. A manual of zoology. Zoology. :>4i CIIOlWArA. Accessor}' structures muy be added to this auditory ajoparatus proper, their ])urpose being to bring sound waves to it. Such structures are but occasionally present in fishes (it is not certain that they hear), since the sound waves are easily carried by the water to the tissues and tlienee directly to the ears. On the other hand, with the change to terrestrial life such a sound-conducting apparatus is necessary on account of the differing densities of the air and the tissues. So we find from Amphibia onwards a vibrat- ing membrane—the tympani
. A manual of zoology. Zoology. :>4i CIIOlWArA. Accessor}' structures muy be added to this auditory ajoparatus proper, their ])urpose being to bring sound waves to it. Such structures are but occasionally present in fishes (it is not certain that they hear), since the sound waves are easily carried by the water to the tissues and tlienee directly to the ears. On the other hand, with the change to terrestrial life such a sound-conducting apparatus is necessary on account of the differing densities of the air and the tissues. So we find from Amphibia onwards a vibrat- ing membrane—the tympanic membrane—which receives the sound vibrations from the air and carries them to a chain of ear bones (ossicula auditus), which in turn transmits them to the inner ear or laljyrhith. These structures are not always functional (cetacea), and they may be wholly or in part rudimentary (urodeles, snakes, Amphisbffiuids). To understand this apparatus it must l)e recalled that the ear lies between the hyoidand mandibular arches in the neighborhood of a canal which leads from the surface to the phar3Tix. In the fishes this canal is the spiracle, a reduced gill cleft. In the Anura and amniotes it consists of an air chamber closed exter- nally by the tympanic membrane, stretched on a tympanic an- nulus, while the opening to the pharynx is retained. The part next the membrane becomes expanded into the tympanic cavity, this with the membrane forming the tympanum or drum. The part connecting with the pharynx is usually narrowed and is called the Eustachian tube. The membranous labyrinth lies in the wall of the tympanic cavit}'and touches it at one or two jDoints where the bony auditory capsule is interrujited, the always present fenestra ovalis, and the fenestra rotunda, lacking in Amphibia. Wlien it is recalled that the mandibular arch lies just in front of the spiracle, and the hyoid close behind it, it is readily under- stood how parts of these arches can enter the tympanum and produ
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