Seatwork: The Eye Question: Why do newborn babies have blue eyes?

Answer: The iris pigment is not yet developed. Although irises come in different colors, they contain only brown pigment. When there is a lot of pigment, eyes appear brown or black. If there isn’t much pigment, the shorter wavelengths of light are scattered from the non-pigmented parts, and eyes appear blue, green, or gray.

a. accessory organs-aid eye in its function (1) eyelids—4 layers (skin, muscle, connective tissue, conjunctiva) (a) orbicularis oculi—closes lids (b) levator palpebrae superioris— opens (raises upper lid) (c) conjunctiva—lines inner surface (not on cornea); protects eye from foreign objects

(2) lacrimal apparatus-produces tears that lubricate & cleanse eye (a) lacrimal gland—secretes tears (moist, & antibacterial due to lysozyme) (b) nasolacrimal duct—collects fluid & eventually enters nose (3) extrinsic muscles—move the eye in various directions—3 pairs: (a) inferior rectus (moves eye down) & superior rectus (moves eye up) (b) inferior oblique & superior oblique (rotate eye) (c) lateral rectus (moves eye outward) and medial rectus (moves eye inward)

b. structure of the eye—fluid filled hollow sphere with 3 layers (1) outer tunic (a.k.a. fibrous tunic) (a) anterior 1/6; transparent bulge (cornea) i. few cells & no blood vessels ii. arranged in regular patterns (what gives the cornea its transparency)

(b) posterior 5/6; white portion (sclera) i. collagenous & elastic fibers ii. protection & attachment for extrinsic muscles iii. posterior portion pierced by optic nerve

(2) middle tunic (vascular layer) (a) choroid coat—posterior 5/6 of the globe of the eye i. many blood vessels—nourishment for other eye tissues ii melanocytes—absorb excess light (darkly pigmented) (b) ciliary body—contains ciliary process & ciliary muscles; forms ring around eye

(c) lens—transparent & biconvex i. held in place by strong suspensory ligaments ii. surrounded by thin capsule attached to suspensory ligaments iii. action of ciliary muscles increases or decreases tension on ligament; lens shape changes in turn iv. accommodation: ability of lens to adjust shape to facilitate focusing example: lens becomes more convex (thickens) to view object closer

(d) iris—colored portion from outside, between cornea & lens; circular & radial set of muscles that adjust light entering pupil i. divides into 2 chambers filled with aqueous humor—make up anterior cavity anterior chamber: between cornea & iris posterior chamber: occupied by the lens (between iris & vitreous humor) ii. aqueous humor—watery fluid in chambers—circulates through pupil [a] provides nutrients & helps maintain shape [b] glaucoma—buildup of pressure

(3) Inner tunic—retina (back of eye to ciliary body) thin, transparent & contains photoreceptors (a) macula lutea—central region, yellowish spot. Has depression: fovea centralis (point of sharpest vision in retina) (b) optic disk—medial to fovea. Where axons converge to form optic nerve (vessels & nerve fibers leave from here) Called “blind spot” (no rods or cones) (c) posterior cavity—space bound by retina, lens & ciliary body. Contains vitreous humor (jellylike)

c. refraction of light (bending of light) (1) cornea refracts light more than lens (2) convex surfaces converge light (focus it to a point) (3) image focused on retina is inverted & reversed

d. visual receptors—modified neurons (rods & cones) rods: elongated; cones: blunt shaped (1) closest to choroids layer (inverted) (2) differences—many more rods than cones in human eye (a) rods—lower threshold of light intensity (more sensitive to light) (b) rods: vision in dim light (c) rods: achromatic vision; cones allow color vision (d) cones—give visual acuity (cones in fovea) allow for sharp, color images

e. visual pigments (1) rods—rhodopsin (a.k.a. visual purple) light sensitive pigment in rods; stable in dim light (2) cones—3 different sets of pigments (a) higher threshold of light intensity (chromatic vision) (b) respond to red, green & blue wavelengths.

f. stereoscopic vision—depth & distance perception (1) possible because pupils 6-7cm apart (requires 2 eyes) (2) objects 20 ft away or closer produce slightly different retinal images (3) interpreted as depth perception by visual cortex of brain; leads to 3D perception

g. visual nerve pathway—axons of ganglion cells in retina leave eyes to form optic nerves (1) optic nerves leave retina (2) go to optic chiasma (a) fibers from nasal (medial) half of retina cross over to opposite side (b) fibers from lateral (temporal) half of retina go to same side of brain

(3) optic tracts—impulses travel to thalamus on optic tracts. (4) optic radiations—from thalamus’ back, impulses enter paths called optic radiations which travel to visual cortex of occipital lobe

h. visual problems (1) nearsighted—myopia—image focuses in front of retina; corrected by concave lens (2) farsighted—hyperopia—image focuses behind retina; corrected by convex lens (3) astigmatism—imperfection of cornea or lens—can lead to eyestrain, headache (4) presbyopia—loss of efficiency (decreased power of accommodation that occurs with aging)

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