ORGANS OP THE SENSES. 285 



to be the principal ossiculum ossified in the tympanum, 

 and the tympanic cavity begins to assume a more 

 lengthened tubular form towards its exterior membrane 

 preparatory to the formation of an external auditory 

 meatus and expanded concha to collect and direct the vibra- 

 tions of the air. In some of the saurian reptiles the mem- 

 brana tympani is still covered, as in many serpents, by the 

 ordinary scales of the head, in most it is quite naked and 

 exposed as it is in the frogs and the salamanders, and in the 

 most elevated forms of the sauria it begins to be protected 

 by external overhanging elastic folds of the skin, forming 

 thus a distinct rudimentary concha, as seen in the crocodiles, 

 the gavials, and the alligators. The internal parts of the ear 

 are nearly in the same condition in the chelonian reptiles, 

 which present a more narrow and lengthened tympanic 

 cavity covered externally by the loose integuments of the 

 head, and communicating with the fauces by a wide semi- 

 cartilaginous and distinct Eustachian tube. The three 

 anchilosed tympanic ossicula still constitute a long stiliform 

 bone with its opercular or stapeal piece applied to the fenes- 

 tra ovalis and attached by its outer rounded malleal ex- 

 tremity to the middle of the membrana tympani as in the 

 other orders of reptiles. The vestibule, enveloped with its 

 sacculus and the semi-circular canals, in the substance of the 

 temporal bone, is provided with solid lapilli supported by 

 the filaments of the acoustic nerves, and with a compara- 

 tively large cochlea, though still undivided by an internal 

 lamina and unconvoluted in its form, so that the cochlea 

 appears to be the last part of the internal ear which acquires 

 its normal and perfect form ; but in the crocodilian reptiles 

 it already presents a slightly curved form, and an internal 

 membrane which supports branches of the acoustic nerves and 

 divides it into a scala tympani communicating with the fe- 

 nestra rotunda and a scala vestibuli continuous with the 

 general cavity of the vestibule and membranous labyrinth. 

 Notwithstanding the gelatinous consistence of the ento- 

 lymph which fills the membranous labyrinth of the cold- 

 blooded as well as the higher vertebrata, resembling the 

 vitreous humour of the eye, no cellular tissue or folds of a 

 hyaloid membrane have been observed to pervade its sub- 

 stance, nor in the more abundant and fluid peri-lymph which 



