Chapter 7 The Sensory Systems 277 The bony labyrinth is filled with a clear fluid called perilymph (PER-i-limf). Inside the bony labyrinth are membrane-covered tubes called the membranous labyrinth, which follow the course of the bony labyrinth. Inside the membranous labyrinth is a thicker fluid called endolymph (EHN-doh-limf). Check Your Understanding 1. What is the purpose of the Eustachian tube? 2. What two types of information are carried by the vestibulocochlear nerve? Functions of the Ear You know that the primary function of your ears is to detect sounds in your environment. But did you know that your ears also help your body to maintain its balance, or equilibrium? This section discusses these two important functions of the ear. Hearing When sound waves enter the ear, they are transmitted through the auditory canal, causing the tympanic membrane to vibrate. This motion stimulates the malleus, incus, and stapes to amplify and transmit these vibrations to the The cochlear nerve and vestibular (vehs- TIB-yoo-lar) nerve transmit sensory information from the cochlea and vestibule, respectively. The cochlear nerve carries information about hearing, and the vestibular nerve carries information about balance. These two nerves join to form a single cranial nerve, the vestibulocochlear (vehs-tib- yoo-loh-KAHK-lee-ar) nerve. The inner ear begins its development with a small area of thickening in the epithelium of the embryo’s head during week 4. This area folds inward and pinches off from the surface to form a fluid-filled vesicle called the otocyst. The otocyst epithelium evolves to form the primitive membranous labyrinth. The labyrinth then divides into the cochlea and semicircular canals. Externally, the auricle and external auditory canal begin to form during the fourth and fifth weeks of pregnancy. They are both fully formed at birth but continue to grow into the size and shape of adult ears through about age nine. Ceruminous glands begin development at about five months gestation. They are anatomically complete at birth but do not fully function until puberty. At what point is the developing baby able to hear? The fetus begins to hear sounds at about four and a half months into the pregnancy. Sensitivity to sound increases and between the sixth and seventh months, fetuses have been observed to respond to voices and other noises. Young children are able to hear an extremely large range of sound frequencies, from low- to high-pitched. As people age, damage due to loud noise progressively accumulates. At advanced age, there are characteristic age-associated deficits in hearing, as described in the next section. Importantly, however, most of these changes in hearing ability may be as much due to repeated exposure to loud noise as to aging. Life Span Review 1. What portions of the ear continue to develop after birth? 2. How early is a developing baby in the womb able to hear? 3. Is age-related hearing loss inevitable? LIFE SPAN DEVELOPMENT: The Ears and Hearing © Body Scientific International Figure 7.13 Structures of the inner ear. What role, if any, do you think the inner ear plays in the development of a phobia such as a fear of heights? Semicircular canals Bony labyrinth (contains perilymph) Membranous labyrinth (contains endolymph) Vestibule Oval window Cochlear nerve Vestibular nerve Cochlea Copyright Goodheart-Willcox Co., Inc.