54:4 CHORD AT A. 



Accessory structures may be added to this auditory apparatus 

 proper, their purpose 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 thence 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 labyrinth. These structures are not always functional (cetacea), 

 and they may be wholly or in part rudimentary (urodeles, snakes, 

 Amphisbaenids). 



To understand this apparatus it must be recalled that the ear 

 lies between the hyoidand mandibular arches in the neighborhood 

 of a canal which leads from the surface to the pharynx. 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 cavity and touches it at one or two points 

 where the bony auditory capsule is interrupted, the always present 

 fenestra ovalis, and the fenestra rotunda, lacking in Amphibia. 



When 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 produce the 

 ear bones. In Anura, reptiles, and birds 

 a columella has one end attached to 

 the stapedial plate, which lies in the 

 fenestra ovalis, while the other is in- 

 serted in the drum membrane, the 

 whole conveying the waves across the 

 tympanum to the labyrinth. In the 

 'Fio. 577. Ear bones of man. mammals the structure is different, since 



in? ; ^SSffiSSf; 1; the columella is replaced by two bones, 

 stapes> the malleus, which is attached to the 



<drum membrane, and the incus, which articulates with the 



