AUDITORY OEGAXS OF VERTEBRATA. 537 



of Corti) are spread out. As tlie scalge arise as spaces in the tissue 

 wMcli accompanies the ductus cochlearis, they are similar to the 

 cavities between the membranous semicircular canals and their 

 bony wall, or between the membranous and osseous vestibules, and 

 are filled with perilymph. 



In the Amphibia, and all higher forms, spaces appear in that part 

 of the walls of the bony labyrinth which lies on the outer surface of 

 the cranium; these in a different fashion effect a communication 

 between it and other arrangements that are connected with the 

 auditory organ. The fenestra ovalis, which is always closed by a 

 plate-like piece of bone, is a hole of this kind in the osseous vesti- 

 bule. A second opening, which appears first in Reptiles, and which is 

 correlated with the further development of the cochlea (fenestra 

 rotunda), lies in the wall of the scala tympani, and is closed by a 

 membrane. 



Both these arrangements are related to the development of 

 external conducting organs. 



Eetzius, G., Anatom. Untersuch. I. Stockholm, 1872.— Hasse, C, Anatomische 

 Studieu. Leipzig, 1870-73. 



§ 402. 



Other pai'ts are gradually added on, as accessory organs, to 

 the auditory organ, although primitively having no relation to 

 it. The first branchial cleft, which in the Selachii and Ganoidei 

 persists as the " spiracle," enters into close relation with the 

 wall of the labyi-iuth in the Amphibia. As it grows over this 

 wall it is converted into a cavity, the wider portion of which forms 

 the tympanic cavity; this is bounded in the middle line by 

 the wall of the labyrinth ; the portion which leads into the pi-imi- 

 tive buccal cavity forms the Eustachian tube. It reminds us 

 of its primitive (spiracular) condition by at first communicating 

 freely with the exterior. The cleft is, however, soon closed, 

 which leads to different arrangements. In the Coeciliae and Urodela 

 the cleft is closed by the superjacent muscles, so that there is no 

 tympanic cavity. One division of the Anura (PelobatidEe) presents 

 the same arrangement, for in it there are only slight indications of 

 the outgrowth of the mucous membrane of the pharynx into this cleft. 

 In most Anura, however, that membrane does form an outgrowth, 

 and leads into a tympanic cavity, which is closed externally by a 

 tympanic membrane. Among the Reptilia, the Ophidii, and 

 Amphisbaenoidea have no tympanic cavity ; in Chamasleo there is 

 no tympanic membrane ; but both these parts are present in all 

 other Reptiles, and in Birds. 



In the Orocodilini and Aves, the inner openings of the Eustachian 

 tubes are united into a common canal, as is the case also in Pipa 

 among the Amphibia. 



Those parts of the visceral skeleton which are connected with the 



