Which structure is considered part of the outer layer of the eyeball?

The cornea is considered part of the outer layer of the eyeball because it serves as the transparent front part that covers the eye. Its primary role is to provide most of the eye's optical power due to its curvature, and it also protects the inner structures of the eye from dust, debris, and potential injury. The outer layer, known as the fibrous tunic, consists of the cornea and the sclera, the white part of the eye. This outer layer plays a crucial role in maintaining the structural integrity of the eyeball while allowing light to enter, enabling vision.

In contrast, the iris and ciliary body are part of the middle layer, known as the vascular tunic, which is responsible for controlling light entry through the pupil and aiding in the focusing of light, respectively. The retina is also not part of the outer layer; instead, it is part of the inner layer where light is converted into neural signals for vision. Understanding these distinctions helps clarify the roles and locations of various structures in the eye.