MIDDLE-EAR REGION OF GALLUS 249 



inward and outward movement. It is perfectly obvious that 

 the motions in the columellar foot-plate must be accompanied 

 by corresponding mass displacement of the perilymph and that 

 the plus or minus pressure developed would be in direct propor- 

 tion to the elasticity of the tissues which lunit that space. Breuer 

 has called attention to the possible function of the M. tensor 

 tympani in maintaining a pliability of the annular ligament, but 

 from the elastic nature of its structure it may be safely assumed 

 that exercise in no way contributes to this property. Whether 

 the foot-plate motion is a direct tilting, after the manner of the 

 basis of the stapes, or whether it is a direct in-and-out displace- 

 ment is not germane at this point. The position of the elastic 

 fibers are indicated in figures 3 and 4. 



The fenestra cochleae in birds is relatively larger in propor- 

 tion to the size of the perilymphatic space and of the fenestra 

 vestibul than in mammals. It is placed on the posterior aspect 

 of the columellar recess in the chicken, and extends almost from 

 the margin of the fenestra vestibuli to the drum attachment in 

 baby chicks, although in older birds the growth of bone sepa- 

 rates it more and more from the drum membrane. The mem- 

 brane which closes in this opening, the membrana tympani 

 secundaria, is composed of long, interlacing elastic fibers, which 

 attach for the most part directly to the lip of the fenestra. This 

 membrane is undoubtedly an adaptation for compensative dis- 

 placements in the perilymphatic fluid, due to the direct applica- 

 tion of a movable skeletal element to this space. This same 

 mechanical requirement would hold for the operculum in the 

 amphibians; the columellar foot-plate in the birds, and the 

 basis of the stapes in the mammals, and would imply that some- 

 what similar functional requirements might obtain in these 

 widely separated forms. It is interesting to note that the mem- 

 brana tympani secundaria in the amphibians seems to be com- 

 posed of elastic tissue, although the membrane occupies a dehis- 

 cence at the base of the otic capsule (as H. S. Harrison ('03) 

 has shown) and is applied to a lymph sinus in this situation. 

 The membrana tympani secundaria in the amphibians therefore 

 has no direct relation to the tympanic cavity. Similarly, the 



