MIDDLE-EAR REGION OF GALLUS 251 



is SO characteristic in the mammal. The amount of medial and 

 lateral excursion of the columellar foot-plate is lunited by a 

 number of factors: 1) The shortness of the stout elastic fibers of 

 the ligamentum annulare which is quite evenly developed about 

 the circumference of the foot-plate, with the possible exception 

 of the extension of fibers toward the drum margin; 2) by a com- 

 pensating tension of the membrana tympani secundaria which 

 allows a mass displacement of inner-ear fluid as an adjustment 

 to movements of the foot-plate; 3) by limitations in a medial 

 and lateral excursion because of the position of the columella 

 in its attachment to the extracolumella and the restrictions in 

 the tilting of the latter. Breuer's experiments on the living 

 pigeon substantiate, what appears obvious from dissections, that 

 contractions of the M. tensor tympani result in displacement of 

 the extracolumella and a slight lateral excursion of the columellar 

 foot plate. The result of this movement will be discussed later. 



The second point of motion is in what may be termed the 

 columellar hinge, located at the point of union of the lateral 

 tip of the columella with the extracolumella, immediately medial 

 to the point where the infrastapedial process is given off. Here 

 the form of the columella changes from a rounded to a flattened 

 spicule — the flattening being in the axis of the infrastapedial 

 process. This hinge represents the spot where the tilting action 

 of the extracolumella is translated to a plunger action of the 

 columella proper and is accompanied by certain resisting forces ; 

 the twisting of the infrastapedial and the tension on the elastic 

 columellar-squamosal ligament. It will be recalled that the 

 infrastapedial has attached at its tip, or inferior end, not only 

 the tendinous fibers of the M. tensor tympani, but the inferior 

 drum-tubal ligament as well. The twist in the infrastapedial is 

 one of the factors which tends to return the columella to a posi- 

 tion of rest on muscular relaxation, as noted by Breschet. 



The third point of motion occurs when the extracolumella, as 

 a whole, swings upon a hinge formed by the attachment of the 

 suprastapedial to the drum margin. This hinge is reinforced 

 bj^ the tip of the infrastapedial and perhaps also by a long drum 

 process of the extrastapedial. The movement consists in an out- 



