256 A. G. POHLMAN 



press the columella into the fenestra vestibuli and thereby to 

 beget increased perilymphatic pressure. 



COMPARISON OF THE EAR REGION IN BIRDS AND MAMMALS 



The region of the external and middle ear in birds differs in 

 certain particulars from the corresponding regions in mammals. 

 The external auditory canal in the bird may be closed to afford 

 a direct protection against violence and pressure from without, 

 whereas in mammals this is not possible. The drum membrane 

 in birds, as indicated in the dead or anesthetized bird, appears 

 to be displaced medially on relaxation of the M. tensor tympani, 

 while in mammals relaxation of the muscle of the same name is 

 accompanied by lateral displacement of the drum, carrying with 

 it the malleus, probably on a rotation of the incus, the nature of 

 which has not been clearly defined. The tympanic cavity and 

 the large air-sinus system associated with it and the tuba audi- 

 tiva, afford a relatively greater surface for air absorption in the 

 bird than in the mammal. The two middle-ear cavities stand 

 in open communication in the bird, while in the mammal they 

 are distinct. The tubae auditivae in birds open directly, by a 

 small common canal, into the posterior oral cavity, while in 

 mammals the separate orifices lie above the level of the soft 

 palate in relation to the posterior nasopharynx. In mammals 

 the opening and closing of the tubal orifices is controlled by 

 voluntary muscles associated with the muscles of the soft palate, 

 while the shearing off of the common tubal recess is probably 

 brought about by approximation of .the bordering palatal folds. 

 The M. tensor tympani and its closely associated M. tensor veli 

 palati of the mammal are wanting in the bird, although the posi- 

 tion of these muscles is occupied by the drum-tubal and colu- 

 mellar-squamosal ligaments. The M. tensor tympani of the bird 

 corresponds to the M. stapedius, both in action and in nerve 

 supply, except that the M. stapedius acts directly on the stapes, 

 while the M. tensor tympani acts indirectly on the columella 

 through a tilting of the extracolumella. Finally, in spite of the 

 quadratosquamosal articulation and its close relation to the 

 ventral drum margin and the tympanic cavity, the bird's middle 



