SENSE OF HEARING. 419 



or less obstructed. The loss of the stapes is much more 

 serious, and, according to Bonnafont, always causes deafness. 

 This fact is easily explained: the stapes is attached at its 

 base to the oval fenestra, closing it entirely, and is so firmly 

 fastened to it that it cannot be removed without tearing the 

 membrane of the oval fenestra, and thus allowing the endo- 

 lymph or liquid in the internal ear to flow out ; it is not, 

 therefore, properly speaking, the loss of the bone which 

 occasions deafness, but rather the escape of the fluid through 

 the aperture made by the ablation (Bonnafont, op. cit., p. 

 264). 



Two organs are attached to the middle chamber of the 

 ear : these are, behind, the mastoid cells, which are irregular 

 cavities, or sinuses hollowed out in the mastoid process of 

 the temporal bone ; and, in front, the Eustachian tube, con- 

 necting the drum of the tympanum with the nasal part of 

 the pharynx. 



The mastoid cells, being full of air, are generally considered 

 as a resonant organ ; this supposition has, however, no other 

 ground than the fact that the air in the drum vibrates, and, 

 consequently, its vibrations are intensified by those of the 

 air in the mastoid cells. "We have just seen that the vibra- 

 tions of the air in the drum are of no importance whatever; 

 neither have diseases of the mastoid cells furnished any 

 indication as to the purpose of these cavities. We incline to 

 the opinion that the mastoid cavities are empty spaces, hav- 

 ing no special function, and intended only to increase the 

 size of the cavity of the tympanum. We shall presently see 

 that in the normal state the tympanum is closed on all 

 sides : now, as the tympanum is an extremely small cavity, 

 any sudden change in the tension of this thin layer of air 

 which covers the inner surface of the membrana tympani 

 would, no doubt, have an injurious effect upon this mem- 

 brane ; this effect would be mitigated by the addition of 

 another cavity, adding its capacity to that of the tympanum 

 properly so called ; thus those animals which are exposed to 

 sudden and considerable changes of atmospheric pressure, as 

 birds which fly very high, have their mastoid cells more fully 

 developed than is the case with other animals, these cells 

 even communicating with other supernumerary bony cavities. 



The Eustachian tube, which is situated in front of the 

 middle chamber of the ear, that is, facing the mastoid cells, 

 is a long tube extending from the tympanic chamber to the 

 pharynx, and thus forming a means of communication be- 



