 |
70% of all sensory receptors are in the eyes
|
the optic tracts that carry information from the retina to the occipital
lobe are the second largest tracts in the body -- only the corticospinal
tracts controlling voluntary muscles are larger
|
the eye is a sphere ~1 inch in diameter. It is cushioned in its socket by
a pad of fat.
|
accessory structures help the eye in its duties
 |
eyebrows : shade eyes from bright light and keep sweat from
trickling in
|
eyelids : also called palpebrae, protect the surface
of the eye. The eyelids meet at the medial and lateral canthi. A fleshy
elevation in the medial canthus, called the caruncle, contains sebaceous
and sweat glands that produce "sleepies." Muscles of the eyelid are programmed
to caused blinking every 3-7 seconds, as well as during threatening situations.
Everytime we blink, oil, mucus and saline solution are spread across the
eye's surface.
|
eyelashes : extend from the free margin of each lid. The follicles
of each lash support many nerves so that when things touch the eyelash, reflex
blinking occurs.
|
conjunctiva : a delicate mucus membrane that lines the eyelids
(palpebral conjunctiva) and folds back (or reflects) over the
anterior surface of the eyeball as the ocular conjunctiva. The ocular
portion covers only the white of the eye, not the cornea.
|
lacrimal apparatus : tear formation. The lacrimal gland
is found above the lateral end of the eye. It releases lacrimal
secretions through several small ducts. Blinking spreads the tears downward,
where they exit via the lacrimal puncta into the lacrimal canals,
where they eventually end up in the nasal cavity
 |
tears contain mucus, antibodies and lysozyme
|
|
extrinsic eye muscles : control the movement of the eyeball.
They originate from the bony orbit and insert into the eyeball's outer surface.
They also help maintain the eyeball's shape.
|
superior rectus, inferior rectus, medial rectus : all originate from
the annular ring and are controlled by III.
|
lateral rectus : originates from the annular ring and is controlled
by VI.
|
superior oblique : runs along the medial wall of the orbit, then makes
a 90 degree turn, passing through the trochlea to insert on the
superolateral portion. It moves the eye down and laterally. Controlled by
IV.
|
inferior oblique : rotates the eye up and laterally. Controlled by
III.
|
problems with these muscles, if they work in an uncoordinated manner, can
cause diplopia (double vision) or strabismus (cross-eyed).
| | | | |
| | | | | |
|
The eyeball itself is covered by "tunics" and is filled with "humors." The
lens divides it into anterior and posterior chambers.
|
fibrous tunic : composed of avascular tissue, it is divided
unto the sclera and cornea. The sclera is the white of the
eye and the cornea forms a "window" in the most anterior section of
the eye. The cornea is also well supplied with nerve endings, especially
nociceptors.
|
vascular tunic : also called the uvea ("grape"), it
is between the fibrous and sensory tunics. It has 3 distinct regions :
|
choroid : the posterior 5/6, its blood vessels provide nutrition to
all tunics and its melanin absorbs light
|
ciliary body : lines the area between the anterior and posterior segments;
it encircles the lens. It contains the ciliary muscles and suspensory ligament
(zonule), which hold and move the lens.
|
iris : the visible, colored part of the eye. It is between the cornea
and the lens.
| | |
|
sensory tunic : the two-layered retina. The outer,
or pigmented, layer abuts the choroid. The inner, or nervous,
layer is transparent and is composed of rods and cones. The spot where
the blind spot occurs, the optic disk, has no photoreceptors; it is
where the optic nerve attaches.
|
lateral to the optic disk is the macula lutae, which has a pit in
the center called the foveae centralis. Everything we view critically
is focused on the foveae, which have the greatest density of cones. The macula
lutae is mostly rods.
|
|
internal chambers and fluids : the anterior segment is filled with
aquaeous humor, which is formed constantly, with the excess draining
out the scleral venous sinus. If that sinus is blocked, pressure in
the eye increases, which is a condition called glaucoma. The posterior
segment is filled with vitreous humor, which is formed at birth and
doesn't drain.
|
the lens : a biconcave, transparent structure, made of lens epithelium
and crystallin lens fibers. The older a lens, the thicker and less flexible
it becomes. Cataracts usually results from age-related lens hardening.
|
rhodopsin : a deep purple pigment found in rods. It is formed and
accumulates in the darkness, and is "bleached" in light. Until the rhodopsin
is bleached in bright light, the rods can not react to the input and so we
are temporarily "blinded."
|
vitamin A deficiency -- leads to rod degeneration.
| | | | | | |
| | | | |
