The external acoustic meatus is an air-filled tubular space that follows a slightly S-shaped course for about 25 mm to the tympanic membrane (eardrum). The wall of the canal
FIGURE 25.1 • Three divisions of the ear. The three divisions of the ear are represented by different colors and consist of the external ear (auricle and external acoustic meatus; pink tone), the middle ear (tympanic cavity, auditory ossicles, tympanic membrane, and auditory tube; green), and the internal ear containing the bony labyrinth (semicircular canals, vestibule, and cochlea; blue) and the membranous labyrinth (not visible).
is continuous externally with the auricle. The wall of the lateral one third of the canal is cartilaginous and is continuous with the elastic cartilage of the auricle. The medial two thirds of the canal is contained within the temporal bone.
The lateral part of the canal is lined by skin that contains hair follicles, sebaceous glands, and ceruminous glands but no eccrine sweat glands. The coiled tubular ceruminous glands closely resemble the apocrine glands found in the axillary  region. Their secretion mixes with that of the sebaceous glands and desquamated cells to form cerumen, or earwax. The cerumen lubricates the skin and coats the meatal hairs to impede the entry of foreign particles into the ear. Excessive accumulation of cerumen can plug the meatus,  however, resulting in conductive hearing loss. The medial part of the canal located within the temporal bone has thinner skin and fewer hairs and glands.
The middle ear is an air-filled space that contains three small bones, the ossicles.
The middle ear is located in an air-filled space, called the tympanic cavity, within the temporal bone (Fig. 25.3). It is spanned by three small bones, the auditory ossicles, which are connected by two movable joints. The middle ear also contains the auditory tube (Eustachian tube) as well as the muscles that attach to the ossicles. The middle ear is bounded anteriorly by the auditory tube and posteriorly by the spongy bone of the mastoid process, which contains the mastoid antrum and other, smaller air-filled spaces called mastoid cells. Laterally, the middle ear is bounded by the tympanic membrane and medially by the bony wall of the internal ear.
The middle ear works as a full-fledged mechanical energy transformer. Its primary function is to convert sound waves (air vibrations) arriving from the external acoustic meatus into mechanical vibrations that are transmitted to the internal ear. Two openings in the medial wall of the middle ear, the oval (vestibular) window and the round (cochlear)  window, are essential components in this conversion process.
The tympanic membrane separates the external acoustic meatus from the middle ear.
The tympanic membrane is in the shape of an irregular cone, the apex of which is located at the umbo that corresponds to the tip of the manubrium of the malleus. During otoscopic examination of the normal ear, tympanic membrane is in a light gray color and a cone of light (light reflex) should reflect off its surface (Fig. 25.4). The malleus is one of three small auditory ossicles residing in the middle ear and the only one that attaches to the tympanic membrane (see Fig. 25.1). Tympanic membrane forms the medial boundary of the external acoustic meatus and the lateral wall of the middle ear (Fig. 25.5). The layers of the tympanic membrane from outside to inside are  The skin of the external acoustic meatus  A core of radially and circularly arranged collagen fibers  The mucous membrane of the middle ear
Sound waves cause the tympanic membrane to vibrate, and these vibrations are transmitted through the ossicular chain of three small bones that link the external ear to the internal ear.
petrous part of temporal bone