MIDDLE-EAR REGION OF GALLUS 261 



tympani, and the displacement of the elastic ligaments against a 

 variable topography in the middle-ear, which is due to drum 

 displacements as a result of fluctuations in air pressure. This 

 part of the problem clearly falls in the province of the physiologist. 



CONCLUSIONS 



The minor mechanical errors suggested by Breuer may prove 

 to be the major factors calling for an adjustment, and in any 

 event, the bird's middle-ear, with its relatively stable surround- 

 ings, its simple columellar apparatus, its single muscle, its elastic 

 ligaments, and the possibility of shearing off its external audi- 

 tory canal, is remarkably well adapted to varying conditions of 

 air pressure. It would appear that the M. tensor tympani pri- 

 marily compensates for variable drum positions in the columellar 

 apparatus, rather than adapting the sound-transmitting mechan- 

 ism to greater acuteness in hearing. The resulting drum tension 

 would, therefore, be an incident to the contraction of the muscle, 

 and in comparing conditions in bird and mammal, the greater 

 the drum tension in the former, the more convex the drum; the 

 greater the drum tension in the latter, the more concave the 

 drum. 



The prime function of the entire mechanism in birds would 

 be represented by a function of the middle-ear, and compensa- 

 tory to drum displacement, due to air absorption, on the one 

 hand, and to fluctuations in barometric pressures on the other. 

 The bendings in the columellar apparatus, while they are quite 

 similar to the behavior of the ossicles in the mammal, probably 

 do not have any effect upon the character of the sound wave. 

 The sound wave, impinging upon the drum, would be focused, 

 as it were, upon the stapedial or columellar foot-plate and beget 

 vibrations of a molecular rather than of a molar character in the 

 perilymph. The tension of the drum, like the shifting of the 

 columellar foot-plate, is probably only a result of the variable 

 topography of the drum in its relation to the inner ear and have 

 little to do with acuteness in hearing. 



