396 



HITCHCOCK'S ANATOMY 



Eustachian tube is evidently to allow of equal atmospheric 

 pressure upon both sides of the membrana tympani. For 

 were the air in the tympanum closely confined, the atmosphere, 

 varying as it does in density, would sometimes create such a 

 pressure on the drum that all vibrations would be indistinctly 

 conveyed to the brain. This is the same arrangement that is 

 found in bass and tenor drums used for musical instruments, 

 a small orifice always being necessary to give free access to 

 the air. Hence, when we have a cold, or especially a sore 

 throat, where the mucous membrane is inflamed, we often find 

 hearing difficult, because the lining membrane of the Eus- 

 tachian tube is so much swollen that air can not pass through. 

 Also when the same condition prevails we can, by swallowing 

 the small amount of saliva in the mouth with considerable 

 force, drive so much air into the tympanum that a sense of 

 fullness occurs in the ear, which continues until the air has 

 found some means of escape. 



724. Functions of the Internal Ear— The uses of all the 

 parts belonging to the internal ear are not perfectly under- 

 stood. Probably, however, the labyrinth and cochlea serve 

 to give as great expansion as possible to the nerve in the 

 small space provided for it. And the use of the fluid filling 

 the labyrinth is to secure all the vibrations coming in through 

 the bones of hearing, and give them greater intensity. Some 

 experiments worked out by Muller, a German physiologist, 

 throw considerable light on the subject. He found that 

 sounds passing directly into water from the air lose consider- 

 able of their intensity ; but if a tense membrane is placed be- 

 tween the two, greater intensity is produced. The intensity 

 is also greatly increased when to this membrane is attached 

 a small solid body which communicates with the water alone. 

 This seems to show why the drum receives all the sound 



723. What is the use of the Eustachian tube ? In what instruments of music is there 

 it similar arrangement? Why does partial deafness sometimes accompany a cold ? 724. 

 What is said of the functions of the internal ear ? Kelate the experiments made by 

 Muller. 



