The size and shape of a dog's ears will depend on his breed. The visible ear, the flap called the pinna, is only part of a complex organ involved with hearing and balance and is best understood if it's divided into three parts. The external ear captures sound, the middle ear transmits the sound vibrations to the cochlea and the inner ear sends auditory information to the brain and maintains balance.
The pinnae -- plural of pinna -- can be erect or pricked like the upright ears of a German shepherd, semi-erect or semi-pricked like a Jack Russell's, or a dog can have pendulous ear flaps, such as the floppy ears of a spaniel or the folded and curved pendulous ears of a bloodhound. The pricked and semi-pricked ears can move independently of each other. This helps the dog to locate where sounds are coming from, and these ear types are better at detecting sounds than floppy ears.
The pinna picks up sounds and the sound waves travel down the ear canal. The ear canal, made of cartilage and specialized skin, has two types of glands that together produce cerumen, or ear wax, to help keep the ear dry by repelling moisture. The ear canal has a long vertical section that becomes horizontal as it joins the eardrum, or tympanic membrane, where the incoming sound waves strike the eardrum and cause it to vibrate.
In the middle ear there are three small bones called the auditory ossicles. The largest, the malleus, is attached to the eardrum and transmits the vibrations from the eardrum to the next bone, the incus, which contacts with the third bone, the stapes, which in turn conducts the vibrations to the inner ear via the oval window membrane. There is also a large air-filled chamber called the bulba which transmits low-pitch sounds. In the bottom of the middle ear, the short Eustachian, or auditory, tube opens into the throat. This equalizes pressure on each side of the eardrum, helping to protect the eardrum when pressure outside the ear increases.
The vibrations from the auditory ossicles create waves in the fluid inside the cochlea, a structure shaped like a snail's shell in the inner ear. The waves move membranes that stimulate hair cells -- different hair cells are stimulated in response to different frequencies -- and this produces nerve impulses that are carried by the vestibulocochlear nerve to the auditory center in the brain. Here the signals are translated into what the dog hears. The dog's cochlea is larger than a human's and the dog's hearing is more sensitive. The human hearing range is around 20 hertz to 20 kilohertz, whereas a dog can hear sound frequencies from 67 hertz to 45 kilohertz. This enables the dog to hear the high-pitched noises of rodents, which in the past would have been part of his diet.
A dog's ears also deal with balance. In the inner ear there are semicircular canals where liquid moves as the dog's head changes position. The movement of the liquid in the canals tells the brain which way the head is moving and by how much. The brain, in turn, sends messages to the limbs and muscles in the dog's body so they respond appropriately to changes in orientation and to maintain his balance.