The Globe

The eye is a very elegant organ, and a wonderful example of the intimate relationship of structure to function. Each part of the eye is designed to achieve or contribute to the special sense of sight. The globe is composed of three basic layers or coats. The outer coat is the fibrous tunic composed of the cornea, the sclera, and the juncture of the two called the limbus. The fibrous tunic gives the eye a constant shape and form which is imperative for a functional visual system. In addition, the anterior portion of the fibrous tunic, the cornea, is transparent, enabling light to pass through, and shaped in a manner that makes it a powerful lens which refracts light rays centrally towards the visual axis of the eye.

The middle layer, or vascular tunic, is the uvea which consists of the iris, the ciliary body, and the choroid. The most anterior portion of the vascular tunic, the iris, extends from the ciliary body centrally just anterior to the surface of the lens. The iris is heavily pigmented and contains muscles which change the shape and size of the iris and the pupillary aperture to control the amount of light that enters the posterior segment to stimulate the retina. The ciliary body is involved in both the production and outflow of aqueous humor, a fluid which flows through the anterior segment. Aqueous humor is secreted from ciliary body processes, which are heavily pigmented central extensions of the ciliary body. Aqueous humor leaves the eye through the iridocorneal angle, a portion of which (the uveal meshwork sinus) is of ciliary body origin. The ciliary body and its processes provide a base on which lenticular zonules are attached. These zonules are fine fibrous bands which attach to the outer portions of the lens and hold it in place. Contractions of ciliary body muscle alter the tension of these zonules and are able to change the shape or position of the lens. This process, called accommodation, alters the degree to which light is refracted. Thus, the lens acts as a fine focusing mechanism, while the cornea serves as the most powerful fixed “lens” of the visual system. The choroid, located in the posterior half of the eye, is found between the outer sclera and the retina. It functions to provide nourishment to the highly metabolic retina and to modify internal light reflection and scatter, as it is either heavily pigmented or reflective. In some species, a special reflective structure, called the tapetum, is located within the choroid and acts to improve photoreceptor stimulation in dim illumination.

The third layer of the eye is the nervous coat which is made up of the retina and associated optic nerve. Briefly, the retina contains light sensitive cells (photoreceptors) which, after a series of intermediate modifying processes, transmit impulses to the brain via the optic nerve.

In addition to the three tunics, additional ocular components fill the interior of the globe: (i) the intraocular fluids (aqueous humor and vitreous humor) and (ii) the crystalline lens.

Aqueous humor is continuously produced by ciliary body processes at a slow rate and fills the anterior and posterior chambers of the eye (between the cornea anteriorly and the lens posteriorly), then drains out of the eye into the bloodstream through the iridocorneal angle to regulate the intraocular pressure of the normal eye. Aqueous humor provides vital nutrients to the avascular lens and cornea and also assists in removing metabolic waste products.

Vitreous humor, a gelatinous fluid, occupies the large chamber in the back of the eye. The vitreous humor helps support and distend the globe and also provides an optically clear medium through which light can pass essentially unaltered.