1. Chapter 8 – Special Senses Eye sphere – 1 inch in diameter – only see 1/6 of eyeball

2. External Structure Eyelids – protection  Come together at medial and lateral canthus  Eyelashes along border  Tarsal glands – lubricate eye

3. External Structrure cont. Conjunctiva – lines eyelids and covers outer surface of eyeball  Ends at edge of cornea  Secretes mucous for lubrication  Conjunctivitis – pink eye

4. External Structure cont. Lacrimal glands – above lateral end of eye  Release tears which move across eye into lacrimal canals → lacrimal sac → nasolacrimal duct → empties into nasal cavity  Tears also contain antibodies and lysozymes  Cleanses and protects

5. External Structure cont. Extrinsic (external) muscles – 6  Lateral rectus – laterally  Medial rectus – medially  Superior rectus – elevates  Inferior rectus – depresses  Superior oblique – elevates and lateral  Inferior oblique – depresses and lateral

6. Internal Structure Tunics = coats; humors = fluids Sclera – outermost – aka fibrous tunic  Thick, white  Central anterior is clear = cornea  Where light enters

7. Internal Structure cont. Vascular tunic  Choroids – posteriorly – dark Prevents light from scattering  Ciliary body and ciliary zonule – hold lens in place  Iris – has opening called pupil Muscles control diameter – regulates light

8. Internal Structure cont. Sensory tunic  Retina – stops at ciliary body Contains photoreceptors – rods and cones Pass signals through bipolar and ganglion cells to optic nerve → optic cortex = vision Are all through retina except where optic nerve leaves – called the optic disk (blind spot)

9. Internal Structure cont. Rods – more dense at edge  Allow peripheral vision  Allow to see at night Cones – more dense in center  Allow color vision  3 types – different wavelengths  Missing cones = colorblindness Fovea centralis – pit next to optic disc  Contains only cones  Point of sharpest vision

10. Internal Structure cont. Lens – focuses light on retina  Divides eye into 2 chambers  Anterior (aqueous) segment Contains aqueous humor – clear, watery Provides nutrients to lens and cornea Reabsorbed through scleral venous sinus (canal of Schlemm) Blocked sinus = glaucoma – increase of intraocular pressure

11. Internal Structure cont.  Posterior (vitreous) segment Contains vitreous humor – gel like Keeps eye from collapsing in – maintains intraocular pressure  Cataracts – lens becomes milky

12. Light Pathway Refraction – bending of light – occurs when light passes through substances with different densities Occurs as light moves through cornea, conjunctiva, aqueous humor, lens, pupil, vitreous humor, retina Lens changes shape – causes more/less bend in light  Greater bulge (convexity) – more bending  Flatter – less bending  Eye at rest is set for distant vision About 20ft and no change is necessary  Closer and lens must bulge – accommodation Ciliary body contracts – lens becomes convex

13. Vision Emmetropia – “harmonious vision” Myopia – nearsightedness – “short vision”  Light is focused in front of retina  Eyeball is too long or cornea is too curved  Correction = concave lenses Hyperopic – farsightedness – “far vision”  Light focuses behind retina  Eyeball is too short  Correction = convex lenses

14. Visual fields & pathways to brain Nerves leave eye via optic nerve At optic chiasma some (medial) fibers cross Optic tracts – contain fibers from both eyes  Joint neurons in thalamus and are sent to occipital lobe of brain Allows binocular vision because the visual fields overlap

15. Eye reflexes Photopupillary reflex – pupil constriction due to bright light Accommodation papillary reflex – constriction for viewing close objects

16. Development Vision is the only sense not fully functional at birth.  No tears until 2 weeks  Newborn sees in gray tones  5 months – close focus, following moving objects  By 5 – color vision well developed, depth perception  6/7 – emmetropia  About 40 – presbyopia may occur – type of farsightedness