Chapter 9: The Eye

Internal eye structures contribute to vision, while accessory eye structures provide protection The accessory structures are: Eye lids and exocrine glands Superficial epithelium of the eye Structures associated with the production, secretion, and removal of tears. Extrinsic eye muscles.

An epithelium called the conjunctiva covers most of the exposed surface of the eye except the transparent cornea. Secretions of the lacrimal gland bathe the conjunctiva and contain an enzyme that attacks bacteria Tears reach the nasal cavity after passing through a series of canals.

Extrinsic Eye Muscles 6 extrinsic eye muscles control external eye movements.

The eye has three layers 1. Outer fibrous layer 2. Vascular layer 3. Inner layer Most of the eye is covered by the sclera( the white of the eye) which is continuous with the cornea, both of which are part of the fibrous layer.

The layers of the eye

Fibrous Layer consists of the sclera and the cornea 1. Provides mechanical support and physical protection 2. Serves as an attachment for the extrinsic eye muscles Sclera is the “white of the eye” Cornea is transparent and continuous with the sclera. It contains no blood vessels and has limited repair abilities.

Vascular layer includes Iris: forms the boundary between the anterior and posterior chambers. Regulates the amount of light entering the eye Ciliary body: contains the ciliary muscle and the ciliary process which attach to the suspensory ligaments of the lens. Choroid

Contains many blood vessels 1. provides route for blood vessels that supply the eye. 2. regulate the amount of light entering the eye 3. secretes and absorbs the aqueous humor 4. controls the shape of the lens.

Eye Color 1. determined by number of melanocytes in the iris. 2. the presence of melanin granules in the pigmented epithelium on the posterior surface of the iris Few melanocytes in iris, light passes through to the pigmented epithelium and eye appears blue.

Retina Inner layer Consists of outer pigmented part and inner neural part Neural part contains two types of photoreceptors Rods: don’t discriminate among colors of light. Cones: give us the sharpest vision but require brighter light. densely clustered at the fovea—site of sharpest vision at the center of the macula

Structure of the eye Ciliary body and the lens divide the interior of the eye into a large posterior cavity and a smaller anterior cavity. The anterior cavity is subdivided into the anterior chamber-cornea to iris- and the posterior chamber-iris and lens Posterior cavity contains gelatinous vitreous body which helps stabilize the shape of the eye and supports the retina.

Rods and cones aren’t distributed evenly Rods and cones aren’t distributed evenly. More rods on the side, more cones at the back. The area of the most cones is called the macula. The highest concentration is at a smaller depression called the fovea. 1 million ganglion cells converge on the optic disc which becomes the optic nerve as it exits the back of the eye.

Blind spot Because the optic disc has no photoreceptors, light striking this area goes unnoticed. It is called the blind spot. Go to figure 9-13(page 320) in your book and follow the directions to demonstrate your blind spot.

Aqueous humor circulates within the eye and reenters the circulation after diffusion through the walls of the anterior chamber and into the veins of the sclera. The lens focuses a visual image on the retinal receptor. Light is refracted when it passes through the cornea and the lens. During accomodation, the shape of the lens changes to focus the image on the retina.

The lens Lies posterior to the cornea and is held in place by suspensory ligaments from the ciliary body. It is made of layers of cells surrounded by by a dense fibrous capsule. Read the section in your book on page 321 about light refraction and accommodation. Also read the Spotlight on 323.

Photoreceptors Photoreceptors detect photons. Rods provide the CNS about the presence or absence of photons. Allows us to see in dim light Cones provide info about the wavelengths of photons. Require brighter light. Three types Blue cones, green cones, and red cones.

Color blindness Two of the genes that control color vision are found on the X chromosome. The most common form of colorblindness is red-green. The gene for blue cones is found on chromosome 7. See if you have normal color vision. http://colorvisiontesting.com/ishihara.htm

Chemistry of vision Visual pigments are able to absorb photons Made of rhodopsin Made of opsin and retinal RETINAL MADE FROM VITAMIN A?

Visual Acuity 20/20: you can see details at 20 feet as clearly as a normal individual would. 20/15: better than average. You can see at 20 feet what a normal person can see at 15. 20/30: worse than average. You can see at 20 feet what a normal person can see at 30.

Accommodation Problem http://www.allaboutvision.com/conditions/myopia.htm

Visual Pathway