MIDDLE-EAR REGION OF GALLUS 259 



inferior drum-tubal ligament; the tilting action of the extra- 

 columella is transformed into a plunger action of the columella 

 at the columellar hinge, accompanied by a twisting of the infra- 

 stapedial on its long axis; the columellar-squamosal ligament 

 becomes tense; the columellar foot-plate moves laterally at the 

 fenestra vestibuli and the membrana tympani secundaria 

 becomes concave to allow for displacement of the perilymph. 

 If the aspiration be increased, the supra- and infrastapedials 

 and columelar squamosal ligament apparently create a stability 

 in the extracolumella, so that further lateral displacement of the 

 entire apparatus is eliminated, and the extrastapedial is drawn 

 downward and backward in the direction of the tendinous fibers 

 of the M. tensor tympani, until the middle drum-tubal liga- 

 ment is under marked tension. At this time the extrastapedial 

 is supported by the erectile auditory pad and the thin portion of 

 the drum membrane applied to the anterior wall of the external 

 auditory canal. This entire result implies that the amount of 

 movement in the columellar foot-plate does not correspond to 

 the amount of movement in the lateral displacement of the 

 drum membrane, partly because of the hinge system in the 

 columellar apparatus and partly because the extracolumella 

 lies obliquely behind the midarea of the drum. Release of suc- 

 tion returns the parts to their former position. 



Reversing the conditions by increasing the pressure in the 

 external auditory canal, the drum moves medially, carrying 

 with it the entire extracolumella on the suprastapedial hinge; 

 the columella is displaced medially at the fenestra vestibuli and 

 the membrana tympani secundaria becomes correspondingly 

 convex. The medial displacement of the extracolumella is, 

 however, limited by the close relation of the suprastapedial to 

 the medial bony wall of the tympanum, and by the position of 

 the intrastapedial and the columellar-squamosal ligament, so 

 that further medial displacement of the drum occurs at the 

 extrastapedial hinge. The extrastapedial buckles forward and 

 inward, so that it may come to lie parallel to the columellar- 

 squamosal ligament and, in the living bird, this is probably not 

 so marked unless the M. tensor tympani relaxes to the utmost. 



