The ear is the sense organ for both hearing and equilibrium (Fig. 7-12). It is divided into three main sections:
* The outer ear includes an outer projection and a canal ending at a membrane.
* The middle ear is an air space containing three small bones.
* The inner ear is the most complex and contains the sensory receptors for hearing and equilibrium.
Figure 7-12 The ear. Structures in the outer, middle, and inner divisions are shown.
The Outer Ear
The external portion of the ear consists of a visible projecting portion, the pinna, also called the auricle, and the external auditory canal, or meatus, that leads into the deeper parts of the ear. The pinna directs sound waves into the ear, but it is probably of little importance in humans. The external auditory canal extends medially from the pinna for about 2.5 cm or more, depending on which wall of the canal is measured. The skin lining this tube is thin and, in the first part of the canal, contains many wax-producing ceruminous glands. The wax, or cerumen, may become dried and impacted in the canal and must then be removed. The same kinds of disorders that involve the skin elsewhere-atopic dermatitis, boils, and other infections-may also affect the skin of the external auditory canal. The tympanic membrane, or eardrum, is at the end of the external auditory canal. It is a boundary between this canal and the middle ear cavity, and it vibrates freely as sound waves enter the ear.
The Middle Ear and Ossicles
The middle ear cavity is a small, flattened space that contains three small bones, or ossicles (see Fig. 7-12). The three ossicles are joined in such a way that they amplify the sound waves received by the tympanic membrane as they transmit the sounds to the inner ear. The first bone is shaped like a hammer and is called the malleus (Fig. 7-13). The handlelike part of the malleus is attached to the tympanic membrane, whereas the headlike part is connected to the second bone, the incus. The incus is shaped like an anvil, an iron block used in shaping metal, as is used by a blacksmith. The innermost ossicle is shaped somewhat like the stirrup of a saddle and is called the stapes. The base of the stapes is in contact with the inner ear.
The Eustachian Tube The eustachian tube (auditory tube) connects the middle ear cavity with the throat, or pharynx (see Fig. 7-12). This tube opens to allow pressure to equalize on the two sides of the tympanic membrane. A valve that closes the tube can be forced open by swallowing hard, yawning, or blowing with the nose and mouth sealed, as one often does when experiencing pain from pressure changes in an airplane. The mucous membrane of the pharynx is continuous through the eustachian tube into the middle ear cavity. At the posterior of the middle ear cavity is an opening into the mastoid air cells, which are spaces inside the mastoid process of the temporal bone.
Figure 7-13 The ossicles of the middle ear. The handle of the malleus is in contact with the tympanic membrane, and the headlike part with the incus. The base of the stapes is in contact with the inner ear. (x30)
The Inner Ear
The most complicated and important part of the ear is the internal portion, which is described as a labyrinth because it has a complex mazelike construction. It consists of three separate areas containing sensory receptors. The skeleton of the inner ear is called the bony labyrinth (Fig. 7-14). It has three divisions:
* The vestibule consists of two bony chambers that contain some of the receptors for equilibrium.
* The semicircular canals are three projecting bony tubes located toward the posterior. Areas at the bases of the semicircular canals also contain receptors for equilibrium.
* The cochlea is coiled like a snail shell and is located toward the anterior. It contains the receptors for hearing. All three divisions of the bony labyrinth contain a fluid called perilymph. Within the bony labyrinth is an exact replica of this bony shell made of membrane, much like an inner tube within a tire. The tubes and chambers of this membranous labyrinth are filled with a fluid called endolymph (see Fig.7-14). The endolymph is within the membranous labyrinth, and the perilymph surrounds it. These fluids are important to the sensory functions of the inner ear.
Figure 7-14 The inner ear. The vestibule, semicircular canals, and cochlea are made of a bony shell (labyrinth) with an interior membranous labyrinth. Endolymph fills the membranous labyrinth and perilymph is around it in the bony labyrinth.
Hearing The organ of hearing, called the organ of Corti, consists of ciliated receptor cells located inside the membranous cochlea, or cochlear duct (Fig. 7-15). Sound waves enter the external auditory canal and cause vibrations in the tympanic membrane. The ossicles amplify these vibrations and finally transmit them from the stapes to a membrane covering the oval window of the inner ear. As the sound waves move through the fluids in these chambers, they set up vibrations in the cochlear duct. As a result, the tiny, hairlike cilia on the receptor cells begin to move back and forth against the tectorial membrane above them. (The membrane is named from a Latin word that means “roof.”) This motion sets up nerve impulses that travel to the brain in the cochlear nerve, a branch of the eighth cranial nerve (formerly called the auditory or acoustic nerve). Sound waves ultimately leave the ear through another membrane-covered space in the bony labyrinth, the round window.
Figure 7-15 Cochlea and the organ of Corti. The arrows show the direction of sound waves in the cochlea.
Hearing receptors respond to both the pitch (tone) of sound and its intensity (loudness). The various pitches stimulate different regions of the organ of Corti. Receptors detect higher pitched sounds near the base of the cochlea and lower pitched sounds near the top. Loud sounds stimulate more cells and produce more vibrations, sending more nerve impulses to the brain. Exposure to loud noises, such as very loud music, jet plane noise, or industrial noises, can damage the receptors for particular pitches of sound and lead to hearing loss for those tones. The steps in hearing are:
1. Sound waves enter the external auditory canal.
2. The tympanic membrane vibrates.
3. The ossicles transmit vibrations across the middle ear cavity.
4. The stapes transmits the vibrations to the inner ear fluid.
5. Vibrations move cilia on hair cells of the organ of Corti in the cochlear duct.
6. Movement against the tectorial membrane generates nerve impulses.
7. Impulses travel to the brain in the VIIIth cranial nerve.
8. The temporal lobe cortex interprets the impulses.
Equilibrium The other sensory receptors in the inner ear are those related to equilibrium (balance). They are located in the vestibule and the semicircular canals. Receptors for the sense of equilibrium are also ciliated cells. As the head moves, a shift in the position of the cilia within the thick fluid around them generates a nerve impulse. Receptors located in the two small chambers of the vestibule sense the position of the head or the position of the body when moving in a straight line, as in a moving vehicle or when tilting the head. This form of equilibrium is termed static equilibrium. Each receptor is called a macula. (There is also a macula in the eye, but this is a general term that means “spot.”) The fluid above the ciliated cells contains small crystals of calcium carbonate, called otoliths, which add drag to the fluid around the receptor cells and increase the effect of gravity’s pull (Fig. 7-16). Similar devices are found in lower animals, such as fish and crustaceans, that help them in balance. The receptors for dynamic equilibrium function when the body is spinning or moving in different directions. The receptors, called cristae, are located at the bases of the semicircular canals (Fig. 7-17). It’s easy to remember what these receptors do, because the semicircular canals go off in different directions. Nerve fibers from the vestibule and from the semicircular canals form the vestibular nerve, which joins the cochlear nerve to form the vestibulocochlear nerve, the eighth cranial nerve.
Figure 7-17 Action of the receptors (cristae) for dynamic equilibrium. As the body spins or moves in different directions, the cilia bend as the head changes position, generating nerve impulses.
Figure 7-16 Action of the receptors (maculae) for static equilibrium. As the head moves, the thick fluid above the receptor cells, weighted with otoliths, pulls on the cilia of the cells, generating a nerve impulse.