AND PHYSIOLOGY. 395 



for protection to the membrana tympani and the delicate or- 

 gans of the ear, which must be placed within the head to 

 such a distance that no harm could come to them from exter- 

 nal violence. 



722. Function of Membrana Tympani— Use of the "Lit- 

 tle Bones." — The membrana tympani is a very thin and elas- 

 tic membrane, not half an inch in diameter, which is designed 

 to receive all the vibrations of the air falling upon it. On 

 this account it is stretched directly across the auditory canal, 

 and is so* arranged, that by means of muscles it may be ren- 

 dered more or less tense, so that if it is desired to catch a 

 very faint sound, the drum may be made so sensitive as to 

 catch the slightest vibration. The sound received by the 

 drum of the ear is transmitted to the auditory nerve by means 

 of the chain of bones already mentioned, the broad side of the 

 malleus resting directly against the drum, and the broad end 

 of the stapes closing the opening in the semicircular canals 

 which contain the nerve of hearing. The use of this arrange- 

 ment seems to be to catch all the vibrations of the drum, and 

 instead of allowing them to expend themselves upon the air 

 in the tympanum, to transmit them directly to the auditory 

 nerve. The use of the cavity of the tympanum seems to be 

 to allow a free vibration of the membrane of the drum. An- 

 other reason for the large size of the tympanum is to prevent 

 the conveyance of sound to any other part of the ear but the 

 semicircular canals, and that by means of the chain of little 

 bones. And in order to convey the sound only to the nerve 

 of hearing the stapes joins on to the canal, not by a solid ar- 

 ticulation, but simply by cartilage, so that none of the vibra- 

 tions can be conveyed to the walls of the canals, but all to 

 the nerve. 



723. Function of Eustachian Tube.— The design of the 



What does this canal protect? 722. What is the function of the membrana tympani ? 

 Of what use are the " little bones V What service is rendered by the tympanum ? How 

 does the stapes unite with the canal ? 



