The Eye

The nearly spherical eyeball sits inside the orbit. Its volume is 4.5 to 5 cm3, its weight is 10 to 11 g, and its horizontal diameter is 19 to 22 mm. The wall of the ocular cavity consists of three layers, or tunicae: - the external tunica (sclera and cornea);- the middle tunica (uvea: choroid, ciliary body, and iris);- the internal tunica (retina with papilla and optic nerve).

The eyeball contains the aqueous humor that fills the anterior cavity and the vitreous humor (2.8 cm3), separated by the lens, a transparent, refractive, biconvex organ (8 mm thick, 0.5 cm3, 1.4 g) that plays an essential role in focusing.

The cornea is the transparent, anterior part of the external tunica. The uvea, a vascular, innervated tunica, includes the anterior lamina of the iris, the ciliary body, and the choroid. The iris, located in front of the lens, separates the posterior chamber of the eye from its anterior chamber.

The iris, an aperture that is greenish-blue, yellow, etc. and marked by many concentric or radiating striations, allows light to pass only through the pupil.The size of the pupil regulates light penetration. In bright daylight, it closes to form a very narrow, vertical slit. In semi-darkness, it opens into a circle.The thin, densely pigmented choroid located between the retina and sclera consists of a network of vessels. It supplies blood to the retina and the anterior segment of the eye. The upper half of the back of the eye is formed by the tapetum lucidum, consisting of cells that reflect and break down light, ensuring good night vision. The optic nerve connects the retina to the brain.

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