36 ORGAN IZATIONOFREPTILES. 



than do those of the eye; the Chelonian and Saurian Reptiles, in some respects, 

 approaching the Birds, Avhile in the Sirens we perceive a similarity of structure to 

 the Fishes. We find no external ear destined to collect and concentrate sound 

 and direct it to an internal organ; even the membrane of the tympanum is not 

 apparent in all the species; it is concealed in the Chelonia, Serpents, Engystoma, 

 &c. by the organs of integumation passing over it, apparently unchanged in their 

 nature. In Frogs and Hyte it is large, smooth, and very apparent; less so in the 

 Toads, and in the Salamander wanting altogether. The cavity of the tympanum 

 is the space between the membrane of the same name, and the labyrinth or internal 

 ear, varying in size and arrangement, and containing the small bones of the organ. 

 This cavity communicates in all the tribes, where it exists with the fauces, by a 

 canal called the Eustachian tube. This tube and canal form a sort of primitive 

 organ of hearing, for in the lower classes it seems to transmit sound to the internal 

 ear;* but as we approach the higher orders, where the membrane of the tympanum 

 becomes more perfect, and the external ear more developed, this canal decreases 

 in size and seems designed for the transmission of air only, to the cavity of the 

 tympanum. In the Chelonian Reptiles, the cavity of the tympanum contains a 

 single bone called columella, having a long stalk and an oval flat portion, attached 

 by the small end to the tympanal membrane, and joined by its oval extremity to 

 the labyrinth; this is admirably well arranged to transmit impressions, made on 

 the drum of the ear without, to the labyrinth within, where the auditory nerve is 

 spent. Some Anatomistsf have considered the columella composed of three 

 portions, an external cartilaginous, a middle bony, and an internal one, again 

 cartilaginous in structure. In the Sauria we find nearly the same arrangement, 

 the cavity of the tympanum being only a little more capacious. In Serpents, this 

 cavity is extremely small, and the bone of the ear instead of going to a tympanal 

 membrane, is attached by its outer extremity to the parts about the articulation 

 of the lower jaw. In Frogs, Hylas, &c. this cavity is large, and contains two bones 



* Scarpa. Disq. de audit, et olfac, p. 27. 

 tBlainville, Princip. d'Aiiat. Comp., torn. i. p. 541. 



