1.0 Photoreception: The Human Eye
The Eye is a highly specialized sense organ for vision. It acts as a biological camera, focusing light onto a light-sensitive screen (the retina) and converting it into electrochemical impulses for the brain to interpret.
1.1 Structural Layers
- Sclera: The outermost tough, white fibrous layer. Its transparent front portion is the Cornea, which serves as the primary refractive surface.
- Choroid: The middle vascular layer, rich in blood vessels and dark pigments to prevent internal reflection of light. It forms the Ciliary Body and Iris in the front.
- Retina: The innermost neurosensory layer containing two types of photoreceptors:
• Rods: Sensitive to low light (scotopic vision); contain Rhodopsin.
• Cones: Sensitive to bright light and color; contain Iodopsin.
1.2 Accommodation and Light Regulation
The eye must constantly adjust to varying light intensities and object distances.
| Feature | Distant Vision | Near Vision |
|---|---|---|
| Ciliary Muscles | Relaxed | Contracted |
| Suspensory Ligaments | Taut (Pulled tight) | Relaxed |
| Lens Shape | Flattened / Less Convex | Rounded / More Convex |
Yellow Spot (Macula Lutea): The area of maximum visual acuity containing only cones. Conversely, the Blind Spot is the point where the optic nerve leaves the eye; it has no photoreceptors and vision is nil.
2.0 Phonoreception: The Human Ear
The Ear performs two distinct functions: Hearing (phonoreception) and Maintenance of Body Balance (equilibrium).
2.1 Structural Divisions
- Outer Ear: Consists of the Pinna (collects sound) and Auditory Canal (secretes wax/cerumen).
- Middle Ear: Contains the Ear Ossicles (Malleus, Incus, Stapes) which amplify sound vibrations. The Eustachian Tube connects it to the pharynx to equalize air pressure.
- Inner Ear (Labyrinth):
• Cochlea: Snail-like structure for hearing; contains the Organ of Corti.
• Semicircular Canals: For dynamic balance while the body is in motion.
• Vestibule: For static balance relative to gravity.
Stereoscopic Vision: Humans have both eyes focused on the same object but from slightly different angles. This allows for Depth Perception (3D vision). In contrast, many birds have Monocular Vision where each eye sees a different field.
Defects of Vision:
• Myopia (Short-sightedness): Eyeball too long; image falls in front of retina. Corrected by Concave Lens.
• Hypermetropia (Long-sightedness): Eyeball too short; image falls behind retina. Corrected by Convex Lens.