Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 35

The globe is composed of three basic layers or coats (Figure 1.17). The outer layer is the fibrous tunic, which is further divided into the cornea and sclera. The fibrous tunic provides shape to the eye. In addition, the anterior portion of the fibrous tunic (i.e., the cornea) is transparent, thus enabling light to pass through, and is shaped in a manner that makes it a powerful lens that refracts light rays centrally, toward the visual axis of the eye.

The middle layer is the vascular tunic, called the uvea (meaning “grape”). The uvea is further divided into the iris, ciliary body, and choroid, and is heavily pigmented and vascularized. It functions to restrict the amount of light entering the eye and to provide nourishment and remove waste products.

The innermost layer is the nervous tunic, which consists of the retina and optic nerve. The three tunics embrace the large, inner, transparent media of the eye: the aqueous humor, lens, and vitreous humor, which collectively function to transmit and refract light to the retina and provide an internal pressure that keeps the globe firmly distended.