68 ANATOMY OF A CHELONIAN. 



m. On the right portion of the divided head, the Ear 

 is to be dissected either now or after completing the 

 dissections described in paragraphs 179-192, inclusive. 

 For this purpose — 



a. Remove the skin over the tympanum (3, /) ; 

 note the white, opaque, fibrous ring surround- 

 ing the aperture, and the circular cartilaginous 

 plate which nearly fills it up, and is united to 

 its margin by a delicate membrane. 

 6. Cut away the uniting membrane, and turn 

 the cartilage (which is the outer expansion of 

 the columella, 26, ^) to one side ; the rod-like 

 portion of the columella will be seen running 

 in from it. Cut off the outer end of the col- 

 umella, and examine again the outer division 

 of the tympanum (27, a) now lined by mucous 

 membrane ; the columella traverses it and 

 disappears on its inner side through a narrow 

 passage. Beneath and behind the columella 

 is a slit bounded by a pair of white prominent 

 lips ; this is the tympanic opening of the £us- 

 tachian tube ; pass a guarded bristle along it 

 to its oral end, which will be found to open 

 on the side of the buccal cavity, just behind 

 the prominence caused by the powerful mus- 

 cles which run from the temporal fossa to the 

 mandible. 

 c. Carefully break away the floor of the tym- 

 panum, and trace the columella through its 

 fibrous canal to the inner dilatation of the tym- 

 panum (26, d), where it ends in a circular car- 

 tilaginous plate, smaller than the outer one, 

 fixed in the foramen ovale. 



