CJ Shuster

Lab Addenum

Eye Anatomy

1

EYE ANATOMY (Adapted from Johnson, Weipz and Savage Lab Book)

Introduction The eyes are the largest and most complex sensory organs in the human body. They function somewhat as a pair of cameras, recording images of the surrounding world that can be interpreted by the brain. In order to record a clear image the eye must be able to focus equally well on both near and far objects (accommodation for distance) and must be able to adjust to different light intensities (accommodation to light). These many varied functions of the eyes require a variety of specialized and intricate functional mechanisms.

The perception of information received by special cells called PHOTORECEPTORS. The stimulus is light energy. In other words, vision is the perception of light energy. The EYE is a special organ that allows us to receive light energy, focus it, and transduce it into an action potential.

The fovea centralis is the region on the photoreceptive layer that receives focused light; here, the cells are the densest.

CJ Shuster

Lab Addenum

Eye Anatomy

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To avoid the recording of a double image the eyes must be able to point directly at the object being viewed (convergence). Muscles control eye position and movement.

CJ Shuster

Lab Addenum

Eye Anatomy

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* ACUITY is the ability of the eye to focus on the image.

The 2 fovea are 5-7.5 cm apart, and the nose and eye socket block the view of the opposite side. Also, your brain learns to interpret “up” as “back”. This is termed DEPTH PERCEPTION.

A mechanism also exists that enables the eye to differentiate between light of different wave lengths (color perception). *low wave height = long wave length = lower power. *Higher wave height (= short length) = higher frequency (closer together); and since it is the m ove m ent o f particles called photons, hi gher frequ en cy = higher pow er (or ability to do work). NOT E: “colors” are different wavelengths in the visible spectrum.

CJ Shuster

Lab Addenum

Eye Anatomy

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Lab Exercises Human Eye Models. Take a look at a model of the eye in the orbit. Study the muscles and exterior anatomy as outlined on your wordlist. See figure #1a on the Data/Analysis Sheet. Here are some highlights: 1. 2. 3. 4. 5. 6. 7.

Eyelids (palebrae). Keep surface lubricated & free of debris. Medial & lateral canthus. The “corners” of the eye. Eyelashes Meibom ian gland. Secretes a lipid-rich produ ct that helps prevent the eyelids from sticking together. Orbicularis oculi & levator palebrae superioris .... closing & raising eyelid. Conjunctiva. Epithelium covering the inner eyelid & outer surface of eye ball. Mucous mem brane. Lacrimal gland. Releases lysosome-filled secretion. Drained by the lacrimal ducts, which lead to the lacrim al sac. From the sac, th rough the nasolacrim al duct to th e inferior m eatus of the nasal cavity. 8. Orbital fat. Insulation & padding.

Obtain one of the "naked" (that is, eye only — not in orbit — no major detail of associated anatomy) models of a human eye. Locate each of the anatomical parts (SEE WORDLIST)on the model, and also on Figure #1B of the Data/Analysis Sheet. In most cases careful reading of the brief description of the structure given here, combined with some good old common sense, should enable you to make an accurate identification. Before opening the eye model, locate the following. The outer coat of the eyeball is a tough layer of dense fibrous tissue divided into two regions. The sclera ("white of the eye") is white and opaque, and covers all but the very anterior portion of the eyeball. The cornea is the transparent anterior portion. The muscles that move the eyeball (the extrinsic eye muscles) attach to the sclera. Light enters the eye through the cornea. At the back of the eye, somewhat off center toward the midline of the body, is the optic nerve. The position of the optic nerve will tell you whether the model is of a right or left eyeball. Now open the eye model and locate the following. Looking at the posterior part of the eyeball note two layers in addition to the sclera. The middle layer is the choroid. This layer is pigmented and is usually shown as dark colored (blue-black). The inner layer is the retina. This layer is by far the thinnest and most delicate of the three layers and is usually shown as light colored (tan or pink). Blood vessels can be seen directly in the retina of the living eyes by use of a special reflecting light called an ophthalmoscope. A nervous tissue layer of the retina contains the photoreceptor neurons that form rods and cones. The continuity of the three layers in the back of the eyeball is interrupted where the optic nerve exits. This point is the optic disc or blind spot, so-called because there are no

CJ Shuster

Lab Addenum

Eye Anatomy

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photoreceptor neurons at this particular part of the retina. Slightly toward the midline of the eyeball from the blind spot is a small, lighter colored spot called the macula lutea. The center of the macula lutea is called the fovea centralis. This is the very center of the retina and is the point of sharpest vision. The entire posterior cavity of the eye (between the lens and the retina) is filled with a thick gelatinous fluid called vitreous humor. On most of the models a clear plastic insert represents the vitreous humor. This fluid helps to maintain the shape of the eyeball and also to hold the retina in place against the choroid. The anterior cavity of the eye (between the lens and cornea) is filled with a more watery fluid called aqueous humor. Aqueous humor is continuously produced and is drained from the anterior chamber through the canal of schlemm, a venous sinus located at the junction of the sclera and cornea. Intraocular pressure is produced mainly by the aqueous humor. Excessive intraocular pressure, called glaucoma, can result in degeneration of the retina and blindness. The lens is a clear, elliptical structure that, together with the cornea, bends light rays to focus and form a clear image on the retina. Normally, the lens is perfectly transparent. A loss of transparency of the lens is known as a cataract. The lens is held in place by suspensory ligaments that attach it to the ciliary body, the thickest portion of the choroid. The ciliary body consists of protrusions or folds called ciliary processes that secrete aqueous humor, and smooth muscles called ciliary muscles (intrinsic eye muscles), that work to alter the shape of the lens. When the ciliary muscles are relaxed, the suspensory ligaments pull on the lens making it flatter and enabling the eye to focus on objects father away. Contraction of the ciliary muscles reduces tension on the suspensory ligaments, allowing the elastic lens to round up and enabling the eye to focus on objects closer to the eye. Immediately in front of the lens lies the pigmented iris, also a modification of the choroid. The opening in the iris is called the pupil. Muscle fibers in the iris (intrinsic eye muscles) can cause the pupil to constrict or dilate, thus regulating the amount of light passing through the lens. Genetically, some persons produce a pigment in the iris that causes it to be brown. Blue-eyed persons do not produce the pigment. Persons with eyes a shade of green have genes to produce pigment (i.e., are genetically brown-eyed), but produce only small amounts of the pigment, causing the unusual shading.

Other Human Eye Models. Locate each of the following structures on Figures #1A & #1B of the Data/Analysis Sheet. Also, find as many of these structures as possible on the other eye models available in the lab. The conjunctiva is a thin mucous membrane that lines the eyelids and covers the exposed surface of the eyeball (mostly cornea) with an epithelial layer of cells. The extrinsic muscles are small and very precisely controlled skeletal muscles that serve to point the eye at an object being viewed. In Figure #2A the muscle which has a portion cut away to reveal the optic nerve is the lateral rectus muscle. On the other side of the eye is its

CJ Shuster

Lab Addenum

Eye Anatomy

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opposing muscle, the medial rectus which is only partially visible. On the superior surface of the eyeball is attached the superior rectus, and opposing it on the inferior surface is the inferior rectus. These four muscles point the eyeball up, down, medially and laterally. Just above the stub of the lateral rectus is seen the insertion of the superior oblique muscle, which passes through a cartilaginous loop, the trochlea. Opposing the superior oblique is the inferior oblique of which only the insertion is seen below the stub of the lateral rectus. These two muscles are used for small rotational movements of the eyeball. The muscle which extends into the upper eyelid is the levator palpebrae. It raises the eyelid. In Figure #2B identify the lacrimal glands located superior and lateral to the eye. Their secretion is spread over the surface of the eye by the opening and closing of the eyelid. Excess tear fluid is collected in the medial corner (medial canthus) of the eye and channeled into the lacrimal ducts. These ducts empty into a larger vessel, the lacrimal sac, which then empties into the nasal cavity via the nasolacrimal duct.

CJ Shuster

Lab Addenum

Eye Anatomy

Figure #1A & B (Exterior Eye & Horizontal Section)

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CJ Shuster

Lab Addenum

Eye Anatomy

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Beef Eye Dissection. Obtain one of the preserved or fresh beef eyes available for dissection. Take a moment to examine it externally before cutting into it. The amount of excess tissue remaining will vary from minimal to lots. Look to see if there are any remnants of the extrinsic muscles, eyelid, or lacrimal glands. Note the tough, white outer sclera and the transparent anterior cornea. Examine the back of the eyeball to locate the stub of the optic nerve. If the cornea is still relatively clear, look through it (a small penlight might be useful) at the iris and pupil within the eye. Holding the eyeball very firmly in your hand, pierce the sclera with a sharp- pointed scissors at a point about 1 cm away from the edge of the cornea. EXERCISE EXTREME CARE IN DOING THIS. The eye is difficult to hold, and the sclera is difficult to penetrate. The difficulty you have in piercing the sclera should give you an appreciation for the protective capabilities of a dense connective tissue. Once you have made an opening through the wall of the eyeball, use a scissors to cut all around the edge of the cornea. Using a forceps, lift the anterior part of the eye away to expose the interior of the eye. (NOTE! Try to keep the thick vitreous humor in the back of the eye as you do this. Dislodging the vitreous humor will detach the retina.) Locate and examine the lens. It may remain attached to the vitreous humor or it may come off with the anterior part of the eye. Use a probe to separate the lens from the other tissue of the eye. Carefully remove the lens from the eye and set it on a scrap piece of printed matter. Is the lens transparent? Feel the lens and note whether the surface feels any different than the central core. Look at the vitreous humor in the back of the eye and note its consistency and clarity. Locate the filmy, light tan retina that lines the inside of the eyeball. Determine if it is firmly attached to the choroid coat or just lying against it. Note the blood vessels in the retina. Remove the vitreous humor from the back of the eye. Note what happens to the retina when you do this. Locate the point at which the retina is firmly attached to the wall of the eyeball and determine if this is the same point where the optic nerve stub was found. Examine the choroid coat and separate it partially from the sclera. Note that most of the choroid coat (including the ciliary body in the anterior part of the eye) is black. This prevents light from reflecting around inside the eye and blurring the images being formed on the retina. Note that a portion of the choroid coat is an iridescent blue-green color that does reflect light. This is called the tapetum lucidum and is an adaptation for night vision. Although the reflection of light tends to blur the images, it also increases the amount of stimulation of the retina. Animals with this adaptation are trading clarity of vision for the ability to at least see something in lower light conditions. This is also the feature that causes some animal's eyes to shine back at you at night when you shine a light at them. Humans do not have a tapetum lucidum.

CJ Shuster

Lab Addenum

Eye Anatomy

9

Examine the ciliary body and iris in the anterior part of the eye. Look at the iris to see if you can find any indications of the radial and circular muscle fibers that control pupil size. When you have completed this dissection, ask your instructor about any structures you were uncertain of. Answer the questions about the dissection that are on the Data/Analysis Sheet.

Remo ve e x c e ss glandular tissue, etc.

musc l e ,

Notice: Sclera, conjunctiva, cornea, optic nerve

Using your scapula and small, pointy scissors, cut around cornea, keeping a couple of mm away from the cornea. Do not drift too far away, or you’ll detach the retina (see later). Notice: iris, pupil, lens, vitreous humor, aqueous humor, ciliary body, suspensory ligaments

CJ Shuster

Lab Addenum

Eye Anatomy

10

Remove lens. Vitreous humor should come along with it. Be careful not to detach retina.

Look into posterior chamber. Notice: retina, choroid, optic disk, tapetum.

CJ Shuster

Lab Addenum

Eye Anatomy

11

DATA/ANALYSIS SHEET Beef Eye Dissection. 1.

What is it about the wall of the eye that makes it so difficult to penetrate with the point of a scissors?

2.

Compare the consistencies of the aqueous humor and vitreous humor of the eye.

3.

What happens to the retina of the eye when the vitreous humor is removed? Why does this happen?

4.

What is the reflective portion of the beef eye called? Do people have this structure? What is the advantage of being reflective? What is the disadvantage of being reflective?

5.

Why is most of the inner wall of the eye pigmented black?

6.

Does the central core of the lens feel any different from the outer edges? What difference do you notice?

CJ Shuster

Lab Addenum

Eye Anatomy

12

Matching. ___outer layer of wall of eye

A.

aqueous humor

___middle vascular layer of wall of eye

B.

blind spot

___inner light-sensitive layer of wall of eye

C.

choroid coat

___fluid between lens and cornea

D.

ciliary body

___fluid between lens and back of eye

E.

cornea

___small pit in retina of eye

F.

conjunctiva

___small area at back of eye that lacks photoreceptors

G.

fovea centralis

H.

iris

I.

lacrimal ducts

J.

nasolacrimal ducts

K.

pupil

L.

retina

M.

scleroid coat

N.

suspensory ligament

O.

vitreous body

___membrane that lines eyelid and covers cornea ___drain tubes for tears in eyelids ___tube that drains tears to nasal cavity ___transparent front portion of eyeball ___circular band of smooth muscle around lens ___connective tissue holding lens in place ___opening in the center of the iris ___region containing muscle to regulate lens shape