238 A. G. POHLMAN 



rial against the intrusion of vermin, and because of the shearing 

 mechanism and the erectile pad, the bird undoubtedly possesses 

 a more adequate protection against violence from without than 

 does the mammal. 



While the fibrous elements of the ventral wall of the external 

 auditory canal swing around the prominence indicating the posi- 

 tion of the tympanic process of the quadratum, to terminate in 

 close relation to the drum attachment, this is not the case at the 

 dorsal drum margin, which is separated from the fibrous plate 

 by a bony lip which contributes to a rudimentary external 

 osseous canal. This lip is perforated by a foramen which trans- 

 mits the tendon of the M. tensor tympani from the external 

 surface of the skull to the dorsal-inferior drum margin. The 

 muscle has been correctly described by Killian, figured correctly 

 by Breschet, and is clearly innervated through the N. facialis as 

 the experiments of Breuer substantiate. The relation of the 

 tendon of this muscle to the drum membrane and the function 

 of the M. tensor tympani will be considered later. The pull of 

 the muscle, judging from its attachments and structure, is prob- 

 ably limited and feeble. 



The drum is attached to the margins of the bones surrounding 

 the external canal, but in the case of the quadratum this does 

 not appear to be the case, although Beyer has noted it exphcitly. 

 The attachment of the drum, at this region, is to a stout fibrous 

 ligament which rides over the rounded surface of the tympanic 

 process of the quadratum near its articulation with the squa- 

 mosum, and at this portion of the drum circumference, a diver- 

 ticulum of the middle ear appears to afford a sort of cushion 

 w^hich insulates the drum membrane from the small movements 

 in the quadratum. 



ELASTIC ELEMENTS OF THE DRUM MEMBRAXE 



The drum membrane has been well described by Breuer and 

 by Beyer. It consists of a delicate oval membrane attached to 

 the bones limiting the tympanic cavity, with the exception of 

 the quadratum anteriorly. The movements of the tympanic 



