1. Vision and Structure of the Eye

3. General Info Our eyes give us our most important sense—sight or vision
Estimated that 90% of the data processed by our brains about the environment is related to vision
Processed in the visual cortex at the posterior of the brain
Eyes contain photoreceptors that are sensitive to various qualities of light

4. Accessory Structures
Several structures assist with the proper functioning of the eye, though not directly related to perceiving light
These include the eyelids, extrinsic muscles, and the lacrimal apparatus which is responsible for producing tears

6. Eyelids Protect anterior surfaces of the eyes
Eyelashes help protect from debris and shade the eyes slightly
The conjunctiva is a mucous membrane which lines the inside of the eyelid and folds back to cover the anterior portion of the eye; secretes mucus to help moisten and lubricate the eye

7. Lacrimal Apparatus
Consists of a lacrimal gland, ducts, and the nasolacrimal duct which channels excess fluid into the nasal cavity
Tears produced by the lacrimal gland are slightly salty and are released continuously
Help to moisten, lubricate, and clear eye of debris
Also help to create a smooth surface for refraction of light

8. Structure of the Eye Approx. 1 inch in diameter
Made of three distinct layers, or tunics, on its outer surface
These are the fibrous tunic, vascular tunic, and nervous tunic listed from superficial to deep
Inside is filled with fluid which is divided into chambers

9. Fibrous Tunic
Made up of two parts; the posterior sclera and the anterior cornea
Sclera is also known as the “white of the eye”
Dense with blood vessel; penetrated on the posterior surface by the optic nerve
Cornea is the clear “window” that bulges out slightly

11. Vascular Tunic
Very dense with blood vessels that supply nourishment to various structures of the eye
Components include the choroid, the ciliary body, and the iris. Associated with the lens
Choroid is a thin, dark brown pigment that absorbs light bouncing around within the eye; reduces distortion of images

13. Vascular Tunic cont’d
Anterior of the choroid are the ciliary body and the iris which are made of smooth muscle tissue
Ciliary body connects to the lens by way of a suspensory ligament
Contraction of muscles in the ciliary body changes the shape of the lens to adjust focus, called accommodation

15. The lens provides separation of the interior of the eye into two chambers
The anterior chamber, located between the lens and the iris, is filled with a fluid called the aqueous humor, which circulates and is continuously recycled
The posterior chamber, located between the lens and the retina, is filled with a fluid called the vitreous humor which is not recycled

16. Vascular Tunic cont’d
The iris is the beautifully colored, donut-shaped portion of the eye.
Contraction of it’s muscle changes the size of the pupil (its blackened middle) which controls how much light enters into the eye

17. The Nervous Tunic
Made up of the retina, which is imbedded with numerous photoreceptors and nerves which transmit the data detected to the brain; rely on photo-reactive pigments
Rod cells – contain an elongated, cylindrical dendrite, sensitive to small levels of light Photoisomerization
Cone cells – contain a dendrite that tapers to a point, color sensitive and produce a sharper image

19. Rods and Cones

20. The Nervous Tunic cont’d
The axons from the rods and cones converge at a spot near the center of the retina called the optic disc
There are no photoreceptors at the optic disc which forms a blind spot
The macula lutea and fovea centralis are areas of the retina which have the highest concentration of cones which provide optimal visual acuity

21. Extrinsic Muscles Humans have six
Move and rotate the eyeball within its socket
Originate from the walls of the orbit and insert upon the outer surface of the eye

23. Muscles that Move the Eye
Superior rectus
Turns eye upward
Inferior rectus
Turns eye downward
Lateral rectus
Turns eye laterally
Medial rectus
Turns eye medially
Inferior oblique
Rotating movements of eye
Superior oblique

24. MR = Medial Rectus LR = Lateral Rectus SR = Superior Rectus
   
LR = Lateral Rectus
SR = Superior Rectus
IR = Inferior Rectus
SO = Superior Oblique
IO = Inferior Oblique

25. Common Vision Conditions
Emmetropia
Normal vision
Hypermetropia
Cannot see close objects clearly; farsightedness
Astigmatism
Inability to focus an image clearly on the retina due to unequal curvature of the lens, cornea, or retina
Nyctalopia
Inability to adjust vision to darkness; nightblindness

26. Common Vision Conditions Cont’d
Strabismus
Inadequate muscle action or an increase in the strength of one muscle over another; cross-eyed
Myopia
Cannot see distant objects clearly; nearsightedness
Cataract
Reduced vision due to the presence of opaque spots on the lens
Glaucoma
Increase in intraocular pressure that interferes with vision