230 A. G. POHLMAN 



requirements of this region are about the same in all forms. 

 Here again we find inadequate explanation of the significance of 

 the opercular muscle and the functional interpretation of a 

 movable skeletal element applied to the perilymphatic space of 

 the anurans. 



The bibliography on the function of the middle-ear region in 

 birds is not surprisingly large. Krause's ('01) monograph on 

 the form of the columella and the relation of its form to acute- 

 ness of hearing is not productive of definite results or convic- 

 tions. He concludes that, while the form of the sound-trans- 

 mitting apparatus has something to do with the acuteness of 

 hearing, much must be referred to the development of the end 

 organs and to the central nervous system. His excellent figures 

 demonstrate that the columella in birds is not subject to wide 

 variations in form. It is impossible in the limited scope of this 

 paper to go into any great amount of comparative detail, and 

 the discussion will be limited practically to the conditions as 

 found in the chicken. A general description of the relations of 

 the structures in the middle-ear region of birds is, however, 

 essential because only one article presents anything like a com- 

 prehensive account and none is adequately illustrated. 



The best general description of the middle-ear region in birds 

 is to be found in the account of Breuer ('08), whose abstracts 

 and deductions read about as follows: 



The drum membrane of birds presents a marked inversion in its 

 relations to that found in mammals. The drum membrane in mam- 

 mals is drawn inward toward the cavum tympani, while in birds, it 

 is pushed outward. The manubrium mallei extends from the upper 

 border of the drum toward its mid-area (in man, from above ventrally) ; 

 in birds, the corresponding cartilage apparatus extends downward and 

 backward from the central area of the drum toward its periphery. In 

 mammals, the uppermost portion of the drum is flaccid; in birds, the 

 relaxed portion is below. If one imagine a human ear drum turned 

 through 180° on an axis represented by a diagonal line drawn across 

 its surface from the postero-superior to the antero-inferior margin the 

 resulting relations would represent roughly the position of the struc- 

 tures found in birds. 



The drum membrane in Ijirds, including the entire sound-transmit- 

 ting apparatus, has but a single muscle attached to it and correspond- 

 ing with the above mentioned inverted relations, the M. tensor tympani 



