25.4 Vibrations enter the outer ear via the pinna (auricle), the portion of the ear visible from the outside, which collects the vibrations and funnels them into the ear canal (external auditory meatus) (How). The ear canal is about an inch long and a quarter inch in diameter, and extends from the pinna to the eardrum. The vibrations then reach the eardrum (tympanic membrane) at the end of the ear canal and the beginning of the middle ear. The middle ear contains the ossicles, three tiny bones which move back and forth with the vibrations, changing the sound waves into mechanical vibrations.
The first bone is called the malleus (hammer), and is connected to the eardrum (How). The malleus is connected to the second ossicle, the incus (anvil), which in turn is connected to the third ossicle, the stapes (stirrup). The mechanical vibration is transmitted through the three bones, and causes an in-and-out movement of the base of the stapes, the stapes footplate, in patterns that match those of the incoming sound waves. The stapes footplate fits into the oval window, which is the beginning of the inner ear.
The vibrations then reach the inner ear which contains the sensory organ for hearing (How). The part of the inner ear related to hearing is called the cochlea. It is a bony structure shaped like a snail and filled with fluid called endolymph and perilymph. The sensory receptor in the cochlea is the organ of Corti, which holds the hair cells which are the nerve receptors for hearing. Mechanical energy from movement in the middle ear pushes in a membrane (the oval window) in the cochlea, and the force moves the fluids in the cochlea, which stimulate the hair cells. Individual hair cells respond to specific sound frequencies (pitches). Signals to the hair cells are translated into nerve impulses and transmitted to the brain by portions of the acoustic nerve (cranial nerve VIII).
25.5 Sounds of different pitch are d...