The Eye and Ear

The special senses — vision, hearing, balance, taste, and smell — are our windows to the world. Unlike general senses (touch, pressure, temperature, pain), which are distributed throughout the body, the special senses are localised in specialised organs in the head. This lesson focuses on the two most clinically important: the eye (vision) and the ear (hearing and balance). Understanding their anatomy explains many important clinical conditions — from cataracts to glaucoma, from otitis media to Menière's disease.

The eyeball is a sphere approximately 2.5 cm in diameter, sitting in the bony orbit (eye socket) of the skull. The orbit is cushioned by fat and anchored by six extraocular muscles that move the eye in all directions. The three layers of the eyeball wall: 1. The fibrous layer (outer) - Sclera — the "white of the eye". A tough, opaque coat of dense connective tissue that maintains the eye's shape and provides attachment for the extraocular muscles. The sclera is continuous anteriorly with the cornea. - Cornea — the clear, dome-shaped window at the front. Refracts (bends) light as it enters the eye — accounting for about 70% of the eye's total focusing power. The cornea has no blood vessels (avascular) — it receives oxygen directly from the air and nutrients from the aqueous humour. This avascularity makes corneal transplants highly successful (no immune attack via blood vessels). 2. The vascular layer (middle) — the uvea - Choroid — a highly vascular layer behind the retina, supplying it with oxygen and nutrients. Its dark pigment absorbs scattered light, preventing internal reflection (like the inside of a camera being painted black). - Ciliary body — contains the ciliary muscle and produces aqueous humour (the clear fluid that fills the anterior chambers of the eye). The ciliary muscle changes the shape of the lens during accommodation (focusing on near objects). - Iris — the coloured part of the eye. Contains circular and radial smooth muscle fibres that control the size of the pupil — the central opening. The pupillary light reflex: bright light → pupil constricts (miosis) via the oculomotor nerve (CN III). A fixed, dilated pupil can indicate CN III compression from a brain herniation — a neurological emergency. 3. The sensory layer (inner) - Retina — the light-sensitive inner lining. Contains two types of photoreceptors: - Rods — sensitive to dim light; responsible for night vision and peripheral vision. Contain rhodopsin (bleached by bright light — why objects are invisible briefly after a camera flash). - Cones — concentrated in the fovea (central retina). Three types sensitive to red, green, and blue light. Responsible for colour vision and fine detail. - Optic disc (blind spot) — where the optic nerve exits the eye. No photoreceptors here — stimulation of this area cannot be detected. The optic disc normally appears pale pink; a blurred or swollen disc (papilloedema) indicates raised intracranial pressure.

The lens A biconvex, transparent, flexible disc suspended behind the iris by suspensory ligaments (zonular fibres) attached to the ciliary body. Its job is fine-focusing. Accommodation — when looking at something close, the ciliary muscle contracts, the suspensory ligaments relax, and the lens becomes more rounded (greater curvature = greater refraction). When looking at distance, the ciliary muscle relaxes, ligaments tighten, and the lens flattens. As we age, the lens loses elasticity — presbyopia, the reason people in their 40s need reading glasses. Cataract — clouding of the lens. The most common cause of blindness worldwide. Risk factors: age, UV exposure, diabetes, steroids. Treatment is surgical replacement of the cloudy lens with an artificial one. Chambers of the eye: - Anterior chamber — between the cornea and iris; filled with aqueous humour - Posterior chamber — between the iris and lens; also filled with aqueous humour - Vitreous chamber — large space behind the lens; filled with vitreous humour (a clear gel) Aqueous humour is produced by the ciliary body and drains through the canal of Schlemm at the iridocorneal angle. If drainage is impaired, intraocular pressure rises — causing glaucoma, which damages the optic nerve and causes irreversible vision loss. Glaucoma is a leading cause of preventable blindness. The visual pathway: Light → cornea → pupil → lens → vitreous → retina → optic nerve (CN II) → optic chiasm (where fibres from the nasal halves cross) → optic tracts → lateral geniculate nucleus → visual cortex (occipital lobe). The chiasm crossing means the left visual cortex processes the right visual field and vice versa. A stroke in the right occipital lobe causes left homonymous hemianopia (loss of the left half of the visual field in both eyes).

The ear has two distinct functions: hearing and balance. It is divided into three anatomical parts. Outer ear - Auricle (pinna) — the visible cartilage that collects and funnels sound waves - External auditory canal — a tube about 2.5 cm long leading to the eardrum. The outer one-third is cartilaginous; the inner two-thirds are bony. Lined with skin containing ceruminous glands producing earwax (cerumen) — which traps dust and has antibacterial properties. - Tympanic membrane (eardrum) — a thin, semi-transparent membrane at the junction of the outer and middle ear. Sound waves cause it to vibrate. Middle ear An air-filled cavity in the temporal bone. Contains three tiny bones — the ossicles (the smallest bones in the body): - Malleus (hammer) — attached to the tympanic membrane - Incus (anvil) — connects malleus to stapes - Stapes (stirrup) — its footplate sits in the oval window, the entrance to the inner ear The ossicles amplify vibrations from the large tympanic membrane and funnel them into the smaller oval window — a mechanical amplification of about 20 times. The Eustachian tube connects the middle ear to the nasopharynx (throat). It equalises air pressure across the eardrum (this is why your ears pop when altitude changes) and drains secretions. In children the Eustachian tube is more horizontal and shorter — making ascending infection from the throat to the middle ear easier, which is why otitis media (middle ear infection) is so common in children. Inner ear (labyrinth) The most complex and delicate part. Consists of two systems: - Cochlea — a spiral, snail-shell-shaped structure for hearing. Filled with fluid (endolymph and perilymph). Sound vibrations at the oval window create pressure waves in the fluid, which bend tiny hair cells in the organ of Corti — the true sensory organ of hearing. Hair cell bending generates electrical signals transmitted to the brain via the cochlear nerve (part of CN VIII). - Vestibular system — three semicircular canals (for detecting rotational movement) and two chambers — the utricle and saccule (for detecting linear acceleration and gravity). The vestibular nerve (the other part of CN VIII) carries balance information to the brain.

Glaucoma — raised intraocular pressure damages the optic nerve, causing gradual peripheral vision loss. Often asymptomatic until advanced. Detected by measuring intraocular pressure (tonometry) and examining the optic disc. Treated with eye drops, laser, or surgery to improve aqueous drainage. Macular degeneration — degeneration of the macula (central retina) causing progressive loss of central vision. The most common cause of blindness in the elderly in developed countries. Two types: dry (more common, slow) and wet (faster, due to abnormal blood vessel growth). The wet form can be slowed by anti-VEGF injections. Retinal detachment — separation of the retina from the choroid. Presents with sudden flashing lights, floaters, and a "curtain" across the vision. A surgical emergency — the retina survives only a few hours without its blood supply from the choroid. Otitis media — middle ear infection, most common in children. Presents with ear pain (otalgia), fever, and hearing loss. The tympanic membrane appears red and bulging on otoscopy. Often viral — antibiotics used selectively. Otosclerosis — abnormal bone growth fixing the stapes in the oval window, causing progressive conductive hearing loss. More common in women. Treated surgically (stapedectomy). Menière's disease — recurrent episodes of vertigo, tinnitus (ringing in the ears), low-frequency hearing loss, and a feeling of fullness in the ear. Caused by abnormal fluid pressure in the inner ear (endolymphatic hydrops). Managed with dietary salt restriction, diuretics, and vestibular rehabilitation. Sensorineural vs conductive hearing loss — a critical distinction: - Conductive loss — problem in the outer or middle ear (wax, otitis media, otosclerosis). Sound can bypass the blockage by bone conduction — the basis of the Rinne and Weber tuning fork tests. - Sensorineural loss — problem in the cochlea or cochlear nerve (noise damage, ageing — presbycusis, ototoxic drugs). Cannot be bypassed by bone conduction.