Sensory Systems | Visual (Sight) System
Objective 7
Label the components of the eye. List the cell types of the retina. Trace the pathway taken by neural information as it passes from retina to brain.
The eye has a number of structures specialized for focusing an image on the retina, which is the receptive sheet of the eye (more about that later).
It is moved by six extraocular muscles.
A thin mucous membrane, the conjuctiva, covers the sclera and is also continuous with the internal surfaces of the upper and lower palpebrae. When the conjuctiva becomes inflamed, the patient has conjuctivitis, often called “pink eye”. This can result from either a bacterial or viral infection.
The eye is divided into three tunics, which are related embryologically and also have clinical importance.
From outermost to innermost, the three tunics and their constituents are:
- The fibrous tunic, consisting of the sclera (“white of the eye”) and cornea (clear covering over the pupil). In both eyeball images shown here, the fibrous tunic is white or blue.
- The vascular tunic, also called the uvea, consisting of the iris (colored part of the eye surrounding the pupil); the ciliary body (controls the shape of the lens) and the choroid (choriocapillaris; supplies blood to the retina and other structures of the eye). When this entire layer is inflamed it is called uveitis and may threaten the patient’s sight. Structures comprising the uvea are shown in pink in these diagrams.
- The nervous tunic, consisting of the retina, an outpocketing of the CNS inside the eye. The nervous tunic or neural retina is shown in yellow in these drawings of the eye.
Two viscous (i.e. goopy) liquids help maintain the shape of the eyeball. The vitreous body is the jelly-like substance in contact with the retina in the larger, central chamber of the eye. The other liquid is called the aqueous humor.
The ciliary body filters blood to secrete aqueous humor into the anterior chamber, located between the lens and inside of the cornea (blue in this diagram). Aqueous humor is a filtrate of the blood that nourishes the lens and cornea.
Aqueous humor is made by the ciliary body and flows along the posterior surface of the iris in the narrow posterior chamber. It then flows through the pupil and fills the anterior chamber. The scleral venous sinus (the old name is much more fun to say: the “canal of Schlemm”) is a drainage system for the aqueous humor.
The lens is a clear structure, shaped like an M&M®, that works with image on the retina. When the eye is focused on a distant object, the cornea to focus an image on the retina pulls on suspensory ligaments (zonular fibers; old name the “Zonules of Zinn” — learning anatomy used to be a lot more fun) to flatten the lens (regular M&M®). When the eye is focused on a near object, the ciliary muscle relaxes, and the lens snaps back into a more spherical shape (peanut M&M®). This process, which changes the shape of the lens to focus on objects near and far, i