Free anatomy Study Guides

• The external layer, formed by the sclera and cornea
• The intermediate layer, divided into two parts: anterior (iris and ciliary body) and posterior (choroid)
• The internal layer, or the sensory part of the eye, the retina

Three chambers of fluid: Anterior chamber (between cornea and iris), Posterior chamber (between iris, zonule fibers and lens) and the Vitreous chamber (between the lens and the retina). The first two chambers are filled with aqueous humor whereas the vitreous chamber is filled with a more viscous fluid, the vitreous humor.
The sagittal section of the eye also reveals the lens which is a transparent body located behind the iris. The lens is suspended by ligaments (called zonule fibers) attached to the anterior portion of the ciliary body. The contraction or relaxation of these ligaments as a consequence of ciliary muscle actions, changes the shape of the lens, a process called accommodation that allows us to form a sharp image on the retina. Light rays are focused through the transparent cornea and lens upon the retina. The central point for image focus (the visual axis) in the human retina is the fovea. Here a maximally focussed image initiates resolution of the finest detail and direct transmission of that detail to the brain for the higher operations needed for perception. Slightly more nasally than the visual axis is the optic axis projecting closer to the optic nerve head. The optic axis is the longest sagittal distance between the front or vertex of the corna and the furthest posterior part of the eyeball. It is about the optic axis that the eye is rotated by the eye muscles. Some vertebrate retinas have instead of a fovea, another specialization of the central retina, known as an area centralis or a visual streak.