The eyeball is made up of three layers of tissue:
1) Sclera, which an external tough layer made of fibrous tissue, wraps up the eye, except at the front where it becomes thin and transparent to form the cornea. The sclera is commonly referred to as the white of the eye.
2) Choroid is the middle vascular layer which lies between the sclera and the retina. It ends at the front with the ciliary processes and the ciliary muscle, which holds the lens of the eye.
3) The retina is a the inner layer of the eyeball. It consists of light, sensitive tissue that lines the choroid. It is on the retina where all images that we see are focused on and transmitted through the optic nerve to the brain.
The cornea is at the front of the eyeball which is a fine transparent extension of the sclera. The iris is located also at the front, but beyond the cornea and it consists of a layer of pigmented muscles, which are composed of two types of fibers, radial fibers and circular fibers. These fibers open and close the pupil when they contract.
The lens is set just behind the iris, blocking the pupilar orifice, and held in place by the ciliary muscles. Between the cornea and the lens there is aqueous humor, which a liquid substance. The inner hollow of the eyeball is filled with the vitreous humor, a transparent, jelly-like liquid.
The images of the objects that we see are refracted by the cornea and then is focused on the retina by the lens, whose shape is altered by the ciliary muscle to make the vision adapt to look at objects situated at different distances.
The eyeball movement is controlled by six types of external muscles attached to the sclera and anchored to the orbital bones of the eye sockets. They are the medial rectus muscle, superior rectus muscle, inferior rectus muscle, lateral rectus muscle, inferior oblique muscle, and superior oblique muscle.