503 THE SPECIAL SENSES. 



of hearing. For the air on the two sides of the membrane of the 

 tympanum should be in the same condition of elasticity in order to 

 allow of the proper vibration of the membrane; and this equilibrium 

 would be liable to disturbance if the air within the tympanum were 

 completely confined, while that outside is subjected to variations 

 of barometric pressure. By means of the Eustachian tube, how- 

 ever, a communication is established between the cavity of the 

 tympanum and the exterior, and the free vibration of the membrane 

 is thus secured. 



The exact tension of the membrana tympani itself is also provided 

 for, as we have already observed, by the action of the two muscles 

 inserted into the malleus and the stapes. By the contraction of 

 the internal muscle of the malleus, or tensor tympani, the membrane 

 of the tympanum is drawn inward and rendered more tense than 

 usual. The action of the stapedius muscle is by some thought to 

 relax the membrana tympani, by others to assist in the tension 

 both of this membrane and that of the foramen ovale, to which 

 the stapes is attached. But there is no doubt that both these mus- 

 cles, by their combined or alternate action, can regulate the tension 

 of the tympanic membrane, to an extraordinary degree of nicety, 

 and thus increase the ease and delicacy with which various sounds 

 are distinguished. For if the membrane be so put upon the stretch 

 that its fundamental note shall be the same with that of the sound 

 which is to be heard, it will vibrate more readily in consonance 

 with the undulations of the atmosphere, and the sound will be 

 more distinctly heard. On the contrary, if the membrane be too 

 highly stretched, very grave sounds may not be heard at all, until 

 its tension is diminished to the requisite degree. 



Contrary to what is sometimes asserted, the communication of 

 sonorous impulses to the internal ear is accomplished altogether by 

 means of the tympanum and chain of bones. It has been thought that 

 sounds were transmitted, in many instances, directly to the internal 

 ear by the medium of the cranial bones. This was inferred from 

 such facts as the following. If a tuning-fork, in vibration, be taken 

 between the teeth, its sound will appear very much louder than 

 if it were simply held near the external ear; and if, while it is so 

 held, one of the ears be closed, the sound will appear very much 

 louder on that side than on the other. The sound will also be heard 

 if the tuning-fork be applied to the upper part of the cranium or 

 the mastoid process, with a similar increase of resonance on closing 

 the ears. Finally our own voices are heard, though the ears be 



