Structure and Physiology

The Ear

The ear can be divided into the following four parts:

1. External ear

2. Middle ear

3. Inner ear

4. Nervous innervation

A cross section through the ear is illustrated in Figure 11-1.

The external ear consists of the pinna and the external auditory canal. The pinna is composed of elastic cartilage and skin. Figure 11-2 illustrates the parts of the pinna.

EXTERNAL EAR

MIDDLE INNER EAR EAR -il-

Temporal bone Mastoid air cells

Malleus Incus

Malleus Incus

Semicircular canals

Acousto-vestibular nerve

Eustachian tube

External auditory canal

Semicircular canals

Acousto-vestibular nerve

Eustachian tube

Stapes Tympanic membrane

External auditory canal

Figure 11-1 Cross-sectional view through the ear.

Figure 11-1 Cross-sectional view through the ear.

Figure 11-2 Landmarks of the pinna.

Figure 11-3 Earwax on the base of hairs. Figure 11-4 Earwax in the external ear canal.

The external auditory canal is about 1 inch in length. Its outer third is cartilaginous, and its inner two thirds are composed of bone. Within the cartilaginous portion, there are hair follicles, pilosebaceous glands, and ceruminous, or wax-producing, glands. Secretions of these ceru-minous glands, debris, and desquamated keratin are "earwax." The glands secrete their product around the base of the hairs, as depicted in Figure 11-3. Figure 11-4 shows earwax in the external ear canal. Earwax color and consistency depend on the type of cerumen secreted, the amount of keratin present, and the presence of debris. The soft, brown form shown here is the most common type. The cartilaginous portion of the ear is continuous with the pinna. The canal curves slightly, being directed forward and downward. The innervation to most of the external canal is through the trigeminal, or fifth cranial, nerve. The innermost portion of the canal is innervated by the vagus, or tenth cranial, nerve.*

The middle ear, or tympanic cavity, consists of connections to the mastoid antrum and to its connecting air cells and, through the eustachian tube, to the nasopharynx. The function of the eustachian tube is to provide an air passage from the nasopharynx to the ear to equalize pressure on both sides of the tympanic membrane. The eustachian tube is normally closed but opens during swallowing and yawning.

The tympanic membrane forms the lateral boundary of the middle ear. The medial boundary is formed by the cochlea. The tympanic membrane is gray, with blood vessels at its periphery. It is composed of two parts: the pars flaccida and the pars tensa. The pars flaccida is the upper, smaller portion of the tympanic membrane. The pars tensa comprises the remainder of the membrane. The handle of the malleus is a prominent landmark and divides the pars tensa into the anterior and posterior folds. The tympanic membrane is set slightly at an angle to the external canal. The inferior portion is more medial than is the superior portion. Figure 11-5 illustrates the left tympanic membrane.

Sound is conducted from the tympanic membrane to the inner ear by three auditory ossicles: the malleus, the incus, and the stapes. The malleus is the largest ossicle. At its upper end is the short process, which appears as a tiny knob. The handle (long process) of the malleus, or manubrium, extends downward to its tip, called the umbo. The short process and the handle of the malleus attach directly to the tympanic membrane. At the other end of the malleus is its head, which articulates with the incus. The incus then articulates with the head of the stapes, the footplate of which attaches to the oval window of the inner ear.

The middle ear also contains two muscles: the tensor tympani and the stapedius. The tensor tympani muscle attaches to the malleus, and the stapedius muscle attaches to the neck of the stapes. The tensor tympani muscle is innervated by the trigeminal nerve, and the stapedius muscle is innervated by the facial, or seventh cranial, nerve. Both muscles contract in response to high-intensity sound.

*On occasion, when the distal external canal is cleaned, coughing may result. This cough reflex is mediated through the vagus nerve.

Short proces" of the malleu

Pars accida

Semicircular canal

Short proces" of the malleu

Anterior fold

Light reflex

Handle of the malleus

Figure 11-6 Cross-sectional view through the cochlea.

Pars accida

Semicircular canal

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