The eye receives light waves that are focused on the retina, producing visual nerve impulses transmitted to the brain via the optic nerve. It has three layers - the outer protective sclera, middle choroid containing blood vessels and inner retina containing rods and cones. The lens focuses light on the retina. Common diseases are cataracts, where the lens clouds over; glaucoma, with abnormal pressure rise; and retinal detachment, where the retina separates from the choroid.

1. THE EYE
The Organ of Vision
Purpose: Recieves light waves that are focused on the retina and produces visual nerve impulses that are transmitted
to the brain by the optic nerve.
Layers: Sclera - The outer protective layer
- The white of the eye
- Maintains eyeball shape
Choroid – The middle layer
- Contains blood vessels that supply nutrients
- Contains the iris (the colored portion) and the ciliary muscle
- The iris has an opening in the middle called the pupil. The iris controls the amount of light
entering the eye through dilation and contraction of the pupil.
- The lens, located behind the pupil, focuses light rays on the retina.
Retina – The inner layer
- Contains two nerve cells, rods and cones, which are responsible for the adaptation to light
- The rods and cones send impulses to the optic nerve, which carries them to the vision center
in the cerebrum.
Anterior and Posterior Cavities
- The small anterior cavity: filled with aqueous humor (a watery substance), which is
constantly formed and drained.
- The large posterior cavity: filled with vitreous humor (a jelly-like substance).
- The fluids maintain the shape of the eyeball.
Diseases: Cataract – The gradual development of cloudiness of the lens
- Usually occurs in both eyes
- Most develop after a person reaches 50 years old
- Symptoms include blurred vision
- Loss of eyesight occurs if not treated
- Treatment includes surgical removal of the lens
Glaucoma – The abnormal increase in intraocular (within the eye) pressure
- The cause of 15% of all cases of blindness
- Most preventable
- Caused by the overproduction of aqueous humor
- Treatment varies but is often treated with drugs (taken for the rest of their life)
Retinal Detachment – The separation of the retina from the choroid
- Allows vitreous humor to leak between the choroid and retina
- Often caused by trauma or aging
- Treatment included surgical procedures

2. THE EAR
The Organ of Hearing and Balance
Parts: TheOuter Ear
- Pinna: the appendage on the outside of the head
- Auditory Canal: the tube that leads from the outer ear to the middle ear, through which sound
waves pass
The Middle Ear
- Tympanic Membrane (eardrum): separates the outer ear from the middle ear
o The eardrum contains three small bones: the malleus, incus, and stapes. These bones
transfer vibrations of the eardrum to the inner ear.
- Eustachian Tube: equalizes pressure on both sides of the tympanic membrane
The Inner Ear (or Labyrinth)
- Cochlea: an organ shaped like a snail, with receptors for hearing
- Semicircular Canals: maintains balance and equilibrium
Diseases: Tinnitus – A symptom in most disorders of the ear
- A ringing, buzzing, or roaring noise in the ears
- May be caused by muscle spasm or an abnormality in blood vessels
- Common causes include chronic infection, head injury, prolonged exposure to loud noise,
hypertension, and cardiovascular disease
- Also caused by drugs that are ototoxic, such as aspirin
- Treatment includes masking ear noises by providing soft background music