ORGANS OF THE SENSES. 283 



the solid cretaceous bodies they contain, and the vestibule and 

 semi-circular canals are the first parts which become imbedded 

 in the substance of the temporal bone. In some fishes, as the 

 lophius, where the skeleton is of a semi-osseous consistence 

 the canals are already partially imbedded in the substance of 

 the cranial parietes, by their passing round a process of the 

 temporal bone. 



The perennibranchiate amphibia being, like the fishes, 

 permanent inhabitants of the water, and receiving their so- 

 norous vibrations through that dense medium, are still 

 destitute of a tympanic cavity, and consequently of a Eus- 

 tachian tube, which are parts of the organ first required and 

 first developed in those species which lose their gills and 

 change their aquatic for an aerial element. In these aquatic 

 species the labyrinth is still imperfectly enclosed in a 

 spacious general cavity of the temporal bone, the vestibule 

 communicates externally by the fenestra ovalis, to which 

 the stapes is applied as in all the higher classes of animals, 

 but is here in form of an operculum as in fishes. There is 

 no tympanic cavity or membrane ; the semi-circular canals 

 and the vestibule with its sacculus and lapilli, are formed 

 like those of osseous fishes, but with the sacculus propor- 

 tionably small, and the muscles and integuments still cover 

 the exterior of the organ without leaving a trace of external 

 meatus ; so that the auditory vibrations from without are 

 here obscurely conveyed, as in fishes, through the solid 

 walls of the cranium. The same simple condition of these 

 acoustic organs is seen in the caecilia, and appears to be 

 possessed by the larvae of all the higher caduci-branchiate 

 amphibia. But in the adult state of the frogs and sala- 

 manders we find the semi-circular canals already imbedded 

 in the substance of the temporal bone, and the rest of the 

 labyrinth still free in the original general cavity of that bone, 

 as we observe in the sturgeon among the cartilaginous fishes. 

 The lapilli here form a soft white pulpy calcareous substance, 

 and a small tympanic cavity filled with air, communicating 

 with the fauces, and confining the small united ossicula, is 

 now found between the labyrinth and the skin of the head. 

 The Eustachiaii tubes leading from the tympanum to the 

 fauces, are generally separated throughout, wide, and short, 

 sometimes they unite to open by a single aperture on the 

 median plain as in several frogs and in the pipa, and in 



