468 Dynamic Theory. 



Coming up a stop into the class of reptiles we find the second sao 

 answering to our scala media present in all. But in some, as the 

 Turtles, it is quite rudimentary while it is not coiled in any of them ; 

 although it may be slightly bent or twisted. There is also a fenestra 

 rotunda which implies a scala tympani accompanying the scala media, 

 the whole constituting a cochlea, except in the feature of the spiral twist, 

 the circumstance on which the name is based. There is still no auditory 

 canal or ear flap. The crocodile, however, has a pair of fleshy lips over 

 his ear drum which are opened and closed at will. A few of the lower 



FIG. 208. Ear of a Crocodile. 



h Flap of Skin which can be closed down to the opposite fold, i t and shut the ear. 



<?. Tympanic membrane or ear drum-head. 



A;. A gristle representing the Malleus. 



/.The columella or ear bone. 



/.Fenestra ovalis (or vestibuli ) with foot of the columella resting upon it. 



m. Tympanic cavity or drum. 



c. Anterior perpendicular Semicircular Canal. 



d. Posterior 



e. External or horizontal 



v. Cochlear division of Auditory Nerve. 



a. Apex of cochlea slightly bent. 



6. Double cartilaginous partition dividing the cochlea into two compartments, except 

 at the apex where they unite. 



n.A. valve partly closing the opening into the pharynx of the Eustachian tubes for 

 both ears. Three branches depart from this place p and p' to the left and right respect- 

 ively, and a middle vertical one o, which sends off a branch q from behind in the median 



s s'. Branches of canals from o to a junction with p and p at t. [line. 



r r'. Branches of g. (After Owen.) 



and retrograde families of the reptiles are destitute of the tympanum 

 (Ophidia and Amphisboenes ). Others, as Turtles, Chameleons and the 

 lizzard, Sphenodon, have tympanic cavities covered by the skin. In liz- 

 zards the tympanic cavities communicate by wide openings with the 

 pharynx. In Crocodiles and Turtles these are narrowed down to 

 eustachian tubes. The bone of the ear drum, extending from the fenestra 

 ovalis to the drum head, or tympanic membrane when there is one, is 

 simply a Columella ( See fig. 208 ). When there is no tympanic mem- 

 brane its outer end is buried in the muscles covering the ear. 



Birds, being modified forms of reptiles, they possess ears substan- 

 tially reptilian. There is, however, usually a short auditory passage 

 outside of the tympanic membrane. The bone of the middle ear is 

 usually only the columella a single bone. There is no ear flap, but 

 the ear is surrounded by a circle of feathery of peculiar formation. 



