8.1: External Structures of the Eye ANATOMY & PHYSIOLOGY The Eye and Vision 70% of all sensory receptors are in the eye Each eye has over a million nerve fibers Protection: ◦ Most of the eye is enclosed in a bony orbit ◦ We only see about 1/6th of the eyes actually size in adults ◦ A cushion of fat surrounds most of the eye External and Accessory structures There are four structures of the eye that we will focus on: ◦ Eyelids ◦ Conjunctiva ◦ Lacrimal apparatus ◦ Extrinsic eye muscles Eyelids Serve as anterior protection for the eye Contain projections known as eyelashes ◦ Two types of glands are associated with the eyelashes ◦ Tarsal glands – produce an oily secretion that lubricates the eye ◦ Ciliary glands – modified sweat glands Conjunctiva Delicate membrane that lines the eyelids and covers part of the outer surface of the eyeball ◦ Secretes mucus which helps with lubrication ◦ Can become inflamed ◦ Conjunctivitis ◦ If this is caused by bacteria or viruses, it is commonly referred to as pink eye Lacrimal apparatus Two parts: ◦ Lacrimal glands ◦ Secrete a dilute salt solution (tears) ◦ Also contains mucus, antibodies, and lysozymes (an enzyme that destroys bacteria) ◦ Ducts ◦ Drain the lacrimal secretions into the nasal cavity ◦ When these increase substantially, nasal congestion can cause the “sniffles” ◦ Colds can cause these to become inflamed and swell ◦ Drainage cannot occur and we get stuck with “watery” eyes Extrinsic Eye Muscles Six muscles that attach to the eye ◦ Lateral rectus (lateral movement) ◦ Medial rectus (medial movements) ◦ Superior rectus (elevates eyes and turns them medially) ◦ Inferior rectus (depresses eyes and turns them medially) ◦ Inferior oblique (elevates eyes and turns them laterally) ◦ Superior oblique (depresses eyes and turns them laterally) For Next Time: Reading: ◦ None ◦ Brain Games Homework: ◦ 8.1 Worksheet: #1-3.