or, localized clusters of receptors (taste buds and olfactory epithelium)
secretes mucus which lubricates the eyeball
often accompanied by redness of the eye
lie between the eyelashes and help lubricate the eyeball.
An inflammation of one of these is called a sty
secrete an oily substance
Lacrimal glands continually release a dilute salt solution (tears) onto the anterior surface of the eyeball through several small ducts. Tears flush across eyeball into lacrimal canals, into lacrimal sac, then into nasolacrimal duct
contains: sclera (white of eye) and cornea (where light enters)
choroid- blood-rich posterior part, contains a dark pigment that prevents light from scattering within the eye. Leads to ciliary body, iris, pupil
circular muscles contract in close vision and bright light (pupil contracts)
Radial fibers contract in distant vision and in dim light (enlarges the pupil)
transparent neural layer-extends anteriorly only to the ciliary body.
contains rods and cones
the photoreceptor cells are distributed all over the neural retina, except here, where the optic nerve leaves the eyeball
lateral to each blind spot
held upright in the eye by a suspensory ligament attached to the ciliary body.
divided into two segments
contains a clear, watery fluid called the aqueous humor (this is continually formed by the capillaries of the ciliary body. helps to maintain the intraocular pressure of the eye, also provides nutrients)
reversed from left to right, inverted, and smaller than the object
the image normal focuses in front of the retina.
can see close objects without difficulty, but distant objects are blurred or indistinct.
correction requires a concave lens, causing the light reaching the eye to diverge
if the image focuses behind the retina.
need convex lenses to boost converging power of lens for close vision
blurred vision problem.
when the elasticity of the lens dramatically decreases with age.
difficulty in focusing for near/close vision.
can be tested with near point of accommodation-about 10cm in young adults, closer in children and farther in old age
generally tested by a Snellen eye chart
extrinsic eye muscles responsible for this–controlled by the somatic nervous system
essential for near vision