284 ORGANS OF THE SENSES. 



some a canal is found prolonged outwards from the fenestra 

 rotunda. The thin and naked skin forms in these air- 

 breathing amphibia a large membrana tympani on a level 

 with the general surface of the head to the centre of which 

 membrane the bent cartilaginous malleus is attached, and 

 from the soft condition of the tympanic ossicula the sonorous 

 vibrations communicated from without to the large expanded 

 membrana tympani are obscurely conveyed to the fenestra 

 ovalis to which the broad base of the stapes is applied. 



In the air-breathing reptiles the organs of hearing present 

 a higher condition of development than in the semi-aquatic 

 tribes of amphibious animals, especially in the parts which 

 more immediately relate to the rare medium through which 

 their sonorous impressions are received. The tympanic por- 

 tion of the ear is still however so imperfectly developed in 

 the serpents that we find it covered externally not only 

 by the skin and muscles, as in fishes and branchiated 

 amphibia, but also by the hard scales of the head. The three 

 tympanic ossicula are united together into a single piece as 

 in the frogs, which is still cartilaginous at its distal or malleal 

 extremity where it is attached to the inner surface of the 

 membrana tympani. The semi-circular canals, with their 

 ento- and peri-lymph as in fishes and amphibia, are imbedded 

 in the dense petrous portions of their solid temporal bones. 

 The vestibule is still lodged in a capacious cavity of the tem- 

 poral bone, and its sacculus contains a large and solid creta- 

 ceous body, so that the solid lapilli of serpents more resemble 

 those of osseous fishes, while the soft and pulpy cretaceous 

 substances of the labyrinth of amphibia are allied to those of 

 cartilaginous fishes, and they are softer in the chelonian than 

 in the saurian reptiles. The cavity of the tympanum is ge- 

 nerally much larger in the saurian than in the ophidian 

 reptiles, and is covered externally by a thin, projected, trans- 

 parent, naked, membranous continuation of the skin, placed 

 on a level with the general surface of the head, so as yet to 

 present no external auditory meatus. This cavity communi- 

 cates freely with the fauces by a separate Eustachian tube 

 and by means of the fenestra ovalis with the labyrinth, which 

 still presents only a rudimentary undivided cochlea closed 

 externally by the membrane of the fenestra rotunda, and a 

 solid calcareous lapillus as in the serpents. The stapes 

 with its base applied to the fenestra ovalis, continues 



