To be able to see everything that surrounds us, including this video, we can count on two very special sense organs: the eyes. The eyes can be easily injured, so each of them is protected by a hard bony structure called the orbit. The orbits also protect the muscles, vessels and nerves of the eyes. And between each eye and the orbit protecting it, there’s a soft cushion of fat to prevent any friction or damage to the eyes. Additional protection is ensured by the lacrimal apparatus, which secretes tears to lubricate the eyes, and the eyelids, which close and open as needed.
Okay, now, if you look at a sagittal cut of the eyeball, you’ll see it’s shaped like two spheres fused, a bigger posterior one and a smaller anterior one, both with liquid inside. To keep this shape, the eyeball needs a solid structure: the fibrous layer, which is made of dense connective tissue and forms the skeleton of the eyeball. This layer has two parts: the one that makes the outer layer of the smaller sphere, called the cornea; and the one that makes the outer layer of the bigger sphere, called the sclera. The cornea is transparent and located at the anterior end of the eyeball. It allows light to pass through to the interior of the eyeball. Have you ever needed to use eye drops? Were you able to do that without instinctively closing your eyes once the drop touches your eye? The involuntary blinking is actually because of the corneal reflex! See, when something touches or irritates the cornea, it is sensed by the ophthalmic nerve, a branch of cranial nerve V. This sensory signal then reaches the brain stem, and signals the facial nerve, cranial nerve VII, to contract the orbicularis oculi to close our eyes. Now, the sclera is opaque and makes the white shell of the eyes. It occupies the majority of the posterior eyeball and serves for attachment of the extrinsic muscles of the eye. Also, the sclera is pierced by the optic nerve at the posterior end of the eyeball. The place where the cornea and the sclera meet, is called the corneoscleral junction.