XIII.] THE FROG. 239 



a. The tympanum of the frog is a funnel-shaped 

 cavity with its wider end turned outwards. Its 

 sides are bounded by a smooth slightly pig- 

 mented mucous membrane, continuous with 

 that of the mouth through the Eustachian 

 recess. 



@. In its roof lies a rod, ossified in the middle, 

 cartilaginous at each end, which is the columella 

 auris. The columella is attached, by its inner 

 end, to the upper and anterior part of the 

 inner wall of the tympanum and, by its outer 

 end, to the middle layer of the tympanic 

 membrane, in the region of the opaque patch 

 mentioned above (1. 6). 



7. Close to the inner attachment of the columella 

 there is a comparatively large oval opening in 

 the wall of the tympanic cavity; this is the 

 outer end of the Eustachian recess, the inner 

 end of which has been already (B. 11. a) seen 

 on the posterior part of the roof of the mouth. 

 Pass a probe through the opening now exposed, 

 and open the frog's mouth to see its passage 

 into that cavity. 



2. The internal ear. 



a. Carefully dissect away the columella auris from 

 its inner attachment. An aperture into which it 

 was inserted will thus be exposed : this is the 

 fenestra ovalis. 



b. Take a pair of scissors and cut through the bones 

 of the side of the skull in a line joining the 

 fenestra ovalis and the "guard" of the para- 



