584 VERTEBRATE LIFE AND ORGANIZATION 



of the tympanic membrane, but the force of the movement is two or 

 three times as great. The increased pressure provides for keener hearing 

 because the sound waves must be converted to waves in the Hquid of 

 the inner ear, antl liquid is much less compressible. The fact that the 

 tympanic membrane has nearly ten times the surface area of the mem- 

 brane in the oval window also increases the pressure of waves in the 

 endolymph. Virtually all of the force that impinges on the tympanic 

 membrane reaches the membrane in the oval window, and, since this 

 membrane is smaller, the force per square millimeter is increased. 



The middle ear cavity, in which the ossicles lie, evolved from the 

 first gill slit, homologous to the spiracle of many fishes. It connects with the 

 jjharynx via the Eustachian tube and hence indirectly with the outside 

 of the body. The pharyngeal opening of the Eustachian tube is nor- 

 mally closed, but if pressures become unequal on the two sides of the 

 tympanic membrane, swallowing is stimulated by a reflex, the Eu- 

 stachian tube opens and the pressures are equalized. 



A long, cochlear duct has evolved from the lagena of fishes, and it 

 contains the actual receptive structure, the organ of Corti (Fig. 29.6 B 

 and C). The cochlear duct is filled with endolymph and is a part of 

 the membranous labyrinth. Vibrations reach the cochlear duct via 

 specialized perilymphatic channels. A scala vestibuli begins at the oval 

 window, extends along the cochlear duct, curves around its apex, and 

 returns as the scala tympani to a fenestra rotunda, or round window, 

 that is separated by a delicate membrane from the middle ear cavity. 

 The round window permits the escape of the vibrations of the peri- 

 lymph induced by the vibrations of the ossicles against the oval win- 

 dow. Since liquids are incompressible the liquids in the inner ear could 

 not vibrate unless there were some mechanism similar to this. The 

 scala vestibuli and scala tympani have a different origin than the coch- 

 lear duct, but all three are in intimate association and collectively 

 form the spiral-shaped cochlea. 



Vibrations or pressure waves induced by the stapes at the oval 

 window pass through the scala vestibuli, cross the cochlear duct, travel 

 back through the scala tympani, and escape at the round window. The 

 basilar membrane, which supports the organ of Corti, is set in vibra- 

 tion and rubs the hair cells of this organ against an overlying tectorial 

 membrane. Sensory neurons of the acoustic nerve extend from the 

 hair cells to the brain. Cochlear mechanisms are very complex, and 

 just how the basilar membrane is activated is uncertain. It is well estab- 

 lished that tones of different frequency are detected in different regions 

 of the cochlea— low notes near the apex and high notes near the base. 

 Presumably a loud sound of a certam frequency is distinguished from 

 a soft sound of the same frequency because the loud sound sets up 

 stronger vibrations that stimulate the hair cells more vigorously, and 

 they initiate more nerve impulses. 



In an organ as elaborate as the ear, many things can go wrong. 

 Infections may enter the middle ear via the Eustachian tube and affect 

 the auditory ossicles. The stapes may become locked in the oval window 

 by an abnormal growth of bone, or the individual ossicles may fuse 



