Anatomy and Physiology: Special Senses – The EAR
OUTER (EXTERNAL) EAR
The outer ear collects sound waves in the air and channels them to the inner parts of the ear. The outer ear along with its canal has been shown to enhance sounds within a certain frequency range. That range just happens to be the same range that most of the characteristics of human speech sounds fall into. This allows the sounds to be boosted to twice their original intensity. Parts of the outer ear are the following:
- Pinna – Also called the auricle. It is the part which protrudes from the side of the skull made of cartilage covered by skin. The pinna collects sound and channels it into the ear canal. The pinna’s shape enables it to funnel sound waves into the external auditory meatus. The various folds in the pinna’s structure amplify some high frequency components of the sound. They also help in the localization of sound in the vertical plane. As sounds hit the pinna from above and below, their paths to the external auditory meatus vary in length. This means that they take different times to reach the meatus.
- External acoustic meatus or external auditory canal – this is a short chamber about 1 inch long by ¼ inch wide. It is carved into the temporal bone of the skull. The canal has bends in both the vertical and horizontal planes. This means that it is difficult for anything poked into the meatus to hit the drum. Any trauma is likely to be to the walls of the canal. In its skin-lined walls are the ceruminous glands, which secrete a waxy yellow substance, called earwax or cerumen.
Sound waves entering the external auditory canal eventually hit the tympanic membrane or eardrum and cause it to vibrate. The canal ends at the eardrum, which separates the outer ear from the middle ear.
The middle ear or tympanic cavity is an air filled space within the temporal bone. It transforms the acoustical vibration of the sound wave into mechanical vibration and passes it onto the inner ear. The three tiny bones of the middle ear act as a lever to bridge the eardrum with the oval window. Incoming forces are magnified by about 30 %. This increased force allows the fluid in the cochlea of the inner ear to be activated.
The tympanic cavity is spanned by the three smallest bones in the body, the ossicles which transmit the vibratory motion of the eardrum to the fluids of the inner ear. These bones, named for their shape, are the following:
- Hammer or malleus
- Anvil or incus
- Stirrup or stapes
When the eardrum moves the hammer moves with it and transfers the vibration to the anvil. In response to this, the anvil passes it on to the stirrup which in turn presses on the oval window of the inner ear. The movement of the oval window sets the fluids of the inner ear into motion, eventually exciting the hearing receptors.