The Eye

Ch.10 pg. 262

10.9 Sense of Sight

• Eyelid: four layers: skin muscle, connective tissue, and conjuctiva.

• Thinnest skin of the body• Orbicularis oculi – muscle that moves the

eyelids.• Conjunctiva: mucous membrane that lines

the inner surfaces of the eyelids and folds back to cover the anterior surface of the eyeball (except cornea)

• Lacrimal gland: secretes tears, secretes tears continuously.

• Two small ducts collect tears, which flow into the lacrimal sac (located in lacrimal bone).

• Nasolacrimal duct (empties into the nasal cavity).

• Lysozyme: enzyme, antibacterial agent.

• Extrinsic muscles: arise from the bones of the orbit and insert by broad tendons on the eye’s tough outer surface.

• Six extrinsic muscles move the eye in various directions.

Diplopia

• One eye deviating from the line of vision may result in double vision (diplopia).

• The brain may suppress the image from the deviated eye.

• Turning eye may become blind.

• Treating eye deviation early in life with exercises, eyeglasses, and surgery can prevent this.

Structure of Eye

• 3 distinct layers– Fibrous (outer) layer– Vascular (middle) layer– Nervous (inner) layer

Outer layer

• Cornea: transparent bulges forward. Window of the eye and helps focus entering light rays.

• Composed of connective tissue with a thin surface layer of epithelium.

• Contains few cells and no blood vessels.

• Sclera (skle rah): white portion of the eye.

• Optic nerve: goes through sclera

Middle layer

• Choroid coat: contains blood vessels, pigment-producing melanocytes.

• Melanin absorbs excess light and keeps the inside of the eye dark.

• (pg. 266)

• Ciliary body: thickest part of the middle layer, extends forward fromt eh choroid coat and forms an internal ring around the front of the eye.

• Ciliary processes – radiating folds, consists of ciliary muscles.

• Suspensory ligaments: extend inward from the ciliary processes and hold the transparent lens in position.

• Accomodation:• The ciliary muscles and suspensory

ligaments, along with the structure of the lens itself, enable the lens to adjust shape to facilitate focusing

• When suspensory ligaments pull, it flattens the lens, focusing on distant objects.

• When ligaments relax the lens becomes more convex focusing on closer objects.

• The ciliary muscles control the actions of the suspensory ligaments in accomodation.when the ciliary muscles relax, tension on the suspensory ligaments increases, and the lens becomes thinner and less convex.

• When the ciliary muscles contract, the choroid coat is pulled forward and the ciliary body shortens, relaxing the suspensory ligaments, lens thickens.

Iris: thin diaphragm composed mostly of connective tissue and smooth muscle fibers.

Colored portion of the eye.

Lies between the cornea and lens.

Aqueous humor: watery fluid, in posterior chamber.

Fluid circulates through the pupil.

Pupil: circular opening in the center of the iris.

Aqueous humor fills the space between the cornea and lens.

Helps nourish these parts, aids in maintaining the shape of the front of the eye.

• Smooth muscle fibers of the iris• 2 groups: circular set, radial set• Control the size of the pupil, through which

light passes as it enters the eye.• Circular set: acts a sphincter, when it

contracts, the pupil gets smaller, letting less light in.

• Radial set: contract the pupil’s diameter increases, more light enters.

• Dim light stimulates the radial muscles to contract, dilates the pupil, allowing more light to enter the eye.

Inner layerConsists of the retina, which contains the visual

receptor cells (photoreceptors).

In the back of the eye, transparent sheet.

Ends just behind the margin of the ciliary body.

Thin and delicate

Macula lutea: central region of the retina, yellowish spot, depression on its center is called the fovea centralis, region that produces the sharpest vision.

• Optic disc: nerve fibers from the retina leave the eye here, join the optic nerve.

• Central artery and vein also pass through the optic disc.

• Supply blood to the cells of the inner layer.

• Optic disc lacks receptor cells, known as the blind spot of the eye.

• Blind spot activity, draw a dot on a piece of scrap paper.

• Space bounded by the lens, ciliary body, and retina is the largest compartment of the eye, called – posterior cavity.

• Filled with a transparent, jellylike fluid called vitreous humor.

• Vitreous body : vitreous humor + collagenous fibers.– Supports the internal parts of the eye and

helps maintain its shape.

Floaters: as a person ages, tiny, dense clumps of gel or deposits of crystal-like substances form in the vitreous humor, cast shadows on the retina, the person sees small, moving specks in the field of vision.

Light Refraction• Bending the light waves to focus light.

• Refraction occurs when light waves pass at an oblique angle from a medium of one optical density into a medium of a different optical density.

• Lens with a convex surface: causes light waves to converge.

• Refracts light waves from outside objects.

• Convex surface: refracts light at a lesser extent.

• Image that forms on the retina is upside down and reversed from left to right.

• Visual cortex of brain interprets the image in its proper position.

Visual ReceptorsModified neurons of two distinct kinds.

Rods: long, thin projections at their ends, provide black and white vision.

Cones: short, blunt projections, and provide color vision.

Located in a deep portion of the retina, pigment epithelium which absorbs light waves not absorbed by the receptor cells, keeps light from reflecting off surfaces inside the eye.

• Visual receptors are stimulated only when light reaches them.

• Rods are more sensitive to light than cones, to provide vision in dim light.

• Fovea centralis lacks rods, only contains cones.

• Person moves the eye so that the important image falls on the fovea centralis.

Special Senses

• Vision

• Hearing and balance

Visual System: Accessory Structures

• Eyebrows: shade; inhibit sweat• Eyelids (palpebrae) with

conjunctiva. • Conjunctiva: thin transparent

mucous membrane• Eyelashes: double/triple row of

hairs.

Lacrimal Apparatus

• Lacrimal gland: produces tears to moisten, lubricate, wash. Tears pass through ducts and then over eye.

• Lacrimal canaliculi: collect excess tears through openings called puncta.

• Nasolacrimal duct: opens into nasal cavity

Extrinsic Eye Muscles

Anatomy of the Eye

• Three coats or tunics:• Fibrous: sclera (whites) and cornea (transparent)• Vascular: choroid (w/in sclera), ciliary body (attaches lens), iris• Nervous: retina

Retina

• Rods – sensory cells used in low illumination

• Cones – sensory cells that detect color – 3 types of pigment – blue, red, green

• Most images focused on fovea centralis (most concentrated area of cones)

• Rods and cones synapse with bipolar cells – which then synapse with ganglion cells – which form the optic nerves

Focusing

The Ear

• External ear: hearing. Terminates at eardrum (tympanic membrane). Includes auricle (pinna) & external auditory meatus

• Middle ear: hearing. Air-filled space containing auditory ossicles • Inner ear: hearing and balance. Interconnecting fluid-filled tunnels

and chambers within the temporal bone

Inner Ear

Auditory Function

• Sound waves funnel down auricle and external acoustic meatus to vibrate tympanic membrane (ear drum)

• Tympanic membrane vibrations passed along auditory ossicles (ear bones) to inner ear

• These vibrations cause lymph to vibrate which moves the stereocilia of inner hair cells

• Bending of stereocilia causes mechanical opening of ion channels – depolarization

• Depolarization leads to action potentials in vestibulocochlear nerve

A

C

B

D

E

F

Practice Questions

The receptors for sound reception and equilibrium are located in the

A. in the outer, middle and inner cavities. 

B. inner ear cavity. 

C. middle ear cavity. 

D. in the middle and inner ear cavities. 

E. outer ear cavity.

When a person cries, he or she tends to keep on wiping their dripping nose. The nose drips because

A. special secretory glands are located in the nose start to produce secretions into the nose. 

B. lacrimal gland secretions wash the eyeball and drain into the lacrimal duct which empty into the nasal cavity. 

C. the nasal mucosa is sensitized to produce mucous into the nose. 

D. the eyeball produces tears that drain into the nasolacrimal duct and then into the nose. 

E. when a person cries olfactory mucosa are sensitized to increase secretions into the nose.

When focusing on a distant object the lens is    

A. more concave. 

B. more flattened. 

C. more convex. 

D. not changed.

The auditory tube allows pressure equalization between the middle ear and the external environment. 

 True False

The semicircular canals respond to rotational movements. 

 True False

The size of the pupil is dependent on the contraction of the muscles of the iris.

 True False

The rods of the retina operate in bright light and provide high acuity vision.

 True False

Impulses from the olfactory epithelium are sent to the brain via cranial II. 

 True False

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The cornea is part of the vascular tunic. 

 True False