ORGAN OF HEARING IN REPTILES. 



349 



230 



'"IP 



IA^ 



Organ of hearing, Crocodile. 



divided into two compartments by a double cartilaginous septum, 

 ib. b, except at the apex, where they communicate ; whilst, at the 

 base, one compartment, f scala vestibuli,' communicates with the 

 vestibule, the other, e scala tympani,' 

 by a small orifice (foramen cochlea?, 

 sen rotundum), with the tympanic 

 cavity, in the dry skull, but closed 

 by membrane in the living animal. 

 The cochlear division of the acoustic 

 nerve is shown at v, fig. 230. The 

 semicircular canals are small com- 

 pared with those of fishes : c is the 

 anterior perpendicular, d the pos- 

 terior perpendicular, and e the ex- 

 ternal or horizontal, canal which 

 curves over the ( foramen ovale,' f. 

 The membrana tympani, ^7, is lodged 

 at the bottom of a deep fissure, 

 and is protected by an opercular flap of the integument, 

 h, fitting to a smaller fold below, i, and accurately closing the 

 passage. This is the sole approach to an external ear known 

 in existing Reptilia. The ear-drum is inclined downward and 

 outward, adapted to the reception of sound from above, and also 

 to the position of the overhanging flap. The gristly represen- 

 tation of the malleus, k, is well-marked, and the ear-drum is 

 thickened at its place of attachment : the columellar part of the 

 stapes, /, extends obliquely downward to the foramen ovale, sen 

 vestibuli, f. The tympanic cavity, m, is singularly extended 

 by air-cells, not only developed in the mastoid, but in the 

 basi-, par-, and super-occipitals, fig. 94, 3, in the alisphenoid and 

 parietal, ib. 7, bones. The communications between the tympanic 

 cavity and fauces are more complex than in other animals, 1 

 although the eustachian faucial opening, n, is single, median, and 

 common to both ears. It is situated a short way behind the pos- 

 terior nostril ; and from it is continued a median, o, and two lateral, 

 p, canals. The median canal rises and enters a bony canal between 

 the basioccipital and basisphenoid, which bifurcates, one branch, q, 

 inclining forward into the basisphenoid, the other, r, continued ver- 

 tically into the basioccipital, both in the medial plane. Each of 

 these branches again bifurcates, transversely, one to the right, 

 the other to the left, opening upon the floor of the tympanic cavity. 



1 CI.XXII. 



