MIDDLE-EAR REGION OF GALLUS 253 



compensatory to drum movements and are probably not pri- 

 marily associated with acuteness in hearing. The variable 

 factor which allows the single voluntary M. tensor tympani to 

 remain in tonus against the constant pull of elastic ligaments is 

 logically, therefore, a function associated with the middle ear. 

 This factor is necessarily a variable one and must involve a 

 change in the topography of the middle-ear region which requires 

 adjustment. 



This attempt to look upon the M. tensor tympani and the 

 columellar system as adjustment factors to a variable middle-ear 

 topography calls for a short account of the adjustment neces- 

 sarily involving inner-ear mechanism. It is not my purpose to 

 consider the inner-ear function except as it pertains to middle- 

 ear mechanics. 



Keith dismisses the mechanical factors in the middle-ear 

 region of birds in a few words. His work deals mainly with the 

 problem of the inner-ear adjustments to columellar movements, 

 and his evidence was obtained largely from the sparrow. 



In the case of the bird, a single bone — the columella — connects the 

 drum with the oval window. While the outer end, of the columella is 

 fixed to the drum, its inner end extends into a foot-plate which is fixed 

 into the margin of the oval window by a ligamentous membrane. 

 The lower and hinder borders of the foot-plate are more tightly fixed 

 in the window than the upper and anterior margin, not unHke the 

 manner in which the stapes is attached in the fenestra ovalis of the 

 mammalian ear. The movements of the columella are more like those 

 of a lever than of a piston; it is hinged to the lower margin of the oval 

 window. Internal to the foot-plate is the cavity of the vestibule, 

 filled with fluid. The horizontal partition is drawn across the floor of 

 the vestibule, stretching from the lower margin of the oval window 

 which is occupied by the foot-plate to the opposite or inner wall of the 

 vestibule (p. 222). 



It will be noted that the middle passage (scala media) of the cochlea 

 is separated from the cavity of the vestil^ule by a thick folded mem- 

 brane containing many blood vessels — the tegmentum vasculosum. It 

 represents a combination of the Reissner's membrane and the vascular 

 body (stria vascularis) of the mammalian ear. 



The horizontal partition just described forms the floor of the vesti- 

 bule and the roof of the lower or tympanic passage of the cochlea— 

 at least the terminal part of that passage. The round window lies 

 immediately below the oval window. It is qlosed by a strong but loose 

 membrane which is placed between the lower end of the tympanic 



