Sense of Sight

  • Eye: Contains visual receptors.

  • Accessory Organs: Aid with vision by protecting and moving the eye.

Visual Accessory Organs


  • Composed of

    • Skin – Thinnest of the body.

    • Muscles – Obicularis Oculi closes the eyelid, Levator raises the eyelid.

    • Conjuctiva – Mucous membrane that lines the inside of the eyelid.

Lacrimal Apparatus

  • Composed of.

    • Lacrimal gland – Secretes tears continuously.

    • Superior and Inferior Canaliculi – Ducts that collect tears in the medial portion of the eye.

    • Lacrimal Sac – Collects tears from the superior and inferior canaliculi.

    • Nasolacrimal Duct – Collect tears from the lacrimal sac and empties the tears into the nasal cavity.

  • Tears lubricate the eye and contain an enzyme that reduces the risk of eye infection.

Extrinsic Muscle

  • Six individual muscles that move the eye in various directions.

Structure of the Eye

  • Eye is hollow with three distinct layers.

  • Inside is filled with fluid to give it a shape.

Outer Tunic (Fibrous Tuic)

Composed of:


  • Helps to focus light.

  • Bulges forward.

  • Transparent because it does not have any blood vessels.


  • White portion of the eye.

  • Protects eye and is an attachment site for extrinsic muscles.

Middle Tunic (Vascular Layer)

Composed of

Chroid Coat

  • Contains blood vessels which nourishes the tissues.

  • Loosely connected to the sclera.

  • Contains melanocytes which asborbs excess light and keeps the eye dark.

Ciliary Body

  • Forms a ring around the front of the eye.

  • Contains folds called ciliary processes and groups of muscle fibers called ciliary muscles.

  • Extended from he processes are suspensory ligaments that hold the lens in place.

  • Altering the tension on the suspensory ligaments causes the lens to thicken to view close objects.

  • Releasing the tension causes the lens to thin to view distant objects.


  • Colored portion of the eye.

  • Found between the cornea and lens.

  • Controls the amount of light entering the eye.

  • Divides the hollow eye into two chambers: anterior and posterior chambers.

  • Chambers are filled with fluid: Aqueous and vitreous humor which maintain the shape of the eye.

  • Aqeuous humor circulates from the anterior to the posterior chamber through the pupil (circular opening in front of the lens).

Inner Tunic

Composed of


  • Contain visual receptors photoreceptors.

  • Attached to the optic nerve.

  • Fovea Centralis: Depression in the retina that produces the sharpest vision.

How do we see?

  • Light waves enter the eye and the image is focused on the retina.

  • Image is upside down and reversed from left to right.

  • Visual receptors (cones and rods) in the retina are stimulated.

  • Rods provide vision in dim light and give a generalized outline of an object.

  • Cones detect color and produce sharp images.

  • The sensory neurons attached to the cones and rods transmit the impulse to visual cortex of the occipital lobe via the optic nerve.