eye; the lens focuses the image on the retina; and the optic nerve transmits the image to the brain through an opening in the skull termed the optic foramen.

Two important mechanisms contribute to the ability to see. As light hits the eye, it passes first through the cornea, which bends the rays of light (refraction) so that they are projected properly onto the receptor cells in the eye. The muscles in the ciliary body adjust the shape of the lens to aid in this refraction. The lens flattens to adjust to something seen at a distance, or thickens for close vision (a process called accommodation). Errors of refraction are the most common reason for lens prescriptions. See Figure 13-6 for an example of refraction in normal vision, as well as in nearsightedness (myopia) and farsightedness (hypermetropia), and how they are corrected through the use of corrective lenses.