Eye Anatomy Flashcards
sclera
- the white part of the eye that surrounds the cornea
- the dense connective tissue of the eyeball
- forms more than 80% of the surface area of the eyeball
- extending from the cornea all the way to the optic nerve
- only a small portion of the anterior sclera is visible
- some blood vessels pass through the sclera to other tissues, but the sclera itself is considered avascular
what accounts for the strength and flexibility of the eyeball?
The sclera is composed of fibrils (small fibers) of collagen that are arranged in irregular and interlacing bundles.
(The random arrangement and interweaving of these connective tissue fibers are what account for the strength and flexibility of the eyeball)
limbus
the junction between the white sclera and the clear cornea
the sclera is continuous with the stroma layer of the cornea
episclera
- a thin, loose connective tissue layer
- that lies on top of the sclera
- and under the transparent conjunctiva that covers the sclera and episclera
choroid
- the vascular layer of the eyeball that is sandwiched between the sclera and the retina
- contains many tiny blood vessels and has the vital role of nourishing the retina
3 functions of the sclera
1) maintains the shape of the eyeball (along with the intraocular pressure (IOP) of the eye)
2) the tough, fibrous nature of the sclera also protects the eye from serious damage — such as laceration or rupture — from external trauma
3) provides a sturdy attachment for the extraocular muscles that control the movement of the eyes
iris
-the thin, circular structure made of connective tissue and muscle that surrounds the pupil
pupil
circular opening (aperture) in the center of the iris which is the opening that allows light into the eye
what determines the color of our eyes
the amount of pigment in the iris
ciliary body function (3)
-holds the lens of the eye in place
(connected to the lens with a network of many tiny ligaments, called ciliary zonules or zonules of Zinn, that suspend the lens in place behind the pupil)
- secretes the clear aqueous fluid that fills the space in the anterior segment of the eye between the cornea and the iris and lens
- and it contains the muscle that controls accommodation of the eye
ciliary body
It surrounds the iris and cannot be seen because it’s located behind the opaque sclera (white of the eye)
iris function (2)
- in addition to giving the eye its color, the iris acts like the diaphragm of a camera and controls the size of the pupil
- the colored nerve tissue which works in conjunction with a muscle (sphincter) which regulates how much light comes in
uvea
- the pigmented middle layer of the eyeball
- 3 segments: the iris, the ciliary body and the choroid
pupil function
-allows light to enter the eye so it can be focused on the retina to begin the process of sight
why are pupils black
because light that passes through the pupil is absorbed by the retina and is not reflected back (in normal lighting)
what could explain a cloudy pupil
the lens of the eye (which is located directly behind the pupil) becoming opaque due to the formation of a cataract
the size of the pupil is controlled by
- muscles within the iris
- one muscle constricts the pupil opening (makes it smaller)
- another iris muscle dilates the pupil (makes it larger)
In low-light conditions, the pupil..
dilates so more light can reach the retina to improve night vision
In bright conditions, the pupil..
constricts to limit how much light enters the eye
If the pupil lets in too much light..
it can cause glare and discomfort, and it may even damage the lens and retina
pupil size
- varies from person to person
- also changes with age — children and young adults tend to have large pupils, and seniors usually have small pupils
accommodative pupillary response
both pupils normally constrict when you focus on a near object
pupil testing
Marcus Gunn pupil testing
While you are looking at a distant object, the examiner will briefly direct the beam of a small flashlight at one of your eyes a few times
“swinging flashlight test” alternately direct the light at each eye and again observe the pupil responses of both eyes
direct response
reaction of the pupil of the eye receiving direct illumination
consensual response
response of a pupil to light that enters the contralateral (opposite) eye
PERRLA
pupils are equal, round and reactive to light and accommodation
retina
the sensory membrane that lines the inner surface of the back of the eyeball. It’s composed of several layers, including one that contains specialized cells called photoreceptors
macula
tiny, highly sensitive area of the retina that controls central vision and color vision
two types of photoreceptor cells (in the human eye)
Rod photoreceptors
- detect motion - provide black-and-white vision - function well in low light - located throughout the retina
Cones
- responsible for central vision and color vision - perform best in medium and bright light - concentrated in small central area of retina (macula)
fovea
-small depression in the center of the macula
- contains only cone photoreceptors
- is the point in the retina responsible for maximum visual acuity and color vision
retina function
Photoreceptor cells take light focused by the cornea and lens and convert it into chemical and nervous signals which are transported to visual centers in the brain by way of the optic nerve.
In the visual cortex of the brain (which, ironically, is located in the back of the brain), these signals are converted into images and visual perceptions.
conjunctiva
the clear, thin membrane that covers part of the front surface of the eye and the inner surface of the eyelids
2 segments of the conjunctiva
- Bulbar conjunctiva* covers the anterior part of the sclera and stops at the junction between the sclera and cornea; it does not cover the cornea.
- Palpebral conjunctiva* covers the inner surface of both the upper and lower eyelids. (also called the tarsal conjunctiva)
This feature makes it impossible for a contact lens (or anything else) to get lost behind your eye
The bulbar and palpebral conjunctiva are continuous
conjunctiva function (5)
- keep the front surface of the eye moist and lubricated
- keep the inner surface of the eyelids moist and lubricated so they open and close easily without friction or irritation
- protect the eye from dust, debris, and infection-causing microorganisms
- has many small blood vessels that provide nutrients to eye and lids
- also contains special cells that secrete a component of the tear film to help prevent dry eye syndrome
cornea
- the clear front surface of the eye
- lies directly in front of the iris/ pupil, and allows light to enter the eye
Viewed from the front of the eye, the cornea appears
slightly wider than it is tall
cornea function
-provides approximately 65 -75% of the focusing power of the eye
Most refractive errors — nearsightedness, farsightedness and astigmatism — are due to
less-than-optimal curvature or symmetry of the cornea
Presbyopia, on the other hand, is due to
an aging change in the crystalline lens
The eye’s crystalline lens
located directly behind the pupil and further focuses light
Through a process called accommodation, this lens helps the eye automatically focus on near and approaching objects, like an autofocus camera lens
