TRANSMISSION OF SOUND IN THE EAR 497 



Politzer attached threads to the malleus and incus and recorded their move- 

 ments on a revolving drum. In this way he was able to show by direct experi- 

 ment what is supported also by theoretical considerations, namely, that sound 

 is transmitted from the tympanic membrane to the labyrinth by molar move- 

 ments of the auditory ossicles and not by molecular movements. 



The round window is closed by a thin membrane bathed on the inside by 

 the perilymph. The perilymph being incompressible, this membrane in all 

 likelihood constitutes an arrangement by which the movement of the stapes 

 inward can be compensated by an equal movement outward. The endolymph 

 has a similar protective device in the ductus endolymphaticus, which is con- 

 nected on the one hand with the utricle and saccule and by these with the 

 scala media of the cochlea, and on the other passes through the petrous bone 

 and terminates on its posterior surface in a little vesicle underneath the dura 

 mater. 



The round window might also serve for the purpose of conveying vibrations 

 to the perilymph, and this in fact has been observed when the oval window 

 was rigidly closed. 



By means of a capillary manometer introduced into the superior semi- 

 circular canal, Bezold was able to determine the extent of the movements 

 described by the conducting apparatus of the human ear with the tympanic 

 cavity open. The maximum movement of the manubrium caused by variations 

 of atmospheric pressure in the external auditory canal was about 0.76 mm. 

 from one extreme to the other, one-third of this being the movement inward 

 and two-thirds the movement outward. 



As Bezold remarks, this difference between the movement inward and out- 

 ward is difficult to harmonize with an exact transmission of sound waves, and 

 probably would not occur under normal circumstances. As a matter of fact 

 we have in the internal muscles of the ear a device which in life might correct 

 this lack of coordination observed after death. 



These are the tensor tympani and the stapedius muscles, the former inner- 

 vated in the main by the trigeminal nerve, the latter by the facial. The 

 tensor tympani draws the manubrium inward and thereby presses the stapes 

 farther into the labyrinth. It serves thus to keep the chain of ossicles " keyed 

 up." Section of its tendon permits a moderate magnification of the movement 

 of the whole chain and the increase is almost exclusively in the outward 

 movement (Bezold). 



Experiments on dogs have shown that the tensor tympani contracts reflexly 

 to acoustic stimuli (Hensen, Hammerschlag et al.) acting through subcortical 

 centers not higher than the posterior corpora quadrigemina (Ostmann). 



Hensen looks upon the tensor tympani as an apparatus for accommodating 

 the ear in listening to faint sounds, and cites as evidence the fact that a weak 

 sound becomes stronger for the moment when strong motor impulses are sent 

 out, say to the muscles of the face or limbs. The explanation would be that 

 impulses are at the same time sent to the tensor tympani muscle. 



Ostmann would ascribe this function to the stapedius. 



3. The Tympanic Cavity and Eustachian Tube. In order that the middle 

 ear may fulfill its purpose of transmitting the vibrations of the atmosphere 



