IV. VERTEBRATA. 479 



This memhranous lahyrinth is partially or entirely enclosed in the otic capsule 

 of the skull, which may ossify to the otic or petrosal bones (p. 458). In the 

 birds and mammals the enclosure is such that the structure is duplicated in bone, 

 so that the membranous labyrinth lies in a bony labyrinth, the two being sepa- 

 rated by lymph spaces. These spaces are developed in the cochlea into two 

 tubes, the scala tyinpani and scala vestibuli, the two connecting only at the tip, 

 being separated elsewhere in part by the membranous cochlea (the ductus 

 cocJilearis or scala media). The spaces of the bony labyrinth are filled by two 

 different fluids: inside the membranous labyrinth an endolymph, and between 

 this and the walls of the bony labyrinth a perilymph. 



Accessory structures may be added to this auditory apparatus proper, 

 their purpose being to bring sound waves to it. Such structures are 

 rarely present in tishes (it is not certain that all 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 a 

 sound-conducting apparatus is necessary on account of the differing den- 

 sities of the air and the tissues. So we find from Amphibia onwards an 

 air space closed by a vibrating tympanic membrane, which receives the 

 sound vibrations from tlie air and carries them to a chain of ear bones 

 {ossiciila auditus), which in turn transmits them to the inner ear or laljy- 

 rinth. These structures are not always functional (cetacea), and they 

 may be wholly or in part rudimentary (urodeles, snakes, Amphisbx-nids) . 



To understand this apparatus it must be recalled that the ear lies 

 between the hyoid and mandibular arches in the neighborhood of a canal 

 which leads from tire surface to the pharynx. In many fishes this canal is 

 the spiracle, a reduced gill cleft. In the Anura and amniotes it consists 

 of an air chamber closed externally by the tympanic membrane, stretched 

 on a tympanic annulus, 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 Eusta- 

 chiati tube. The membranous labyrinth touches the wall of the tympanic 

 cavity at one or two places where the bony auditory capsule is interrupted. 

 One of these openings (foramen ovale) is always present and is occupied 

 by the inner end of the ear bones. 



As the mandibular arch lies just in front of the spiracle, and the hyoid close 

 behind it, it is readily understood how parts of these arches can enter the tym- 

 panum 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 ovale, while 

 the other is inserted in the drum membrane, the whole conveying the waves 

 across the tympanum to the labyrinth. In the mammals the structure is differ- 

 ent, since the columella is replaced by two bones, the malleus, which is attached 

 to the drum membrane, and the inens. which articulates with the stapes (fig. 534). 

 Most students believe incus and malleus to be parts (quadrate and articulare) 



