258 A. G. POHLMAN 



ossicular chain on the perilymph during the minus pressure 

 phase (air absorption) in that it would hold the weight of the 

 displacing incus from the stapes by transferring some of its 

 thrust to the M. stapedius. The writer feels that, while Wales 

 may have speculated on the changes in the pressure of the 

 middle ear, the deductions in his article are both clever and 

 suggestive. 



The apparent double displacement factor in the mammahan 

 middle ear makes difficult an actual observation of what takes 

 place, under relatively normal conditions, although the cat seems 

 to afford a material in which this may be done. The displace- 

 ment of the drum membrane in the bird does not offer this 

 objection, because the active muscular contraction operates in 

 only one direction. The assumed plus pressure phase in the 

 middle-ear region may therefore be constructed by creating a 

 minus pressure in the external auditory canal, while the minus 

 pressure phase may be imitated by increasing the pressure in the 

 external canal. In either instance the middle ear may be thor- 

 oughly exposed without disturbing the drum or columellar 

 apparatus, although the push-and-pull element on the colu- 

 mellar foot-plate and the membrana tympani secundaria is 

 naturally diminished. This factor of error may be entirely 

 eliminated by a simple change in technique in constructing a 

 transparent roof for the tympanum and creating the pressure 

 directly through the tuba auditiva. However, the conditions 

 are fairly well reproduced in a gross dissection and are easily 

 studied. 



EVIDENCES OF ADJUSTMENTS TO DRUM MEMBRANE 

 DISPLACEMENTS 



By placing a carefully dissected specimen of a bird's middle-ear 

 under binocular magnifier and gently aspirating the air from the 

 external auditory canal, the following changes may be observed: 

 The lateral displacement of the drum membrane is accompanied 

 by a tilting outward and downward of the extracolumella on the 

 suprastapedial hinge, supported in part by the infrastapedial, 

 which is held from backward displacement by the tension on the 



