Which is the smallest bone in the human body?

The stapes is the smallest and lightest bone in the human body. It is located in the middle ear and is about 3 millimetres long and weighs only about 3-4 milligrams. Its name comes from the Latin word for "stirrup", because it is shaped like the stirrup of a horse's saddle.

The stapes is one of three tiny bones in the middle ear, together called the ear ossicles:

• Malleus (hammer): attached to the eardrum (tympanic membrane).
• Incus (anvil): the middle bone.
• Stapes (stirrup): connected to the oval window of the inner ear.

These three bones form a chain that transmits sound vibrations from the eardrum to the inner ear. When sound waves enter the ear canal, they make the eardrum vibrate. The vibrations pass through the malleus, incus and stapes, which amplify them by about 20 times. The stapes pushes on the oval window, setting up waves in the fluid of the cochlea in the inner ear. Hair cells in the cochlea (in the organ of Corti) convert these waves into nerve signals, which travel through the auditory nerve to the brain, where they are interpreted as sound.

The ear has three parts:

Outer ear: the pinna and ear canal, which collect sound.

Middle ear: the eardrum and ossicles, which transmit sound. The Eustachian tube connects the middle ear to the throat and balances air pressure on both sides of the eardrum.

Inner ear: the cochlea, for hearing, and the semicircular canals and vestibule, which help maintain the body's balance.

The stapedius, attached to the stapes, is the smallest skeletal muscle in the body. A condition called otosclerosis, in which the stapes becomes fixed, can cause hearing loss and is treated by an operation called stapedectomy.

Why not the other options?

• ✗The malleus and incus are the other two ear ossicles, slightly larger than the stapes.
• ✗The patella (kneecap) is a much larger bone at the front of the knee.