Image on right is a plastic model of a left ear (photos by Orin James). The structures of the external and middle ear include the following:

1. Pinna or Auricle (1) - The outermost region that is visible to the world is called the pinna or auricle. It is easily recognized in a German Shepherd dog, as it will perk up as the dog attempts to perceive sound. This structure's primary function is to funnel sound into the auditory acoustic meatus. Number one represents both the pinna and the acoustic meatus.
2. Auditory Acoustic Meatus (1) - This is a very narrow chamber that penetrates the temporal bone of the skull and is lined with wax-secreting glands called ceruminous glands. The ear wax is designed to trap or prevent insects from entering the inner ear. The auditory acoustic meatus terminates at the ear drum or tympanic membrane.
3. Tympanic Membrane (Eardrum) (2) - This structure marks the beginning of the middle ear. As sound waves are funneled through the pinna and passes through the auditory acoustic meatus it encounters the tympanic membrane and sets it into vibration at the same frequency as the sound waves. Behind the tympanic membrane are the body's three smallest bones, the auditory ossicles. These are circled and labled as number 2.
4. Auditory Ossicles (2) - Comprised of the malleus,incus, and stapes, the auditory ossicles form a lever system, which amplifies the sound detected by the tympanic membrane via vibrations. These vibratory motions of the stapes will rock against the oval window (not visible in image). The rocking oval window will then set the perilymph (fluid contained in the cochlea) of the cochlea in motion. Before we look at cochlea or inner ear, let's look at another conspicuous structure of the middle ear, namely the pharyngotympanic tube (aka eustachian tube). This structure, located infero lateral to auditory ossicles, is connected to the nasopharynx of the throat and is used to equalize internal pressure with atmospheric pressure as to avoid damage to the eardrum when external pressure changes. Think about what happens when you ascend high altitudes. You will have to "pop" your ear. This tube is also the reason why a throat infection can lead to an ear infection and vise versa. Now we begin a discussion of the inner ear.

The image on the right has in the upper left inset a painted wooden sample of the inner ear (photos by Orin James). Number 1 stands above the cochlea in bottom left image and number two stands above the semicircular canals in both the bottom and upper left images. Between these structures, as can be better seen in the bottom image, resides the vestibular apparatus. All these structures can be described as being comprised of a bony labyrinth. This simply means that each of these structures contain within them a fluid called perilymph. Suspended in the perilymph is another membranous labyrinth, which usually follows the contours of the bony labyrinth. Lastly, within the membranous labyrinth is another fluid substance known as endolymph.

1. Cochlea (1) - This organ is exclusively used for hearing. Its spiral structure contains the spiral organ of corti. The spiral organ of corti contains hair cells that will receive sound stimuli via bending (see physiology of hearing below). All sound stimuli detected by the organ of corti will innervate the cochlear nerve, part of the vestibulocochlear nerve.
2. Semicircular Canals (2) - As seen in the image, there are three semicircular canals, an anterior, a posterior and a lateral. At the base of each semicircular canal is an ampulla, within which, one will find a crista ampullaris surrounded by endolymph. These structures are designed to detect rotary stimuli or dynamic equilibrium (see physiology of equilibrium below).
3. Vestibular Apparatus (located between semicircular canals and cochlea) - This structure is responsible for detecting acceleration along the verticle and horizontal planes (static equilibrium). The vestibular apparatus contains the utricle and saccule. Within these structures are maculae. Maculae are lined with hair cells along the horizontal plane in the utricle, with hairs standing vertically, and the verticle plane, with hairs positioned horizontally in the saccule. Both the utricle and saccule hair cells are covered with a gelationous layer called the otolithic membrane. Bending of the hair cells will innervate the vestibular nerve, part of the vesibulocochlear nerve (see physiology of equilibrium below).

-O. James