254 A. G. POHLMAN 



passage and the cavity of the middle ear or tympanum. Now, in con- 

 sidering the mechanism of the bird's ear, we may omit from our cal- 

 culations the movements of the tegmentum vasculosum; it is a slack 

 membrane and may be regarded for our present purposes as forming 

 part of the fluid which fills the vestibule. When, then, the outer end 

 of the columella is set into vibration by the drum, its foot-plate will 

 carry the impulses of the drum to the fluid filling the vestibule. As in 

 the mammalian ear, we maj^ distinguish four phases in each vibrational 

 cycle. In phase I the foot-plate, starting from its point of rest, moves 

 or rotates inwards, displacing a minute quantity of the vestibular fluid. 

 The vestibule has firm walls everywhere except that part of its floor 

 formed by the basilar membrane. The membrane yields, displacing 

 the fluid contents of the lower passage and forcing out the round mem- 

 brane. In phase II, the foot-plate returns to its starting point, and the 

 basilar membrane and organ of Corti rise to their equatorial level. 

 In phase III, the outward excursion of the foot-plate continues and the 

 basilar membrane rises so as to become convex upwards; the round 

 membrane is drawn in. In phase IV all these parts return to rest. 



Both Wrightson ('18) and Keith ('18) in their discussion have 

 attempted to make the anatomy and physiology conform to the 

 theory rather than analyze the theory on the basis of structure 

 and function. It would appear from Wrightson's account that 

 the pressure of the fluid in the perilymphatic and endolymphatic 

 spaces is atmospheric and that the opposed action of the M. 

 tensor tympani and M. stapedius preserves this condition. It 

 has already been pointed out that the most recent investigations 

 declare for a synergistic action (Kato, '13) of the muscles named. 

 Keith, in any event, makes no suggestion as to the opponent for 

 the single M. tensor tympani in birds, nor does he even mention 

 the unusual mammals in which only one muscle is found. It 

 must be conceded that the intralabyrinthine pressure is probably 

 controlled by secretion and by blood pressure, and that the con- 

 trol exerted by the muscles of the middle ear is merely transient 

 and secondary. 



This again brings up several points which have been dis- 

 regarded by Keith : The bony labyrinth is not a closed container, 

 but communicates freely to the outside through the ductus 

 endolymphaticus and through blood-vessels, with which the 

 region is richly supplied, with the general venous system. Many 

 bird forms show no direct relation of the membrana tympani 

 secundaria to the tympanic cavity, but to the vena jugularis. 



