Special Senses - Eyes

External Anatomy of the Eye Eyelids Eyelashes Conjunctiva – A membrane that lines eyelids and covers part of the outer surface of the eyeball Conjunctivitis Inflammation of the conjunctiva (example – pinkeye) Caused by a bacterial or viral infection

External Anatomy of the Eye Lacrimal glands Above the lateral end of the eye Secrete dilute salt solution (tears) onto the anterior surface of the eyeball

Internal Anatomy of the Eye The eyeball is consisted of three tunics (coats) Sclera – outer tunic (white of the eye) Choroid – middle tunic (has a dark pigment that prevents light from escaping the eye) Retina – inner tunic (has photoreceptors called rods and cones)

Sclera Choroid Retina

Internal Anatomy of the Eye (cont.) Photoreceptors – respond to light Rods Detect black and white Process dim light Used for peripheral vision 120 million rods in one eye Cones Detect color Process bright light Most are found in the center of the retina 7 million cones in one eye

Internal Anatomy of the Eye (cont.) Optic nerve – sends information from the eyes to the brain Optic disk – part of the eye where optic nerves travel through Does not have any photoreceptors Known as the blind spot Fovea centralis Near the optic disk that contains only cones The area of sharpest vision

Internal Anatomy of the Eye (cont.) Cornea – Clear membrane that forms the front of the eye Iris – pigmented portion of the eye Pupil – The expanding and contracting opening in the iris through which light passes to the retina Cornea Pupil Iris

Internal Anatomy of the Eye (cont.) Lens Allows light to reach the retina to focus an image Held in place via suspensory ligaments Lens Suspensory ligaments

Internal Anatomy of the Eye (cont.) The lens divides the eye into two chambers Aqueous humor – Anterior segment that contains a clear watery fluid Provide nutrients for the lens and cornea which lack a blood supply Vitreous humor – Posterior segment that’s filled with a gel-like substance Helps prevent the eyeball from collapsing inward by reinforcing it internally

Pathway of Light Through the Eye Cornea Aqueous humor Pupil Lens Vitreous humor Retina Choroid Optic disk Optic nerve

Malfunctions of the Eyes Cataract As one ages, the lens becomes hard and cloudy. Causes vision to become hazy and possibly blindness

Malfunctions of the Eyes Glaucoma If drainage of aqueous humor is blocked, pressure within the eye increases dramatically compressing the retina and optic nerve. This can cause pain and possibly blindness.