Special Senses and Eye Anatomy

Eye Structure and Function

• Special sense receptors: large, complex sensory organs or localized clusters of receptors
• Accessory structures: extrinsic eye muscles, eyelids, conjunctiva, and lacrimal apparatus

Eyelids and Eye Muscles

• Canthus: medial commissure and lateral commissure
• Palpebral fissure: space between the eyelids in an open eye
• Tarsal glands: modified sebaceous glands associated with eyelid edges, producing oily secretions to lubricate the eye
• Ciliary glands: modified sweat glands between eyelashes, with ducts opening at eyelash follicles
• Lacrimal caruncle: raised area containing sebaceous and sweat glands, producing oily, whitish secretion to lubricate the eye

Lacrimal Apparatus

• Lacrimal glands: located above the lateral end of each eye, continually releasing dilute salt solution (tears) onto the anterior surface of the eyeball
• Lacrimal canaliculi: tears flush across the eyeball into these canaliculi
• Lacrimal sac: after canaliculi
• Nasolacrimal duct: finally enters the inferior meatus of the nasal cavity

Extrinsic Eye Muscles (External Eye Muscles)

• Lateral rectus: moves eye laterally (cranial nerve VI: Abducens)
• Medial rectus: moves eye medially (cranial nerve III: Oculomotor)
• Superior rectus: elevates eye and turns it medially (cranial nerve III: Oculomotor)
• Inferior rectus: depresses eye and turns it medially (cranial nerve III: Oculomotor)
• Inferior oblique: elevates eye and turns it laterally (cranial nerve III: Oculomotor)
• Superior oblique: depresses eye and turns it laterally (cranial nerve IV: Trochlear)

Eyeball Structure

• Fibrous layer: outermost layer
• Sclera: protective layer, visible as the "white of the eye"
• Cornea: transparent, crystal clear, well-supplied with nerve endings
• Vascular layer: middle of the eyeball, divided into three distinguishable regions
• Choroid: blood-rich nutritive tunic containing dark pigment, preventing light scattering inside the eye
• Ciliary body: attached to the lens by the ciliary zonule, and the iris
• Pupil: pigmented iris with a rounded opening, allowing light to pass

Retina

• Sensory layer: delicate two-layered retina, extending anteriorly only to the ciliary body
• Pigmented layer: composed of pigmented cells, absorbing light and preventing scattering inside the eye
• Neural layer: transparent inner of the retina, containing millions of receptor cells (rods and cones)
• Fovea centralis: lateral to each blind spot, a tiny pit containing only cones, with the greatest visual acuity
• Rhodopsin: purple pigment found in rods, formed from opsin (protein) and retinal (modified vitamin A product)

Lens and Accommodation

• Lens: flexible biconvex crystal-like structure, focusing light onto the retina
• Anterior (aqueous) segment: anterior to the lens, containing aqueous humor
• Posterior (vitreous) segment: posterior to the lens, containing vitreous humor
• Accommodation: the eye's ability to focus specifically for close objects