The Eye

External Anatomy of the Eye The eye is protected by the bony orbit and cushioned by fat. Bony orbit consists of: ethmoid, sphenoid, lacrimal, frontal, zygomatic, and maxillary bones Accessory structures include: Eye muscles Eyelids Moisture makers Lacrimal apparatus

Eye Muscles There are 6 skeletal muscles that control eye movement. They are attached to the outer surface of each eye. The muscles are named according to their position.

Name of Muscle Eye Movement Lateral rectus . . . eye moves lateral Medial rectus . . . eye moves medial Superior rectus . . . eye rolls up Inferior rectus . . . eye rolls down Inferior oblique . . . eye rolls up and lateral Superior oblique . . . eye rolls down and lateral

Eyelids The eye is protected by eyelids, AKA: palpebrae. The eyelids meet at the medial and lateral canthus or corners of the eye. The medial canthus is different from the lateral one in that is houses a reddish tissue called the caruncle. Eye residue collects in this location.

The thicker edge or border of each eyelid is referred to as the tarsal plate. Projecting from each eyelid are eyelashes which prevent particles from falling into the eye. Eyebrows serve the same purpose.

Moisture Makers Three different products keep the surface of the eye lubricated and moist. 1. Sweat 2. Oil 3. Mucus Cranial nerve V (the trigeminal nerve) allows us to detect when particles have landed on the surface of the eye and these lubricants prevent them from adhering.

The eye has modified sweat glands, known as ciliary glands The eye has modified sweat glands, known as ciliary glands. They lie between the individual eyelashes. A thin membrane called the conjunctiva lines the underside of the eyelids and covers part of the outer surface of the eyeball. It secretes a thin mucus that lubricates the eyeball. Modified sebaceous glands called meibomian glands are found under each lid. These glands produce an oily substance that lubricates the eye.

Without a healthy layer of moisture, good vision is not possible.

Lacrimal Apparatus The lacrimal glands are located above the lateral end of each eye underneath the upper eyelids. They release tears through several small ducts. Tears are flushed across the eyeball and collect in the medial canthus. There, they drain through tiny openings called puncta, into the lacrimal canals and then the liquid moves down the nasolacrimal duct which empties into the nasal cavity.

Lacrimal Apparatus Figure 16.3