Overview of the Visual System
Chapter 1 of 5
The eye is often referred to as a window to the soul. While this is a statement about our emotions, in reality, the eye is an extension of the central nervous system that converts light signals into electrical signals for brain processing.
The Eye Has 3 Main Layers
The Outer Layer: The Sclera
The sclera is a white, tough tissue that is the outer coating of the eyeball visible as “the white” of the eye. The sclera is protective, and also functions as the insertion point for the extra-ocular muscles that control eye movement.
The Middle Layer: The Choroid
The middle layer, the choroid, or uveal tract consists of the iris, the ciliary body and the choroid. The iris controls the amount of light entering the eye through the pupil, and is noticeable due to its variety of colors. The ciliary body controls the focusing ability of the eye, and it also produces the aqueous fluid in the front chambers of the eye. The choroid is full of vessels, and provides nourishment for the other structures.
The Inner Layer: The Retina
The retina is the most internal layer of the eye. It is composed of ten distinct layers, including specialized light-sensitive neurons: rods and cones. Cones are located centrally in the macular region, with the highest concentration found in the fovea (the center of the macula). Cones are responsible for color perception and details. Therefore, when we discuss visual acuity, we also discuss cone function. Rods, which are more concentrated in the periphery of the retina, are responsible for low-light vision and movement detection. Eventually, all of the electrical signals created by these photoreceptors travel through a series of interconnecting neurons to finally reach the optic nerve. That bundle of nerve fibers is the beginning of the visual pathway, where the visual signal travels from the eye to the occipital (more posterior) part of the brain, where the sense of vision comes to life.
The Crystalline Lens Is the Central Part of the Eye
The crystalline lens, or simply lens, is transparent, biconvex, and can change its shape inorder to adjust the focusing of the eye – a process known as accommodation.
The Inside of the Eye Is Divided Into 3 Different Regions
The eye is divided into different anatomical landmarks or regions. The anterior segment consists of the anterior and posterior chambers. Past the lens is the vitreous body. The back of the eye is referred to as the posterior segment.