THE EYE   CHAPTER 10

THE EYE There are 4 cranial nerves: Optic (II), Oculomotor (III), Trochlear (IV), and Abducens (VI). People of Asian descent have an EPICANTHIC FOLD in the upper eyelid; no functional difference.

The eye 3 layers (tunics/coats) 70% of sensory receptors in the body are found here. Housed in the orbit of skull and surrounded by fat, blood vessels, nerves, and connective tissues. 2.5cm in diameter 3 layers (tunics/coats) Outer layer (fibrous) Protective, refracts light Includes cornea and sclera Middle layer (vascular) Contains pigments to keep eye dark inside Includes choroid coat, ciliary body, lens, iris, pupil, aqueous humor. Inner layer (nervous/sensory) Visual receptors Contains retina, fovea, optic disc, vitreous humor

External/Accessory Structures Eyelids – covers & protects/thin skin Eyelashes – fine hairs grow from eyelid; protect/ catch foreign particles Eyebrow – coarse hairs catch foreign particles/ sweat

PINK EYE - also known as CONJUNCTIVITIS CONJUNCTIVA is a clear, thin covering around the eye and under the eyelids.   PINK EYE - also known as CONJUNCTIVITIS    (from bacteria, very contagious) Pink Eye Slide Show from Web MD

GLANDS OF THE EYE LACRIMAL GLANDS are the largest set. They are on the superior lateral eyelid and they produce tears, which drain into the nasal cavity via the LACRIMAL DUCT. The function is to moisten and lubricate the eye surface, and it has enzymes (lysozyme) to kill bacteria (which thrive in warm, moist conditions).

Figure 16.5b

Extrinsic/External Eye Muscles Lateral rectus – moves eye laterally Medial rectus – moves eye medially Superior rectus – elevates eye Inferior rectus – depresses eye Inferior oblique – elevates & turns eye laterally Superior oblique – depresses & turns eye laterally Orbicularis oculi – opens & closes eyelid

Extrinsic Eye Muscles 5 1 3 4 6 2

Outer Tunic Cornea – transparent covering; focuses light rays. It has many nerve endings with pain fibers; can repair itself (Only tissue in body that can be transplanted from one person to another w/o rejection b/c it has no blood vessels) Sclera – protection, thick white connective tissue, outer layer Optic Nerve – connection from eye to occipital lobe; exits at the optic disk and transmits visual info from eye to brain Keratoconus is an eye condition in which the normally round cornea thins, causing a cone-like bulge to develop. Source: NPR http://n.pr/1a1oSx6

Middle Tunic Choroid coat – supplies blood vessels Ciliary body – 2 smooth muscle structures connected by suspensory ligaments (changes shape of lens & secretes aqueous humor) Lens – disc-shaped structure that bends/refracts light to be focused on the retina Iris – pigment (color) around pupil contains melanin (blue eyes = no melanin) Pupil – round opening through which light passes Aqueous humor – watery fluid b/w cornea & iris; helps maintain intraocular pressure & provides nutrients for lens & cornea

Figure 16.7a

Inner Tunic Vitreous humor – jellylike substance that gives the eye its shape & prevents it from collapsing inward Retina – innermost layer that contains rods & cones (visual receptor cells) Fovea Centralis – area of retina where only cones are found (clearest vision) Optic disc – area of retina lacking cones & rods/where nerve fibers leave the eye creating blind spot Canal of Schlemn – drains fluid that accumulates in eye located at the junction of the sclera & cornea

Retina The retina is made up of PHOTORECEPTORS, which are sensors for light.

Photoreceptor cells – found in retina Rods Cones 125 million 7 million Coarse detail Fine detail Black and white Color More sensitive to low light Less sensitive to low light Used for peripheral and night vision Used for central and detailed vision

Rods = monochromatic (b&w) Cones  = color vision

**Visual cortex of occipital lobe interprets the images** Pathway of Light Cornea Aqueous humor Pupil Lens Vitreous humor Retina **Visual cortex of occipital lobe interprets the images**

EYE PHYSIOLOGY Refraction: the cornea and lens bend light waves to focus an image on the retina Normal vision Image viewed by the eye are upside down Visual cortex of brain interprets it in its proper position

R.O.Y.G.B.I.V   Rainbows are seen after rain because light is passing through water droplets.  This separates the white light into the individual colors of the spectrum

Refraction Visual Nerve Pathway Journey Through the Visual System

Accommodation: Lens changes shape to focus on objects. Physiology cont… Accommodation: Lens changes shape to focus on objects. Ciliary muscles and suspensory ligaments aid in doing this When the ligaments pull outward, the lens flattens and focuses on distant objects (muscle relaxed) When ligaments are relaxed, the lens is more convex (thickens) focusing on closer objects (muscle contracts)

Accommodation Lens Accommodation

VISUAL ACUITY Ophthalmoscope: instrument that illuminates the interior of eyeball allowing the inside to be seen. Snellen eye chart: measure acuity 20/20 normal vision The higher the bottom number, the poorer the vision

Eye problems Myopia: nearsighted Hyperopia: farsighted Emmetropia: normal vision Astigmatism: cornea/lens developed an irregular shape Strabismus: cross-eyed Diplopia: double vision (can be caused by one eye deviating from line of vision).

Myopia = nearsightedness You can see near objects fine, but distant objects appear blurry

Hyperopia = farsightedness You can see distant objects fine, but close objects appear blurry

ASTIGMATISM is when the cornea has an irregular shape. ‘ Part of the field of view is out of focus.

Eye Videos Lasik Surgery: Eye Surgery Video Myopia vs. Hyperopia Vision Problems

Cataracts: lens becomes cloudy (laser surgery to fix) Cataracts video Glaucoma: increase in intraocular pressure, caused by an increase in aqueous humor Tonometer: instrument measuring pressure

Blindness: totally sightless in both eyes Common Causes of blindness include diabetes, macular degeneration, traumatic injuries, infections, glaucoma, and inability to obtain any glasses, loss of transparency of the cornea Treatments depends on cause Cornea transplant (high success rate b/c no blood vessels) Night blindness: poor vision in dim light resulting from a vitamin A deficiency.

What causes red-eye? The flash on a camera is bright enough to cause a reflection off of the retina -- what you see is the red color from the blood vessels. Many cameras have a "red eye reduction" feature. In these cameras, the flash goes off twice -- once right before the picture is taken, and then again to actually take the picture. The first flash causes people's pupils to contract, reducing "red eye" 

Why are babies born with blue eyes?   Melanin is a brownish pigment that adds color to your hair, eyes, and skin. At the time babies are born, melanin hasn't yet been "deposited" in the eyes' iris. Hence, they appear blue.  After about six months, eyes change color depending on the amount of melanin. If you have a lot of it, your eyes will turn dark brown.  If you have little, they'll stay blue. And if you have no melanin, your eyes may appear pink (albino). .

Color Blind   Normal Vision Color Blindness: Genetic trait carried on the X chromosome. Most common is red/green color blindness.  Normal Vision Color Blind 

 Normal Vision Color Blind 