What are the accessory structures of the eye?
-eyelids & associated exocrine glands
-superficial epithelium of the eye
-Structures associated w/ the production, secretion, & removal of tears
-extrinsic eye muscles
Eyelids (palpebrae)
Continuation of the skin
-Their blinking movements keep the surface of the eye lubricated & free from dust & debris
-protect the surface of the eye
-Upper & lower eyelids are connected at the medial/lateral canthus
-eyelashes are hairs that prevent partic
Lacrimal caruncle
A soft mass of tissue at the medial canthus that contains glands that produce thick secretions that contribute to the gritty deposits occasionally found after a night's sleep
Sty
An infection in the sebaceous gland of one of the eyelashes, in a tarsal gland, or in one of the sweat glands between the eyelash follicles produces a painful localized swelling
Conjunctiva
Mucous membrane; the epithelium covering the inner surfaces of the eyelids and the outer surface of the eye
-extends to the edges of the cornea
-conjunctivitis (pinkeye) results from damage to & irritation of the conjunctival surface
Cornea
a transparent part of the outer layer of the eye
-covered by a delicate corneal epithelium which is continuous w/ the conjunctiva
Tears
-Keep the surface of the eyeball moist & clean
-Reduce friction, remove debris, prevent bacterial infection, & provide nutrients/oxygen to the conjunctival epithelium
Lacrimal apparatus
Produces, distributes, & removes tears
Lacrimal gland
Tear gland
-Has many ducts that empty into the pocket between the eyelid and the eye
Lacrimal canal
Two small pores direct the tears from the medial canthus into passageways (lacrimal canals) that end at the lacrimal sac
Nasolacrimal duct
From this sac, the nasolacrimal duct carries the tears to the nasal cavity
What are the 6 extrinsic eye muscles (oculomotor muscles)?
They originate on the surface of the orbit and control the position of the eye
-Inferior rectus, medial rectus, superior rectus, lateral rectus, inferior oblique, and superior oblique
Eyeball
-Hollow; divided into 2 cavities, the posterior & anterior cavity
-posterior cavity (vitreous chamber) contains the gelatinous vitreous body
-smaller anterior cavity is subdivided into the anterior chamber & posterior chamber
What stabilizes the shape of the eye?
The vitreous body & the aqueous humor, a clear fluid that fills the anterior cavity
What are the three distinct layers (tunics) of the wall of the eye?
1. an outer, fibrous tunic
2. an intermediate vascular tunic
3. an inner neural tunic
Fibrous tunic
Outermost layer of the eye; consists of the
sclera
& the
cornea
1. provides mechanical support & some degree of physical protection
2. serves as an attachment site for the extrinsic eye muscles
2. assists in the focusing process
Sclera & Cornea
Sclera
: "White of the eye"
-layer of dense fibrous CT containing collagen & elastic fibers
-6 extrinsic eye muscles insert on the sclera
-surface contains small blood vessels
Cornea
: transparent; continuous w/ the sclera
-collagen fibers are organized i
Vascular tunic
Contains numerous blood vessels, lymphatic vessels, & the intrinsic eye muscles
-includes the iris, ciliary body, & the choroid
1. provides a route for blood vessels & lymphatic vessels that supply tissues of the eye
2. regulates the amount of light enter
Iris
Contains blood vessels, pigment cells, loose CT, & 2 layers of intrinsic smooth muscle fibers
-When these pupillary muscles contract, they change the diameter of the central opening, or
pupil
of the iris
-One layer of these muscles forms concentric circle
What determines eye color?
1. the number of melanocytes in the iris
2. the presences of melanin granules in the pigmented epithelium on the posterior surface of the iris
-no melanocytes: blue eyes
-more melanocytes: green, brown, black eyes
Ciliary body
-consists of the ciliary muscle, a ring of smooth muscle that projects into the interior of the eye
-begins at the junction between the cornea & the sclera & extends to the scalloped border that also marks that anterior edge of the neural tunic
-posterior
Choroid
A layer that separates the fibrous & neural tunics posterior to the ciliary body
-Contains a capillary network that delivers oxygen & nutrients to the neural tunic
Neural Tunic (Retina)
Innermost layer of the eye
-Consists of a thin outer pigment layer called the pigmented part & a thick inner layer called the neural part
-the pigmented part absorbs light after it passes through the neural part
-the neural part contains (1) the photorece
Photoreceptors
The cells that detect light; 2 types of photoreceptors are rods & cones
Rods
Do not discriminate among colors of light; very light-sensitive receptors enable us to see in dimly lit rooms
-give us peripheral vision b/c most are located on the sides & bottom of teh eye
Cones
Provide us w/ color vision; 3 types of cones are present & their stimulation in various combinations provides the perception of different colors
-give us sharper, clearer images but they require brighter light that do rods
-most are concentrated in the
ma
Macula lutea
The area where the visual image arrives after passing through the cornea & lens
-where cones are concentrated
Fovea Centralis
The center of the macula lutea; the area where the highest concentration of cones is found
Bipolar cells
The rods & cones synapse w/ many bipolar cells that synapse within the layer of ganglion cells
Ganglion cells
layer of cells adjacent to the posterior cavity
-create action potentials
-the axons deliver the sensory info to the brain
Horizontal & amacrine cells
Can regulate communication between photoreceptors & ganglion cells thus adjusting the sensitivity of the retina
Optic disc
A circular region just medial to the fovea
-The origin of the optic nerve
-from this point, the axons turn, penetrate the wall of the eye & process toward the diencephalon
-Has no photoreceptors or other retinal structures, thus it is known as the blind s
The Chambers of the Eye
The ciliary body & lens divide the interior of the eye into a small anterior cavity & a larger posterior cavity, or vitreous chamber
1.
Anterior cavity
: divided into the anterior/posterior chambers; filled w/ aqueous humor
2.
Posterior cavity
: filled w/
Aqueous Humor
-Secreted into the posterior chamber by epithelial cells of the ciliary processes
-pressure exerted by this fluid helps maintain the shape of the eye & the circulation of aqueous humor transports nutrients & wastes
Glaucoma
Increase in pressure exerted by the aqueous humor leads to glaucoma which can eventually produce blindness by distorting the retina & the optic disc
Lens
Lies posterior to the cornea & is held in place by suspensory ligaments extending from the ciliary body of the choroid
-The primary function of the lens is to focus the visual image on the photoreceptors
-the lens does so by changing its shape
The Structure of the Lens
The transparent lens consists of concentric layers of cells wrapped in a dense fibrous capsule
-the cells making up the interior of the lens lack organelles & are filled w/ transparent proteins
-The capsule contains many elastic fibers that contract make
Accommodation
The process of focusing an image on the retina by changing the shape of the lens. During accommodation, the lens either becomes rounder (to focus the image of a nearby object on the retina) or flattens (to focus the image of a distant object on the retina
Image formation
The image of an object reaching the retina is a miniature image of the original, but it is upside down & backward. The brain compensates for both aspects of image reversal without our conscious awareness
Visual Pathway
1. Cornea
2. Aqueous humor
3. pupil/iris
4. lens
5. vitreous humor
6. photoreceptors > bipolar cells > ganglion cell
7. axons from the ganglion cells converge on the optic disc & proceed toward the diencephalon as the optic nerve
8. the two optic nerves m