10 Mr. Home's Lecture 



municates freely with the cochlea and semicircular canals ; but 

 these cavities are filled with a watery liquor, and have no com- 

 munication (as the tympanum has) with the external air. 



This fact was ascertained in the horse, by the following ex- 

 periment, repeated several times. The organ of hearing was 

 separated from the skull immediately after death, and the ca- 

 vity of the tympanum exposed ; the parts were then immersed 

 in water, and the stapes removed ; by which means, the mem- 

 brane of the foramen ovale was destroyed, but no globule of 

 air was seen to escape through the water.* 



The following uses have generally been assigned to the parts 

 now mentioned. 



The membrana tympani was supposed to be adapted to re- 

 ceive impressions, by the combined action of the tensor and 

 laxator muscles varying the degree of its tension, so as to bring 

 it in unison with different sounds : these impressions were con- 

 ducted, by the chain of bones, to the vestibulum, cochlea, and 

 semicircular canals ; in which cavities, particularly the cochlea, 

 they were supposed to undergo some modification, before they 

 were impressed upon the nerves spread upon the linings of 

 these cavities. 



The function of modifying impressions of sound was assign- 

 ed to the cochlea, partly from the delicacy of its internal struc- 

 ture, supposed to resemble a musical instrument, and partly 

 from there being no other part of the organ apparently suited 

 for repeating the variety of delicate sounds which pass into the 

 ear : the changes that couid be produced upon the membrana 



* This experiment was made by Mr, Clift, who superintends Mr. Hunter's col- 

 lection, and who has afforded me material assistance in the different parts of this 

 investigation. 



