Lecture #24
The Special Sense Organs
Textbook Chapter 17
The Eyeball
• The eyeball has 6 primary components:
1. The fibrous tunic:
•
Consists of the sclera (white of the eye),
the cornea (transparent portion of the eye)
• Both are avascular connective tissue
•
Also consists of the conjunctiva which covers the anterior surface of the sclera
• A vascular mucous membrane
• Dilation of blood vessels in this layer results in blood shot eyes
2. The vascular tunic:
•
•
•
The iris (the colored part of the eye)
The choroid is highly vascular
• Contains melanin which prevents the
reflection and the scattering of light
Also contains the ciliary body
• Consists of the ciliary muscle and the
ciliary processes which control the shape
of the lens and secrete aqueous humor
3. The nervous tunic:
The Eyeball
• The retina
• Sensory portion of the eye
• Consists of the outer pigmented layer and an inner
neural layer consisting of three layers of neurons
i.
Photoreceptor neurons:
• Rods: for black and white vision
•
Cones: for colored vision
ii.
Bipolar cells
iii.
Ganglion cells:
• Axons of these cells form cranial nerve II: the optic nerve
• The fovea centralis is where light focuses
• The greatest visual acuity is achieved here
• The optic disc is the blind spot
• This is where the optic nerve and blood vessels exit the eye
The Eyeball
4. The lens:
• A transparent, avascular layer
• Cataracts occur when the lens becomes cloudy
•
Can be caused by a number of different things
5. The anterior chamber:
• Located anterior to the lens
• Contains the aqueous humor
•
•
Very similar to blood plasma
Replaced every 90 minutes
• Drains into a sinus located at the
junction between the sclera and
the cornea
6. The posterior segment:
• Located posterior to the lens
• Contains the gel-like vitreous humor
•
This humor is not replaced
The Ear
• The structure of the ear includes the external ear, middle ear and inner ear
1. The external ear:
•
Consists of the:
• Auditory canal (meatus)
• Tympanic membrane (eardrum)
• Auricle: elastic cartilage covered in skin
•
Functions to conduct sound
2. The middle ear:
•
Consists of the auditory ossicles
• Smallest bones in the body
• Malleus: located against the tympanic
membrane of the external ear
• Incus
• Stapes: against the oval window of the inner ear
•
Also contains the eustachian tube (auditory tube)
•
Functions to conduct sound
The Ear
3. The inner ear:
• Contains the vestibular nerve
• Also contains the bony and the membranous labyrinth
• Surrounds and protects the ear
a. The bony labyrinth:
b.
•
Contains the:
• Semi-circular canals
• Cochlea
• Vestibule
•
Lined with periosteum and contains perilymph
The membranous labyrinth:
•
Contains the:
• Semi-circular ducts within the canals: connect with the utricle
• Cochlear duct within the cochlea
• Utricle and the saccule within the vestibule
•
Contains endolymph
The Ear
• The cochlea:
•
•
This is a coiled structureà there are 2.5 coils
There are three channels within each of the coils
1. The scala vestibuli: located above the cochlear duct
2. The scala tympani: located below the cochlear duct
• Both 1 and 2 contain perilymph/are part of the bony
labryinth
3. The scala media: the cochlear duct
• A continuation of the membranous
labyrinth: contains endolymph
• The vestibular membrane: separates
the cochlear duct from the scala vestibuli
• The basilar membrane: separates the
cochlear duct from the scala tympani
• The tectorial membrane: covers hair cells
of the spiral organ
• All of the three components contain endolymph
• The receptors are the hair cells that synapse with the
neuronsà like cilia, extend into the endolymph
The Ear
• The functions of the ear include:
• Conduction of soundà the middle
and external ear
• Hearing and equilibriumà the inner
ear
• The ear directs impulses to the
brain via cranial nerve VIIIà the
vestibulcochlear nerve
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