OF THE SENSE OF SIGHT. Ill 



ear; by it they are reflected and strengthened, and directed 

 towards the middle ear by the auditory passage ; but in man, 

 probably from the smallness of the external ear, the loss of it 

 does not greatly affect the hearing. The vibrations excited 

 in the external ear pass internally by the walls of the canal, 

 but chiefly through the air which fills the canal, and so reach 

 the middle ear. 



225. The membrane of the tympanum is the chief agent 

 in facilitating the transmission of the sonorous vibrations ol 

 the external air towards the acoustic nerve; this has been 

 proved by many experiments. 



226. The vibrations are transmitted from the membrane 

 of the tympanum to the small bones of the ear, to the walls 

 of the cavity, and to the air it contains, and thus they reach 

 membranes stretched over apertures leading to the internal 

 ear. Now the posterior surface of these membranes is in con- 

 tact with the aqueous liquid filling the internal ear, and in 

 this liquid are suspended the membranous pouches, which, in 

 their turn, are filled with another liquid, into which plunge 

 the terminating filaments of the acoustic nerve. Thus the 

 vibrations reach the nerve itself by which the sensation is 

 communicated to the brain. 



227. The air contained in the cavity of the tympanum 

 seems to play an important part in the phenomena of hear- 

 ing, for it has been observed that when the Eustachian tube 

 has been closed by disease, deafness is almost sure to follow. 

 But the integrity of the drum of the ear is not so essential 

 for the due performance of the function. 



228. However important the chain of small bones of 

 the tympanum may be in moderating the intensity of sounds 

 striking the drum of the ear, or by assisting in its tension so 

 as to enable us to perceive remote or feeble sounds, certain it 

 is that the loss of the hammer, the anvil, and lenticular bone, 

 does not necessarily cause the loss of hearing, which however 

 is said to be sure to happen when the stapes has also been 

 lost. 



229. Admitting that all the parts just described serve 

 to perfect the ear of man, it must also be admitted that they 

 are not all essential to the organ as an instrument of sense. 

 They also gradually disappear as we descend in the scale of 

 beings. Thus, in birds, we have no longer an external ear. 

 In reptiles the external auditory canal also disappears ; the 

 drum of the ear becomes external, and the cavity of the tym- 



