252 A. G. POHLMAN 



ward, backward, and downward tilting of the entire extra- 

 columella and is limited, as far as the downward movement is 

 concerned, by the infrastapedial and the direction of the force 

 exerted by the M. tensor tympani. The outward and downward 

 motion is controlled through the attachment of the columellar 

 squamosal ligament, plus the indirect pull of the M. tensor 

 tympani on the extrastapedial, and the thickness of the adjacent 

 drum quadrant. 



The fourth hinge member is formed at the point of attach- 

 ment of the extrastapedial to the common cartilage stalk, just 

 lateral to the confluence of the supra- and infrastapedial proc- 

 esses. This process is directed laterally and is responsible for 

 the drum prominence, because it acts like a strut which is sup- 

 ported by the supra- and infrastapedial and the columellar 

 squamosal ligament. The cartilage bends downward and back- 

 ward as it touches the drum, and may continue in the direction 

 of the attached tendinous fibers of the M. tensor tympani as far 

 as the drum margin. The pull of the muscle is directed mainly 

 on the extrastapedial, and when stability is attained in the plane 

 of the three members — supra- and infrastapedial and columellar 

 squamosal ligament — further displacement may occur by a tilt- 

 ing of the extrastapedial at its basal attachment. This motion 

 is limited, in part, by the form of the extrastapedial and by the 

 middle drum-tubal ligament. The movement is pronounced 

 in the chick, but poorly developed in the turkey, where the 

 extrastapedial is relatively heavy and the middle drum-tubal 

 ligament well developed. 



FUNCTIONAL RELATION OF COLUA'IELLAR APPARATUS AND 

 M. TENSOR T^TVIPANI TO THE INNER EAR 



The entire elastic-ligament system is directly related to the 

 movements in these four areas, and must therefore be set in 

 opposition to the forces which bring about these movements. 

 The mechanism is one which allows for marked alterations in 

 drum position, and therefore in the sound-transmitting appa- 

 ratus, without materially influencing the pressure within the 

 perilymphatic space. It would appear these bending areas are 



