The eyeball is the sensory organ of the human body placed in the skull that provides sight. The eyeball is slightly asymmetrical. Its shape is maintained by constant intraocular pressure of 2 kPa, which slightly increases with age. The eyeball is set in an eye socket, and is attached by a total of six muscles that allow it to move. The eyeball consists of several sub-parts, each of which has its own function, and their joint functioning provides visual perception. The outer layer of the eyeball is protected by a rigid fibrous matter, known as the sclera. It passes in front of the eye on the cornea, a transparent, glossy layer allowing transmission of light rays towards the lens. Another part is the iris, which contains pigment cells and pupil, which, depending on the amount of light, changes its diameter. Between the sclera and the interior of the eyeball is a thin, fibrous layer called the choroid that provides nourishment to the surrounding tissues.
The interior of the eyeball is mostly filled with a gelatinous mass, which is made up of the vitreous humor and intraocular fluids. There is also a suspended lens, which ensures the breaking of light beams, which then appear on the back of the eye, or the retina, where it produces visual perception.
The lens is fixed with the ciliary body and attachment apparatus. The retina contains light-sensitive cells (rods) which allow the perception of contrasts and lighting effects. It also includes cells to distinguish between colors (cones). The place where the density of cones is greatest is called the yellow spot. The blind spot is the place where the optic nerve comes out. Both of these places, the yellow and blind spot, are also part of the retina.