Special Senses: The Eye

1. 16 Special Senses: The Eye

2. Learning Outcomes • State the description and primary functions of the eye. • Analyze, build, spell, and pronounce medical words. • Comprehend the drugs highlighted in this chapter. • Describe diagnostic and laboratory tests related to the eye. • Identify and define selected abbreviations.

3. Multimedia Directory Slide 6 Virtual Tour of the Eye Animation Slide 10 Structures of the Eye Animation Slide 35 Retina Animation Slide 36 Rods and Cones Animation Slide 40 Optic Nerve and Optic Disk Animation 1 Slide 41 Optic Nerve and Optic Disk Animation 2 Slide 50 Cataracts Video Slide 52 Conjunctivitis Video Slide 65 Macular Degeneration Video

4. Anatomy and Physiology Overview • Eye • Composed of special anatomical structures that work together to facilitate sight: • Cornea • Pupil • Lens • Vitreous body • Light stimulates sensory receptors (rods and cones) in the retina or innermost layer of the eye.

5. Anatomy and Physiology Overview • Vision is made possible through the coordinated actions of nerves that control: • the movement of the eyeball • the amount of light admitted by the pupil • the focusing of that light on the retina by the lens • the transmission of the resulting sensory impulses to the brain by the optic nerve.

6. Virtual Tour of the Eye Animation Click on the screenshot to view an animation of the tour of the eye. Back to Directory

7. Insert table 16-1

8. Insert table 16-1

9. Figure 16.1 Internal structure of the eye.

10. Structures of the Eye Animation Click on the screenshot to view an animation of the structures of the eye. The animation may take a moment to begin playing. Back to Directory

11. External Structures of the Eye • Orbit • A cone-shaped cavity in the front of the skull that contains the eyeball. • Formed by the combination of several bones and is lined with fatty tissue that cushions the eyeball. • This cavity has several foramina (openings) through which blood vessels and nerves pass. • Largest opening is the optic foramen for the optic nerve and ophthalmic artery.

12. External Structures of the Eye • Muscles of the Eye • Six eye muscles control the movement of the eye. • Four are rectus muscles, and two are oblique muscles. • Rectus muscles allow a person to see up, down, right, and left. • Oblique muscles allow the eyes to turn to see upper left and upper right, lower left and lower right.

13. External Structures of the Eye • Muscles of the Eye • Eye muscles help maintain the shape of the eyeball.

14. Figure 16.2 (A) Lateral view, left eye.

15. Figure 16.2 (continued) (B) Anterior view, left eye.

16. External Structures of the Eye • Eyelids • Protect the eyeball from: • Intense light • Foreign particles • Impact • Keep the eyeball’s surface lubricated and free from dust and debris.

17. External Structures of the Eye • Eyelids • Canthus • Palpebral fissure • Eyelashes • Meibomian Glands

18. External Structures of the Eye • Conjunctiva • A mucous membrane that lines the underside of each eyelid and reflects onto the anterior portion of the eyeball. • This membrane acts as a protective covering for the exposed surface of the eyeball.

19. External Structures of the Eye • Lacrimal Apparatus • Those structures that produce, store, and remove the tears that cleanse and lubricate the eye: • Lacrimal gland • Lacrimal canaliculi (ducts) • Lacrimal sac • Nasolacrimal duct

20. Figure 16.3 Lacrimal glands and lacrimal canaliculi (ducts).

21. External Structures of the Eye • Lacrimal Gland • Located above the outer corner of the eye. • Secretes tears through approximately 12 ducts onto the surface of the conjunctiva of the upper lid. • This fluid washes across the anterior surface of the eye and is collected by the lacrimal canaliculi (ducts).

22. External Structures of the Eye • Lacrimal Canaliculi • Two ducts at the inner corner of the eye that collect tears and drain into the lacrimal sac. • Lacrimal Sac • The enlargement of the upper portion of the lacrimal duct. • Tears secreted by the lacrimal glands are pulled into this sac and forced into the nasolacrimal duct by the blinking action of the eyelids.

23. External Structures of the Eye • Nasolacrimal duct • The passageway draining lacrimal fluid into the nose. • The lacrimal sac is the enlarged upper portion of this duct.

24. Life Span Considerations • The eyes begin to develop as an outgrowth of the forebrain in the 4-week-old embryo and are complete at 24 weeks. • At 28 weeks, eyebrows and eyelashes are present, and the eyelids open. • The newborn can see, and visual acuity is estimated to be around 20/400.

25. Life Span Considerations • Most newborns appear to have crossed eyes because their eye muscles are not fully developed. • At first, the eyes appear to be blue or gray. • Permanent coloring becomes fixed between 6 and 12 months of age. • Tears do not appear until approximately 1 to 3 months because the lacrimal gland ducts are immature.

26. Life Span Considerations • Depth perception begins to develop around 9 months of age. • Visual acuity improves with age, and, by the age of 2 or 3 years, it is around 20/30 or 20/20. • Children are farsighted until about 5 years of age.

27. Internal Structures of the Eye • The eyeball, its various structures, and the nerve fibers connecting it to the brain make up the internal eye.

28. Internal Structures of the Eye • Eyeball • The organ of vision, globe shaped and divided into two cavities: • The ocular cavity • The space in front of the lens. • Further divided by the iris into anterior and posterior chambers. • Anterior chamber is filled with a watery fluid known as the aqueous humor. • A much larger cavity behind the lens is filled with a jellylike material, the vitreous humor, which maintains the eyeball’s spherical shape.

29. Internal Structures of the Eye • Eyeball’s Outer Layer • Composed of: • Sclera, or white of the eye • Cornea, or transparent anterior portion of the eye’s fibrous outer surface. • The curved surface of the cornea is important because it bends light rays and helps to focus them on the surface of the retina.

30. Internal Structures of the Eye • Eyeball’s Middle Layer • Known as the uvea, it lies just below the sclera and consists of the: • Iris • Ciliary body • Choroid

31. Internal Structures of the Eye • Middle Layer: Iris • A colored membrane attached to the ciliary body and suspended between the lens and cornea in the aqueous humor. • Has a circular opening in its center called the pupil. • Has two muscles that contract or dilate to regulate the amount of light admitted by the pupil.

32. Internal Structures of the Eye • Middle Layer: Ciliary body • A thickened portion of the vascular membrane to which the iris is attached. • Smooth muscle forming a part of the ciliary body governs the convexity of the lens. • Secretes aqueous humor that nourishes the cornea, lens, and surrounding tissues.

33. Internal Structures of the Eye • Middle Layer: Choroid • A pigmented vascular membrane that prevents internal reflection of light. • Inner Layer • The retina contains photoreceptive cells (rods and cones) that translate light waves into nerve impulses.

34. Figure 16.4 Retina as seen through an ophthalmoscope.

35. Retina Animation Click on the screenshot to view an animation on the retina of the eye. Back to Directory

36. Rods and Cones Animation Click on the screenshot to view an animation on the rods and cones of the eye. Back to Directory

37. Internal Structures of the Eye • Inner Layer • Most of the approximately 6 million cone cells are grouped into a small area called the macula lutea. • In the center of the macula lutea is a small depression, the fovea centralis, which is the central focusing point within the eye and contains only cone cells.

38. Internal Structures of the Eye • Inner Layer • The eye contains approximately 120 million rods that are sensitive to dim light. • The rods contain rhodopsin, a pigment necessary for night vision. • Optic disk: the point at which nerve fibers from the retina converge to form the optic nerve.

39. Internal Structures of the Eye • Inner Layer • Blind spot: the absence of rods and cones in the area of the optic disk creates a blind spot on the retina’s surface; the only part of the retina that is insensitive to light.

40. Optic Nerve and Optic Disk Animation 1 Click on the screenshot to view an animation on the optic nerve and optic disk. Back to Directory

41. Optic Nerve and Optic Disk Animation 2 Click on the screenshot to view an animation on the optic nerve and optic disk. Back to Directory

42. Internal Structures of the Eye • Lens • A colorless crystalline body biconvex in shape and enclosed in a transparent capsule. • Suspended by ligaments just behind the iris. • Contraction and relaxation of the ciliary muscle control the tension of the suspensory ligaments to change the shape of the lens. • The function of the lens is to sharpen the focus of light on the retina.

43. Internal Structures of the Eye • Lens • Accommodation keeps the image in the same place on both retinae by combining changes in: • The size of the pupil. • The curvature of the lens. • The convergence of the optic axes.

44. How Sight Occurs • Light rays strike the eye and pass through the cornea, pupil, aqueous humor, lens, and vitreous humor. • Light rays then reach the retina and stimulate rods and cones. • An upside-down image is relayed along nerve impulses to the optic nerve. • The images are transferred to the brain, which turns the images right-side-up.

45. Figure 16.5 Light entering the eye.

46. Figure 16.6 Image inverted on the retina.

47. Audio Pronunciations • Accommodation • Amblyopia • Anisocoria • Aphakia • Astigmatism a- = lack of, without stigmat = point -ism = condition

48. Audio Pronunciations • Bifocal • Blepharitis • Blepharoptosis • Cataract • Chalazion • Choroiditis blephar/o = eyelid -ptosis = prolapse, drooping

49. Figure 16.7 Cataract of the right eye.

50. Cataracts Video Click on the screenshot to view a video on the topic of cataracts. Back to Directory