The Eye and the Ear: Quick Revision

A brief note on The Eye and the Ear. Read it before a test. Try each question at the end before you open its answer. This note is for the boards, A-Level and IB. The structure of the eye and the ear is not in the NEET syllabus.

The eye

• Outer layer: sclera, with the transparent cornea in front. Middle layer: choroid, ciliary body, iris. Inner layer: retina.
• Lens: held by ligaments of the ciliary body. Pupil: the opening in the iris, set by the iris muscles.
• Aqueous humour: watery, between the cornea and the lens. Vitreous humour: gel, between the lens and the retina.
• Retina, inside to outside: ganglion cells, bipolar cells, photoreceptor cells.
• Blind spot: no photoreceptors, the optic nerve leaves. Fovea: only cones, the greatest acuity.

Vision

• Cones: daylight and colour, three kinds (red, green, blue). Rods: twilight, rhodopsin from vitamin A.
• Photopigment: opsin and retinal. Light separates them, the membrane permeability changes and a potential difference arises.
• Path: photoreceptors, bipolar cells, ganglion cells (action potentials), optic nerve, visual cortex.

The ear

• External ear: pinna, canal, ear drum. Middle ear: malleus, incus, stapes. The Eustachian tube joins it to the pharynx.
• Inner ear: cochlea with the scala vestibuli, scala media (endolymph) and scala tympani. The organ of Corti on the basilar membrane has the hair cells, with the tectorial membrane above.
• Balance: crista ampullaris in the three semicircular canals, and the macula of the utricle and the saccule.
• Path of sound: drum, ossicles, oval window, cochlear fluid, basilar membrane, hair cells, auditory nerve.

A worked example

A pupil is 2 mm wide in the sun and 6 mm wide in the dark. How many times larger is its area in the dark?

1. The width grows $6 \div 2 = 3$ times.
2. The area grows as the square: $3^2 = 9$ times.

Practice with new numbers

A program to try

The program prints the path of a sound in order.

Mistakes to avoid

• Saying that rods give colour vision. The cones give colour.
• Putting the vitreous humour in front of the lens. It lies behind the lens.
• Mixing up the order of the ossicles. Malleus, incus, stapes.
• Saying that the cochlea senses balance. It senses sound.

Check yourself

1. Which cells work in daylight?

2. What are the two parts of a photopigment?

3. Which tube joins the middle ear with the pharynx?

4. Where is the organ of Corti?

Sources

• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 36, Sensory Systems.
• NCERT (2023). Biology, Textbook for Class XI, chapter 18.