1. CHAPTER 10
THE EYE AND VISION

2. EYES- sense organs that provide us with the greatest knowledge of our environment
Light energy  Nerve impulses  Optic nerve  Sight

3. ACCESSORY ORGANS
There are several accessory organs that house and protect the eye from external hazards
1. Orbital cavities- bony sockets of skull
- surrounded by 7 different bones: Frontal, maxillary, zygomatic, sphenoid, lacrimal, ethmoid, palatine

4. 2. Eye muscles
- Intrinsic- found in the eye
- Extrinsic- control eye movements
* inferior rectus
* lateral rectus
* medial rectus
* superior rectus
* inferior oblique
* superior oblique

5. - all of these work to control the position of the eye
3. Lacrimal Gland- responsible for the production of tears
- tears that do not evaporate are drained to nasal cavity by lacrimal ducts

6. - when tear production increases, lacrimal ducts cannot keep up and tears flow over the bottom lid
4. Eyebrows- protect upper part of orbital cavity
5. Eyelids
- controlled by the orbicularis oculi

7. - are under voluntary control
6. Eyelashes
- secretions from the tarsal glands prevent the eyelashes from sticking together
7. Conjunctiva- thin, transparent mucous membrane
- Conjunctivitis (pink eye)- caused by irritants or lack of sleep

8. EYE CHAMBERS The interior of the eye can be divided into 2 chambers:
1. Posterior cavity- contains gelatinous VITREOUS HUMOR
2. Anterior cavity- subdivided into anterior chamber and posterior chamber
- filled with fluid called AQUEOUS HUMOR

9. - The shape of the eye is stabilized by both the vitreous humor and the aqueous humor

10. COMPONENTS OF THE EYE
1. SCLERA- white, opaque membrane that also helps keep the shape of the eye
- extrinsic eye muscles insert here
2. CORNEA- transparent and colorless part of the eye through which light waves pass
- contains no blood vessels; its cells obtain nutrients from tears

11. - often called the “window of the eye”
3. UVEA- made up of choroid coat, ciliary body, and iris
- also contains the intrinsic eye muscles
Functions of the uvea include:
- regulating amount of light entering eye

12. - secreting and reabsorbing aqueous humor
- controlling shape of lens

13. *CHOROID- supplies blood vessels to the layers of the eye, especially to retina
- contains pigment granules that prevent reflection of light WITHIN the eye (black interior of a camera)
*CILIARY BODY- contains one intrinsic muscle (ciliary muscle)

14. - fibers of this muscle support and modify the shape of the lens
*IRIS- attached to ciliary body and is the colored portion of the eye
- opening in the center of the iris is the PUPIL

15. - amount of light entering the pupil is regulated by muscles of the iris
- color of iris determined by number of melanocytes, and the pigmented epithelium on its posterior surface
- when melanocytes are absent, light passes through the iris and bounces off of this epithelium  eyes are BLUE

16. - individuals with increasing number of melanocytes in iris have gray, brown, or black eyes
- no pigment at all- iris appears pink  albinos

17. Why do many newborns have blue eyes?
- It takes several days for melanin to develop in melanocytes
- The amount of melanin is inherited from parents

18. Components cont. 4. RETINA- innermost layer of eye
- completely lines the inner surface of the eye
- contains cells responsible for converting light into impulses  called RODS and CONES

19. RODS AND CONES IN RETINA
- Rods do not discriminate among colors of light; they enable us to see in dimly-lit rooms, at twilight, or in pale moonlight
- Cones are responsible for color vision; give us sharper, clearer images, but require more light to do this
In the center of the retina is a yellow disk called the MACULA LUTEA

20. - in the center of the disk is a small depression called the FOVEA CENTRALIS
- this is the region for sharp vision as well as color sensitivity (contains only cones)
- area away from the fovea is the EXTRAFOVEAL REGION
- contains more rods

21. - when you look directly at an object, its image falls on the fovea centralis
OPTIC DISC- found where optic nerve leaves the eye
- also called BLIND SPOT- no rods or cones

22. 5. LENS- found just behind the pupil and iris
- disc-shaped and somewhat elastic
- held in place by a suspensory ligament
- ligament is attached to ciliary muscle, which controls amount of tension on the lens

23. THE LENS

24. STRUCTURE OF LENS
Lens consists of organized layers of cells wrapped in a dense fibrous capsule
- capsule is elastic: unless an outside force is applied, it will contract and make the lens spherical
- tension in the suspensory ligaments can overpower the elastic capsule and change the shape of the lens into a flattened oval

25. Lens is CONVEX- uniformly curved and thicker in the middle than at the edge
- parallel rays of light will CONVERGE and be brought to a single FOCAL POINT

26. REFRACTION AND ACCOMMODATION
In order for an image to produce useful information, it must be in focus
* meaning that rays of light from an object strike the retina in a precisely ordered manner to form a miniature image of the object
Focusing occurs in 2 steps:
- through the cornea
- through the lens

27. REFRACTION- bending of light when it passes from one medium to another with a different density
- in the eye, the greatest amount of refraction occurs when light passes from the air into the cornea
- the lens provides the extra refraction needed to focus light rays from an object toward a specific FOCAL POINT on the retina

28. FOCAL DISTANCE- distance between the center of the lens and the focal point
- determined by
* distance of object from the lens (closer the object, the longer the focal distance)
* shape of lens (rounder the lens, the shorter the focal distance)
- the lens changes shape to keep the focal distance constant

29. ACCOMMODATION- process of focusing an image on the retina by changing the shape of the lens
- lens either becomes rounder or flattens
When you view a nearby object, ciliary muscles contract, reducing tension in the suspensory ligaments and making the lens rounder

30. When you view a distant object, ciliary muscles relax, increasing tension in the suspensory ligaments and making the lens flat

31. IMAGE FORMATION The image of an object that arrives on the retina is:
- a miniature of the original
- upside down and backward
Our brain compensates for the reversal without our conscious awareness

32. VISUAL ACUITY VISUAL ACUITY- clarity of vision
- rated on basis of the sight of a “normal” person
- 20/20 vision means that a person can see details at a distance of 20 feet as clearly as a “normal” individual would
- 20/15 is better than average- at 20 feet, this person is able to see details that a normal person could only see at 15 feet

33. - 20/30 is below average- this person can see details at 20 feet that a “normal” person could see at 30 feet
- when visual acuity falls below 20/200, the individual is considered to be legally blind

34. ACCOMMODATION PROBLEMS
A normal eye condition is called EMMETROPIA- when ciliary muscles are relaxed and the lens is flattened, a distant image will be focused on the retina
IRREGULARITIES:
1. ASTIGMATISM- irregular shape of lens or cornea; can usually be corrected with glasses or contacts

35. 2. MYOPIA (nearsightedness)
- eyeball is too deep; image of a distant object will form in FRONT of the retina, picture will be blurry
- these individuals can see objects that are close
- can be corrected by placing a CONCAVE lens in front of the eye- rays of light diverge (go away from each other)

36. 3. HYPEROPIA (farsightedness)
- eyeball is too shallow; lens cannot provide enough refraction
- these individuals can see objects at a distance
- can be corrected by placing a convex lens (converging) in front of eye
- older individuals become farsighted as their lenses lose elasticity  PRESBYOPIA

37. VISUAL PHYSIOLOGY

38. PHOTORECEPTORS
Rods and cones are called PHOTORECEPTORS because they detect PHOTONS- the basic units of visible light
- light is a form of energy, which is radiated in waves and described in terms of WAVELENGTHS
- we can see wavelengths that make up the visible spectrum

39. WHAT IS
ROY G BIV?
- an acronym for the spectrum of visible light: Red, Orange, Yellow, Green, Blue, Indigo, Violet
- color depends on the wavelength of light
- longer wavelengths = less energy

40. MORE ABOUT RODS AND CONES
Rods provide the CNS with information about the presence or absence of photons, not about wavelength
- so, they do not discriminate among color
Cones do provide information about the wavelength of photons
- they are less sensitive than rods, so function only in bright light
- there are 3 types of cones: blue, green, red

41. COLORBLINDNESS- inability to distinguish certain colors
- occurs because one or more classes of cones are absent or do not function
- in most common condition, red cones are missing, and individual cannot distinguish red light from green light
- colorblindness is common in men, but very rare in women
Test

42. STRUCTURE OF A PHOTORECEPTOR
The outer segment of a rod or cone contains hundreds to thousands of flattened membranous discs
- names “rods” and “cones” refer to the shape of the outer segment
The inner segment synapses with other cells and releases neurotransmitters
Rods and cones

43. The discs of the outer segment in both rods and cones contain special organic compounds called VISUAL PIGMENTS
- the absorption of photos by visual pigments is the first step in the process of photoreception- the detection of light
- these pigments are derivatives of the compound RHODOPSIN- consists of the protein opsin bound to the pigment retinal

44. PHOTORECEPTION
Photoreception begins with a photon strikes a rhodopsin molecule in the outer segment of a rod or cone
- shortly after this, the rhodopsin molecule begins to break down into retinal and opsin  called BLEACHING
- the rhodopsin must then be regenerated from the retinal and opsin

45. - this process takes some time
- bleaching contributes to the lingering visual impression that you have after a camera flash goes off
- after this exposure to light, a photoreceptor cannot respond to further stimulation until its rhodopsin molecules have been regenerated (so you see the “ghost” image)