Final answer:
The semicircular canals are part of the inner ear with two main parts: the fluid-filled canals and the ampullae, which contain hair cells that detect head movement.
Step-by-step explanation:
The semicircular canals are a crucial part of the inner ear involved in the body's sense of balance and head movement. They consist of two main components: the tubular canals filled with fluid and the sensory ampullae. Each of the three semicircular canals, arranged in different spatial planes, contains an enlarged region called the ampulla at their base.
Within the ampulla are hair cells with stereocilia that extend into a gelatinous structure known as the cupula. The hair cells in the ampulla respond to rotational movements of the head by detecting the deflection of the cupula caused by the lag of the fluid in the canals. This system allows the brain to perceive the direction and speed of head movements in three-dimensional space.