REPTILIA, 



581 



or internal ear through the intervention of an ossicle that represents 

 the stapes of Mammalia. 



The drum of the ear is situated immediately beneath the skin, 

 the parts composing 



the external ear of Fig. 263. 



quadrupeds being as 

 yet entirely deficient. 

 The membrana tym- 

 pani, that now for 

 the first time makes 

 its appearance in the 

 series of animals, is 

 tensely stretched across 

 the tympanic aperture; 

 being covered exter- 

 nally by the integu- 

 ment of the head. In 

 the Turtle (Jig. 263) 

 the tympanic mem- 

 brane is represented by a cartilaginous plate (a). The ossicle, or 

 columnella as it is here called, is single and trumpet-shaped : it 

 passes quite across the tympanic cavity (b), its external extremity 

 being inserted into the drum ; while at its opposite end it expands 

 into a disc (c), which closes an aperture (foramen ovale) that com- 

 municates with the membranous vestibule of the internal ear. It 

 is obvious therefore that every tremor impressed upon the mem- 

 Irana tympani will be conveyed by the columnella to the fora- 

 men ovale, and thus communicated to the fluid contained in the 

 labyrinth, upon which, as in Fishes, the auditory nerve is dis- 

 tributed. 



The cavity of the tympanum communicates with the interior 

 of the mouth by a wide opening, that represents the Eustachian 

 tube ; a circumstance evidently intended to prevent air or fluid 

 from being pent up in the tympanic chamber, and thus interfering 

 with the free vibration of the drum. 



In Serpents, on account of the peculiar disposition of the 

 pieces of the temporal bone before described (J 606), there is no 

 tympanic cavity, and the columnella (Jig. 249, v) is absolutely im- 

 bedded in the flesh ; the arrangement, however, in other respects is 

 the same as in the generality of Reptiles. 



The lower tribes of Amphibia, as we might be led to expect 



