Chapter 15
the retina contains...
(1) millions of photoreceptors that transduce (convert) light energy (2) other neurons involved in processing responses to light (3) glia
the main types of neurons of the neural layer
(from posterior to anterior): photoreceptors, bipolar cells, and ganglion cells
the two layers of the retina
- an outer pigmented layer - an inner neural layer
T or F: The pigmented and neural layer are fused.
False; although they are very close together, they are not fused.
choroid
a blood vessel rich, dark brown membrane that forms the posterior five-sixths of the vascular layer - its blood vessels nourish all eye layers - its brown pigment, produced by melanocytes, helps absorb light, preventing it form scattering and reflected within the eye - the choroid has a posterior opening where the optic nerve leaves the eye
vitreous humor
a clear gel that binds tremendous amounts of water; it: - transmits light - supports the posterior surface of the lens and hold the neural layer of the retina firmly against the pigmented layer - contributes to intraocular pressure, helping to counteract the pulling force of the extrinsic eye muscles
ciliary glands
a number of smaller, more typical sebaceous glands are associated with the eyelash follicles; modified sweat glands called _________ lie between the hair follicles
pigmented layer
a single-cell-thick lining; next to the choroid; extends anteriorly to cover the ciliary body and the posterior face of the iris - these pigment cells, like those of the choroid, absorb light and prevent it from scattering in the eye - also act as phagocytes participating in photoreceptor cell renewal, and store vitamin A needed by the photoreceptor cells
eyeball
a slightly irregular hollow sphere - its wall is composed of three layers: the fibrous, vascular, and inner layers
external sheet
a stratified squamous epithelium that protects the cornea from abrasion, merges with the bulbar conjunctiva at the corneoscleral junction - epithelial cells that continually renew the cornea are located here
conjunctiva
a transparent mucous membrane - major function is to produce a lubricating mucus that prevents the eyes from drying out
lysozyme
an enzyme that destroys bacteria
scleral venous sinus
an unusual venous channel that encircles the eye in the angle at the corneoscleral junction
blind spot
another name for the optic disc; this is because it lacks photoreceptors, so light focused on it cannot be seen
ciliary body
anteriorly, the choroid becomes the ____________, a thickened ring of tissue that encircles the lens
eyelids, or palpebrae
anteriorly, the eyes are protected by the ______
cones
are our vision receptors for bright light and provide high-resolution color vision
anterior chamber
between the cornea and the iris
posterior chamber
between the iris and the lens
lacrimal canaliculi
blinking spreads the tears downward and across the eyeball to the medial commissure, where they enter the paired _______________
the cornea has no ______
blood vessels - so, it is beyond the reach of the immune system - as a result, the cornea is the only tissue in the body that can be transplanted from one person to another with little risk of rejection
corneal endothelium
composed of simple squamous epithelium - lines the inner face of the cornea - its cells have active sodium pumps that maintain the clarity of the cornea by keeping its water content low
the fovea contains only _____
cones
the macula contains mostly _________
cones
lens epithelium
confined to the anterior lens surface, consists of cuboidal cells that eventually differentiate into the lens fibers that form the bulk of the lens
lacrimal apparatus
consists of the lacrimal gland and the ducts that drain lacrimal secretions into the nasal cavity
epithelial sheets
cover both faces of the cornea
tarsal glands
embedded in the tarsal plates, and their ducts open at the eyelid edge just posterior to the eyelashes; these modified sebaceous glands produce an oily secretion that lubricates the eyelid and the eye and prevents the eyelids from sticking together
oculomotor nerves
except for the lateral recuts and superior oblique muscles, the ______________ serve all extrinsic eye muscles
neural layer
extends anteriorly to the posterior margin of the ciliary body
ciliary zonule
extends from the ciliary processes to the lens - this halo of fine fibers encircles and helps hold the lens in its upright position
accessory structures
eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles
sclera
forming the posterior portion and the bulk of the fibrous layer, is glistening white and opaque - tough, tendon-like - protects and shapes the eyeball and provides a sturdy anchoring site for the extrinsic eye muscles - posteriorly, where the _____ is pierced by the optic nerve, it is continuous with the dura matter of the brain
vascular layer
forms the middle coat of the eyeball - also called the uvea, this pigmented layer has three regions: choroid, ciliary body, and iris
anything we wish to view critically is focused on the __________
foveae
only the _____ have a sufficient cone density to provide detailed color vision
foveae
lacrimal sac
from the lacrimal canaliculi, the tears drain into the ________
nasolacrimal duct
from the lacrimal canaliculi, the tears drain into the lacrimal sac and then into the ______________, which empties into the nasal cavity at the inferior nasal meatus
sphincter pupillae
in close vision and bright light, the ____________ (circular muscles) contracts and the pupil constricts - controlled by parasympathetic fibers
dilator pupillae
in distant vision and dim light, the ___________ (radial muscles) contracts and the pupil dilates, allowing more light to enter - controlled by sympathetic fibers
posterior segment
is filled with the vitreous humor
lacrimal fluid function
it cleanses and protects the eye surface as it moistens and lubricates it
from the edge of the macula toward the retina periphery, what happens to the cone density?
it declines gradually
From the retina periphery to the macula, what happens to the density of the rods?
it gradually decreases
Why is the optic disc a weak spot in the fundus?
it is not reinforced by the sclera
macula lutea
lateral to the blind spot of each eye is an oval region called the ______ with a minute put in its center called the fovea centralis - in this region, the retinal structures next to the vitreous humor are displaced to the sides
lacrimal gland
lies in the orbit above the lateral end of the eye and is visible through the conjunctiva when the lid is everted
oblique muscles of the eye
move the eye in the vertical plane when the eyeball is already turned medially by the rectus muscles
lacrimal fluid contains ...
mucus, antibodies, and lysozyme
ciliary processes
near the lens, its posterior surface has radiating folds called __________, which secrete the fluid that fills the cavity of the anterior segment of the eyeball
the cornea is well supplied with _____
nerve endings (most of which are pain receptors)
only the ______ layer of the retina plays a direct role in vision
neural
superior oblique muscle
originates in common with the recuts muscles, runs along the medial wall of the orbit, and then makes a right-angle turn and passes through a fibrocartilaginous loop called the trochlea suspended from the frontal bone before inserting on the superolateral aspect of the eyeball - rotates the eye down and somewhat laterally
rods
our dim-light and peripheral vision receptors; they are more numerous and far more sensitive to light than cones, but they do not provide sharp images or color vision - this is why colors disappear and the edges of objects appear fuzzy in dim light and at the edges of our visual field
for us to visually comprehend a scene that is rapidly changing....
our eyes must flick back and forth to provide the foveae with images of different parts of the visual field
fundus
posterior wall of the eye
eyelashes
projecting from the free margin of each eyelid are the _________ - the follicles of the eyelash hairs are richly innervated by nerve endings (hair follicle receptors), and anything that touches the eyelashes (even a puff of air) triggers reflex blinking
the retina periphery contains mostly _____
rods
inferior oblique muscle
rotates the eye up and laterally
eyebrows
short, coarse hairs that overlie the supraorbital margins of the skull - help shade the eyes from sunlight and prevent perspiration trickling down the forehead from reaching the eyes
photoreceptors
specialized receptor cells that respond to light energy; rods and cones
the four rectus muscles
superior, inferior, lateral, and medial rectus muscles
lacrimal puncta
tears enter the paired lacrimal canaliculi via two tiny openings called ________, visible as tiny red dots on the medial margin of each eyelid
when lacrimal secretion increases substantially...
tears spill over the eyelids and fill the nasal cavities, causing congestion and the "sniffles"
ganglion cell
the __________ axons make a right-angle turn at the inner face of the retina, then leave the posterior aspect of the eye as the thick optic nerve
lens
the adjustable focusing apparatus of the eye
cornea
the anterior sixth of the fibrous layer is modified to form the transparent ______ , which bulges anteriorly from its junction with the sclera - the crystal-clear _____ forms a window that lets light enter the eye, and is a major part of the light-bending apparatus of the eye
ciliary muscles
the ciliary body consists chiefly of interlacing smooth muscle bundles called ____________, which control lens shape
iris
the colored part of the eye, is the most anterior portion of the vascular layer; shaped like a flattened donut it lies between the cornea and the lens and is continuous with the ciliary body posteriorly
bulbar conjunctiva
the conjunctiva folds back over the anterior surface of the eyeball as the _________ - covers only the white of the eye, not the cornea - very thin, and blood vessels are clearly visible beneath it
palpebral conjunctiva
the conjunctiva lines the eyelids as the ___________
aqueous humor
the entire anterior segment is filled with the __________, a clear fluid similar in composition to blood plasma - unlike the vitreous humor, the aqueous humor forms and drains continually - normally forms and drains at the same time, maintaining a constant intraocular pressure of about 16mmHg - supplies nutrients and oxygen to the lens and cornea and to some cells of the retina, and it carries away metabolic wastes
humors
the eye's internal cavity is filled with fluids called _____ that help maintain its shape
3 - 7 seconds
the eyelid muscles are activated reflexively to cause blinking every ____________ and to protect the eye from foreign objects
palpebral fissure
the eyelids are separated by the ________
the medial and lateral commissures
the eyelids meet at the _______
common tendinous ring
the four rectus muscles originate from the _________ at the back of the orbit and run straight to their insertion on the eyeball
central artery and central vein of the retina
the inner two-thirds of the retina is served by the _________ which enter and leave the eye thought the center of the optic nerve - radiating outward from the optic disk, these vessels give rise to a rich vascular network - this is the only place where small blood vessels are visible in a living person
retina
the innermost layer of the eyeball, which develops from an extension of the brain
anterior segment
the iris divides the _________ into the anterior chamber and the posterior chamber
ora serrata
the junction of the neural layer and ciliary body; name also means the saw-toothed margin
lacrimal secretion
the lacrimal gland continually releases a dilute saline solution called ___________ (aka tears) into the superior part of the conjunctival sac through several small excretory ducts
the two regions of the lens
the lens epithelium and the lens fibers
anterior and posterior segments
the lens is supported vertically within the eyeball, dividing it into ________
lacrimal caruncle
the medial commissure sports a fleshy elevation called the _______ - it contains sebaceous and sweat glands and produces the whitish, oily secretion that sometimes collects at the medial commissure, especially during sleep
fibrous layer
the outermost boat of the eyeball is composed of dense avascular connective tissue; it has two obviously different regions: the sclera and the cornea
fovea centralis
the pit in the center of the macula lutea
horizontal cels and amacrine cells
the retina also contains other types of neurons, ____________, which play a role in visual processing
pupil
the round central opening of the iris; it allows light to enter the eye - made up of two smooth muscle layers with bunches of sticky elastic fibers that congeal into a random pattern before birth - its muscle fibers allow it to act as a reflexively activated diaphragm to vary pupil size
extrinsic eye muscles
the sic skeletal muscles that attach to and move each eye - allow the eyes to follow a moving object, help maintain the shape of the eyeball, and hold it in orbit
if the amount of pigment is small and restricted to the posterior surface of the iris....
the unpigmented parts simply scatter the shorter wavelengths of light and the eyes appear blue, green, or ray
although irises come in different colors...
they contain only brown pigment
when eyes have a lot of pigment...
they eyes appear brown or black
What is the effect of the retinal structures next to the vitreous humor in the macula lutea being moved to the sides?
this allows light to pass almost directly to the photoreceptors rather than through several retinal layers, greatly enhancing visual acuity (the ability to resolve detail)
enhanced tearing
washes away or dilutes the irritating substance
filling in
we do not usually notice the gaps in our vision due to the blind spot because the brain uses a sophisticated process called _______ to deal with absence of input
conjunctival sac
when the eye is closed, a slitlike space occurs between the conjunctiva-covered eyeball and the eyelids - this is where a contact lens lies
optic disc
where the optic nerve exits the eye
