Special Senses

vision

vision

• Only the anterior one sixth of the eye's surface is visible; the rest of the eye lies in the cone-shaped bony orbit, where it is surrounded by a protective cushion of fat

• Behind the eye, posterior half of orbit contains the optic nerve, the arteries and veins to the eye, and the extrinsic eye muscles.

eyebrows

consist of course hairs in the skin on superciliary arches

• they shade the eyes from sunlight and prevent perspiration running down the forehead from reaching the eyes. 

Eyelids (palpebrae) 

upper and lower eyelids protect the eyes and are separated by a palpebral fissure that meet each other at the medial and lateral angles (eye corners)

meibomian glands

• are tiny oil glands which line the margin of the eyelids (the edges which touch when the eyelids are closed)

• these glands secrete oil which coats the surface of our eyes and keeps the water component of our tears from evaporating (drying out)

conjunctiva

• is a transparent mucous membrane that covers the inner surface of the eyelids and folds back over the anterior surface of the eye

• Produces a lubricating mucus that prevents the eye from drying out. Inflammation of the conjunctiva is called conjunctivitis that causes irritation to the eye and makes it red

• A highly contagious form of conjunctivitis caused by bacteria or viruses is called pinkeye. 

lacrimal apparatus

includes a series of ducts and lacrimal glands that produce a saline (salt) solution called tears that flow across the eye to wash and lubricate the eyeball

nasolacrimal duct

• passes through the nasolacrimal canal and into the nasal cavity under the inferior nasal concha

• it carries tears into the nasal cavity.

extraocular skeletal muscles

that move the eyeball and the intrinsic smooth muscles of the eye are both necessary for reflexes of the eyes

extraocular eye muscles

are designed to stabilize and move the eyes, skeletal muscles are controlled by Cranial Nerves (C.N.) III, IV and VI

Superior rectus (SR)

Oculomotor nerve (C.N. III)

Medial rectus (MR)

Oculomotor nerve (C.N. III)

Inferior rectus (IR)

Oculomotor nerve (C.N. III)

Lateral rectus (LR)

Abducens nerve (C.N. VI)

Superior oblique (SO)

Trochlear nerve (C.N. IV)

Inferior oblique (IO)

Oculomotor nerve (C.N. III)

Levator palpebrae superioris (LPS)

Oculomotor nerve (C.N. III)

sclera

is the outer coat, tough, protective tunic. Its anterior portion is the cornea, which is transparent to allow light to enter the eye

choroid

is the middle coat, which provides nutrition to the internal eye structures and prevents light's scattering in the eye.

• Anterior modifications include two smooth muscle structures:

• 1.) ciliary body and

• 2.) iris (which controls the size of the pupil and therefore the amount of light which passes through the lens)

retina

is the innermost (sensory) coat, which contains the photoreceptors (rods and cones)

rods

are dim light receptors (do not provide sharp images or color)

cones

are receptors that provide for color vision and high visual acuity

fovea centralis

on which acute focusing occurs, contains only cones

bipolar and ganglion

the light must pass through _ and _ cells before striking the photoreceptor cells.

blind spot

(optic disc) is the point where the optic nerve leaves the back of the eyeball

pupil

is the opening in the eye where light enters

lens

is the major light-bending (refractory) structure of the eye. Its convexity is increased by the ciliary body for close focus

cataract

the clouding of the lens

aqueous humor

anterior to the lens

• provides nutrients to the avascular lens

• reinforce the eye internally

vitreous humor

posterior to the lens

• reinforce the eye internally

glaucoma

is a group of eye conditions that damage the optic nerve, the health of which is vital for good vision. This damage is often caused by an abnormally high pressure in your eye

• is one of the leading causes of blindness for people over the age of 60.

pathway of light through eye

Cornea ---> aqueous humor ---> through pupil ---> aqueous humor ---> lens ---> vitreous humor ---> retina

distant point

Light rays from a _ point are parallel to one another as they reach the eye

nearby point

Light rays from a _ point diverge markedly as they enter the eye

to see clearly

the eye must be able to bend all these light rays so they converge on the retina at a single focal point. 

light bending parts of eye

called refractory media

• cornea - does most

• lens - does some

• humors - does least

lens

not as powerful as the cornea in bending light, its curvature is adjustable, which allows the eye to focus on nearby objects, a process called accomodation

ciliary bodies

a thickened ring of tissue that encircles the lens, and consist chiefly of smooth muscle called ciliary muscle which acts to focus the lens 

distant

Nearby parallel rays from a _ object, ciliary muscle flattens the lens for distant vision 

close

Divergent rays from a _ object, ciliary muscle bulges the lens for close vision

• controlled by parasympathetic innervation

myopia

a vision condition in which close objects are seen clearly, but objects further away appear blurred

• This is also called nearsighted

hyperopia

a vision condition in which distant objects are seen clearly, but close ones do not come into proper focus

• This is also called farsighted

astigmatism

Vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina

• Occurs due to the irregular shape of the cornea or the lens inside the eye.

• Refractive errors are correctable with specially ground lenses.

photopupillary eye reflex

• reflex controlling diameter of pupil in response to light intensity.

• causes a pupillary constriction in response to light.

• purpose is shielding the eye from too much and sudden light.

sphincter and dilator pupillae muscles

act to vary size of pupil (round central opening in the eye) and muscles located in the iris (colored part of the eye surrounding pupil).

sphincter pupillae

• bright light and close distant vision

• contracts to constrict pupil, controlled by parasympathetics of the oculomotor nerve (C.N. III).

dilator pupillae

• dim light and far distant vision

• contracts to widen the pupil, allowing more light to enter the eye, controlled by sympathetic innervation.

optic nerve

Axons of ganglion cells exit the eye in the _ (CN II) and travel to optic chiasma

external hearing

• consists of auricle and external acoustic meatus

• sound waves entering the acoustic meatus hit the tympanic membrane (eardrum) which forms the boundary between the external and middle ears.

• sound waves travel through the air cause the eardrum to vibrate transferring the vibrations to the tiny bones of the middle ear. 

auricle (pinna)

is a shell shaped projection surrounding the external acoustic meatus and functions gathering/amplifying sound waves coming into the ear

external acoustic meatus

• is a short tube (canal) that runs from the auricle to the eardrum.

• canal is lined with skin and modified sweat glands that secrete earwax trapping dust and repelling insects keeping them out of the auditory canal