Human Eye — Set 5

Correct Answer: A. Rods and cones

• **Diplopia (double vision)** = seeing two images of one object, usually caused by misalignment of the eyes (strabismus) or weakness of eye muscles. • **Binocular diplopia** disappears when one eye is closed — confirming a muscle/alignment cause, not a retinal one. • 💡 Wrong-option analysis: Myopia: blurring of distant single objects — not doubling; Cataract: scattering causes haziness, not two separate distinct images; Glaucoma: loss of peripheral visual field — not image doubling.

Correct Answer: B. Fovea centralis

• **Strabismus (squint)** = misalignment of the eyes so they do not point at the same object simultaneously — one eye turns in, out, up, or down. • **Treatment** includes corrective glasses, patching the stronger eye, or surgery to re-balance the extraocular muscles. • 💡 Wrong-option analysis: Myopia: a refractive error — both eyes still aligned; Presbyopia: loss of near accommodation — no eye deviation; Astigmatism: meridional blur — not an alignment defect.

Correct Answer: B. Rod cells for dim-light vision

• **Extraocular muscles** = 6 muscles per eye (medial/lateral/superior/inferior recti and superior/inferior obliques) that rotate the eyeball in all directions. • **Oculomotor (CN III)** controls 4 of the 6 muscles; trochlear (CN IV) controls superior oblique; abducens (CN VI) controls lateral rectus. • 💡 Wrong-option analysis: Ciliary muscles: internal — adjust lens curvature; Sphincter pupillae: controls pupil size — inside the eye; Iris dilator: regulates pupil — not involved in eyeball movement.

Correct Answer: A. Suspensory ligaments (zonules)

• **Optic chiasm** = the X-shaped crossing point below the hypothalamus where nasal retinal fibres from each eye cross to the opposite side. • **Visual field result**: each hemisphere receives input from the contralateral visual field from both eyes, enabling depth perception. • 💡 Wrong-option analysis: Optic disc: the exit point of the optic nerve within the eye — not the chiasm; Corpus callosum: connects the two cerebral hemispheres — not specifically visual fibre crossing; Lateral geniculate nucleus: the thalamic relay — receives fibres after the chiasm.

Correct Answer: B. Relax to make the lens thinner

• **Visual cortex** = located in the occipital lobe (Brodmann area 17/V1) at the back of the brain, it processes the nerve signals from the retina into perceived images. • **Retinotopic mapping** means adjacent retinal areas map to adjacent cortical areas — a spatial representation of the visual field. • 💡 Wrong-option analysis: Temporal lobe: processes memory and language — higher-level object recognition, not primary vision; Parietal lobe: spatial awareness and touch — not primary visual processing; Frontal lobe: motor control and planning — not the primary visual area.

Correct Answer: B. Ciliary muscles contract and reduce ligament tension

• **Depth perception** = the ability to perceive the three-dimensional structure of the world and judge distances, primarily through binocular disparity. • **Binocular disparity**: the two eyes are ~6.5 cm apart and receive slightly different images; the brain fuses them to compute depth. • 💡 Wrong-option analysis: Monocular vision: provides some depth cues (size, overlap) but not stereo disparity; Single-eye focusing: changes focal length — not a distance-measurement mechanism; Colour vision: wavelength detection — independent of distance.

Correct Answer: D. 60 D

• **Lens of the eye** = a transparent, biconvex, flexible structure made of crystallin proteins, suspended by zonular fibres from the ciliary body. • **Refractive index** ≈ 1.40 at centre; focal length varies from ~17 mm (far) to ~14 mm (near) as ciliary muscles act. • 💡 Wrong-option analysis: Cornea: fixed curvature, not adjustable, does not account for accommodation; Retina: photosensitive screen — not a focusing lens; Vitreous humour: maintains eye shape, transparent gel — no focusing power.

Correct Answer: A. Avascular and very sensitive

• **Rods and cones** = the two types of photoreceptors in the retina; rods for dim/achromatic vision, cones for bright/colour vision. • **Distribution**: cones densest at fovea (~150,000/mm²); rods absent at fovea but ~120 M spread across retina. • 💡 Wrong-option analysis: Bipolar cells: second-order neurons — not photoreceptors; Ganglion cells: output neurons forming the optic nerve — not light-sensitive directly; Amacrine cells: interneurons modulating signals within the retina.

Correct Answer: C. Choroid

• **Sclerotic (sclera)** = the hard, white, opaque outer coat of the eyeball that maintains its shape and provides attachment for extraocular muscles. • **Cornea** is the transparent anterior portion of the same outer fibrous coat — same layer, different properties. • 💡 Wrong-option analysis: Choroid: the dark vascular middle layer — not the outer structural coat; Retina: the innermost neural layer; Conjunctiva: the mucous membrane over the sclera — a different structure on top.

Correct Answer: B. Ciliary body (ciliary processes)

• **Zonular fibres (suspensory ligaments)** = fine fibres that connect the lens capsule to the ciliary body, holding the lens in position. • **Tension mechanism**: ciliary muscle relaxes → fibres taut → lens flattened (far vision); muscle contracts → fibres relax → lens rounds (near vision). • 💡 Wrong-option analysis: Optic nerve fibres: carry visual signals, not lens support; Extraocular muscles: move the eyeball — do not touch the lens; Corneal stroma: fibrous layers of the cornea — separate structure.