Anatomy of the Eyeball

Structural Parts of the Eyeball

• A spherical structure, the eyeball captures light and sends visual data to the brain.
• It comprises three layers and internal components.

Fibrous Tunic (Outer Layer)

• Sclera: The white, opaque part giving structure and protection, maintaining shape, and connecting to eye muscles.
• Cornea: The transparent front part allows light in, avascular with many nerves, and refracts light contributing to acuity.

Vascular Tunic (Uvea/Middle Layer)

• Choroid: A vascular layer feeding the retina and contains melanocytes to absorb stray light, reducing internal reflection.
• Ciliary Body: Contains the ciliary muscle, which controls the lens shape, and ciliary processes, which make aqueous humor.
• Iris: Controls pupil size using the sphincter pupillae (constriction, parasympathetic) and dilator pupillae (dilation, sympathetic) muscles.
• Pupil: The iris's central opening regulates how much light enters.

Retina (Inner Layer)

• Neural Retina: Includes photoreceptors (rods for low light, cones for color), bipolar cells, and ganglion cells.
• Retinal Pigment Epithelium (RPE): Supports photoreceptors and absorbs light to prevent scattering.
• Optic Disc (Blind Spot): The exit for the optic nerve (CN II), lacking photoreceptors.
• Macula Lutea: Central area for detailed vision, contains the fovea centralis with many cones.

Chambers of the Eye

• Anterior Chamber: Located between the cornea and iris, filled with aqueous humor.
• Posterior Chamber: Located between the iris and lens, also filled with aqueous humor.
• Vitreous Chamber: Behind the lens, filled with vitreous humor, a gel that supports the retina.
• Lens: A biconvex structure that focuses light on the retina through accommodation.

Optic Nerve (CN II)

• Originates from the retina’s ganglion cells.
• Carries visual information from the eye to the brain.

Optic Chiasm

• Where optic nerve fibers partially cross via decussation.
• Nasal retinal fibers cross to the opposite side; temporal retinal fibers stay on the same side (ipsilateral).

Optic Tract

• Continues from the optic chiasm.
• Contains visual information from the opposite (contralateral) visual field.

Lateral Geniculate Nucleus (LGN) of the Thalamus

• The main relay center for visual information.

Optic Radiations

• The geniculocalcarine tract carries signals to the visual cortex.
• Meyer's Loop carries upper visual field information through the temporal lobe.
• The Dorsal Stream carries lower visual field information through the parietal lobe.

Primary Visual Cortex (V1, Brodmann Area 17)

• Located in the occipital lobe (calcarine sulcus).
• Processes basic visual details such as shape, color, and motion.

Extraocular Muscles (Movement of the Eyeball)

• Superior Rectus: Elevates and adducts (CN III – Oculomotor).
• Inferior Rectus: Depresses and adducts (CN III).
• Medial Rectus: Adducts (CN III).
• Lateral Rectus: Abducts (CN VI – Abducens).
• Superior Oblique: Depresses, abducts, and rotates medially (CN IV – Trochlear).
• Inferior Oblique: Elevates, abducts, and rotates laterally (CN III).

Intrinsic Muscles (Within the Eye)

• Ciliary Muscle: Controls lens shape (parasympathetic, CN III).
• Sphincter Pupillae: Constricts pupil (parasympathetic, CN III).
• Dilator Pupillae: Dilates pupil (sympathetic).

Protective Structures

• Eyelids (Palpebrae): Protect and lubricate the eyes.
• Conjunctiva: Transparent membrane lining the eyelids and sclera.
• Eyebrows and Eyelashes: Protect against debris.

Lacrimal Apparatus

• Lacrimal Gland: Produces tears (CN VII – Facial nerve).
• Lacrimal Puncta: Drains tears into the canaliculi.
• Lacrimal Sac and Nasolacrimal Duct: Drain into the nasal cavity.

External Ear

• Auricle (Pinna): Captures sound waves.
• External Acoustic Meatus: Channels sound to the tympanic membrane.
• Tympanic Membrane (Eardrum): Vibrates in response to sound.

Middle Ear

• Ossicles: The malleus (hammer), incus (anvil), and stapes (stirrup) transmit vibrations.
• Oval Window: Transmits sound to the inner ear.
• Eustachian Tube: Equalizes pressure.

Bony Labyrinth (in Temporal Bone)

• Cochlea: Converts sound to neural signals.
• Vestibule: Detects linear acceleration.
• Semicircular Canals: Detect angular acceleration.

Membranous Labyrinth

• Cochlear Duct (Scala Media): Contains the organ of Corti (hearing receptor).
• Utricle and Saccule: Detect static equilibrium.
• Semicircular Ducts: Detect dynamic equilibrium.

Cochlear Nerve (Part of CN VIII)

• Transmits sound from the cochlea.

Cochlear Nuclei (Medulla)

• First synapse in the auditory pathway.

Superior Olivary Complex (Pons)

• Involved in sound localization.

Lateral Lemniscus (Midbrain)

• Ascending auditory pathway.

Inferior Colliculus

• Integrates and relays auditory reflexes.

Medial Geniculate Nucleus (Thalamus)

• Projects to the auditory cortex.

Primary Auditory Cortex (Heschl’s Gyrus, Area 41/42)

• Responsible for sound perception.

Vestibular Nerve (Part of CN VIII)

• Transmits balance information.

Vestibular Nuclei (Brainstem)

• Process equilibrium data.

Projections

• Cerebellum: Fine-tunes balance.
• Thalamus and Cortex: Provides conscious awareness of balance.
• Spinal Cord: Controls posture through the vestibulospinal tract.
• Oculomotor System: Manages eye movement through the vestibulo-ocular reflex.