Learning to walk not only requires appropriate nerve connections and musculature, but also balance. The organ of balance -- the vestibular apparatus or labyrinth -- exists within the inner ear. It consists of two distinct systems. Firstly, there are three semi-circular canals, each filled with fluid and each set in a different plane. All three act as a receptor system, with the contained fluid accelerating or decelerating, enabling the brain to detect and record head movement. Secondly, there are the saccule and the utricle. These also work in a mechanical fashion. They possess chalky particles (otoliths or ear dust) which respond to gravity. The particles come to rest upon sensitive hairs, enabling the organ's owner to know which way is up and which is down. These two systems permit balance to be achieved.