10.9

1. 10.9 Sense of Sight

2. Visual Accessory Organs • Each eyelid has four layers • Skin (thinnest of the body) • Muscle • Connective tissue • Conjunctiva (mucous membrane that lines the inner surface of the eyelid) • Associated muscles • Orbicularisoculi – closes the lids when in contracts • Levatorpalpebraesuperioris – raises the upper lid

3. Visual Accessory Organs • The lacrimal apparatus consists of • Lacrimal gland – continuously secretes tears • Series of ducts • Superior and inferior • Lacrimal sac • Nasolacrimal duct • Nasal cavity • Moistens and lubricates the surface of the eye • Contains lysozyme which is antibacterial

4. Visual Accessory Organs • Extrinsic Muscles • 6 muscles that move the eyes in various directions • Origin from the bones of the orbit and insert into the broad tendons of the eyes tough outer surface

5. Structure of the Eye • Outer (fibrous) layer • Cornea – front 1/6 helps focus entering light rays • Sclera – white portion that protects the eye and acts as an attachment for muscles • Optic nerve – pierce the sclera along with blood vessel at a point at the back of the eye

6. Structure of the Eye • Middle (vascular) layer • Choroid coat - Loosely joined to the sclera • Honeycombed with blood vessels (nourishes) • Melanocytes help absorb excess light (keeps inside dark) • Ciliary body – extends forward from choroid coat • Controls the action of the suspensory ligaments which holds the lens in place • Allows accommodation (focusing of the lens)

7. Structure of the eye • Middle (vascular) layer • Iris – extends forward from ciliary body and lies between the cornea and the lens • Connective tissue and muscles that make up the colored part of the eye • Separates the anterior chamber and the posterior chamber • Filled with aqueous humor which nourishes • The pupil is the circular opening in the iris • The circular set of muscles makes the pupil smaller (lets less light in), while the radial set makes the pupil bigger (lets more light in)

8. Structure of the Eye • Inner (nervous) layer • Retina – contains visual receptor cells (photoreceptors) • The fovea centralisin the center of the macula lutea produces the sharpest vision • The optic disc is where the nerve fibers from the retina leave the eye and join with the optic nerve (blind spot) • The posterior cavity is filled with vitreous humor which supports the internal parts of the eye and helps maintain its shape

9. Light Refraction • To see an object, an object must be giving off light or reflecting light • Light waves enter the eye and are focused on the retina using refraction (bending light waves) at the cornea and the lens • The image on the cornea is upside down and reversed left to right • The visual cortex interprets the image in its proper position

10. Visual Receptors • Rods provide black and white vision • There are three different types of cones which provide color vision • When light hits these receptors, impulses are sent to the brain, but many must be stimulated to see a full image

11. Visual Pigments • Both rods and cones contain light-sensitive pigments that decompose when they absorb light energy • Rods have rhodopsin (broken down into opsinand retinal) • Cones have retinal bound to three different opsin molecules • Erythrolabe – red light waves • Chlorolabe – green light waves • Cyanolabe – blue light waves

12. Visual Nerve Pathways • Visual nerve pathways bring nerve impulses from the retina to the visual cortex, where they are perceived as vision • Axons of the retinal neurons leave the eyes to form the optic nerve • Some nerve fibers cross at the optic chiasma • Continue as the right and left optic tract • A few fibers enter nuclei that function in reflexes • The rest continue through the thalamus as optic radiations which lead to the visual cortex of the occipital lobe