260 A. G. POHLMAN 



The delicate part of the drum is then appUed to the medial wall 

 of the tympanum. In other words, after a limited motion of the 

 drum medially and laterally, all further movements are com- 

 pensated for in the extracolumella itself, and therefore displace- 

 ment of the drum does not give rise to corresponding displace- 

 ment at the columellar foot-plate. It may be that the lever 

 action ascribed to the three ear ossicles has the same purpose, 

 rather than to function as transformers, because, in the bird, a 

 similar arrangement is present and because, in the bird, the 

 course from the tip of the extastapedial to the columellar foot- 

 plate is practically a straight line. 



The displacement which comes about through a minus pres- 

 sure in the external auditory canal is practically equivalent to 

 the action of the M. tensor tympani, while the plus pressure 

 corresponds with its exaggerated relaxation. Therefore, the 

 muscle might function to combat plus pressures in the external 

 auditory canal, while the elastic-ligament system would passively 

 counteract a plus pressure in the middle ear. It must be admit- 

 ted, because of the relatively small tubal orifice and the large 

 air content of the tympanum and its related sinuses, that birds 

 are subject to as great, if not greater, displacements in the drum 

 membrane as a result of barometric variations and altitudinal 

 fluctuations. Relative increases in pressure against the drum 

 would be shunted from the extracolumella through the contrac- 

 tion of the M. tensor tympani, while the reverse would be taken 

 care of by the elastic ligaments. Therefore, because the drum 

 membrane in birds normally tends to displace medially and 

 press the columellar system against the perilymphatic space, a 

 single muscle may operate. While in the mammal, according 

 to Wales and Mangold, there is normally a tendency for the 

 drum to be displaced outward which is controlled by the JNI. 

 tensor tympani, while a medial displacement of the drum, carry- 

 ing with it the ossicular chain, would press the stapes into the 

 fenestra vestibuU and would be counteracted by the M. sta- 

 pedius. It would appear, therefore, that the bird affords an 

 excellent material for proving or disproving the importance of 

 the columellar movements, the contraction of the M. tensor 



