Eyeball

The eyeball, together with the optic nerve, forms part of the eye. It is the essential organ of vision.

It is bounded by a spheroid wall composed of three concentric layers, from the outside inward:

• The fibrous layer, which gives the eye its strength and spheroid shape. It includes the sclera and the cornea.

• The vascular layer or uvea, which is richly vascularized and consists of three parts: the choroidea, the ciliary body, and the iris.

• The inner layer, nervous or sensory, which lines the internal surface of the vascular tunic and forms the retina.

This three-layered wall encloses the ocular chambers, which contain the refractive media. These include the aqueous humor, the lens, and the vitreous body.

The eyeball varies in size and shape according to species: slightly flattened in equids, globular in domestic carnivores.