Chapter 15

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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


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