The outer shape of the eye comprises of two spheres of different radii, one set into the other (Figure 11.1). The anterior sphere, the cornea, is the smaller and more curved of the two and is completely transparent. The posterior sphere or sclera is a white, opaque, fibrous shell which encloses the ocular structures. Both tissues are relatively non-distensible and protect the eye from physical damage. The ring where the two areas join is called the limbus.
The outer tissues of the eye consist of three layers
1. the outermost layers, the sclera and cornea, provide protection for the delicate structures within
2. the middle layer, the uveal tract, has a nutritive function, being mainly vascular and consisting of the choroid, ciliary body and the iris
3. the innermost layer is the retina containing photoreceptors and is concerned with the reception of visual stimuli. The inner eye is divided by the lens that separates the aqueous and vitreous humors. The iris separates the aqueous humor into the anterior and posterior chambers.
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