MULTICELLULAR ANIMALS 231 



cavity. The tympanic cavity is connected with the oral cavity by 

 means of the eustachian tube, the connection serving to maintain 

 equal air pressure on each side of the tympanum. From the tym- 

 panum inward across the tympanic cavity is a chain of bones, 

 named respectively the malleus, incus, and stapes. These bones 

 are so set together that they transfer the vibrations of the tympanic 

 membrane with increased intensity but without change in fre- 



FiG. 160. — Scheme of the organ of Corti, the end organ of the human ear sensitive 



to sound. (After Wiedersheim.) 



quency, to a window. Behind this window, the fenestra ovale, is 

 a liquid, the perilymph. This liquid efficiently transfers the vibra- 

 tions to another liquid, called the endolymph, which is enclosed in 

 a membranous spiral structure termed the cochlea. In the mem- 

 branous spiral cochlea is a shelf that bears the distributed ends of 

 the auditory nerve. This shelf with associated structures is called the 

 organ of corti (Fig. 160) ; it is generally regarded as an analyzing 

 organ, so constructed that certain nerve endings are stimulated by 



