VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 571 



num has, with the external air. This fact was ascertained in the horse, by the 

 following experiment, repeated several times. The organ of hearing was separated 

 from the skull immediately after death, and the cavity of the tympanum exposed; 

 the parts were then immersed in water, and the stapes removed; by which means, 

 the membrane 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 receive 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 conducted, 

 by the chain of bones, to the vestibulum, cochlea, and semicircular canals; in 

 which cavities, particularly the cochlea, they were supposed to undergo some modi- 

 fication, before they were impressed on the nerves spread on the linings of these 

 cavities. The function of modifying impressions of sound was assigned to the 

 cochlea, partly from the delicacy of its internal structure, supposed to resemble a 

 musical instrument, and partly from there being no other part of the organ ap- 

 parently suited for repeating the variety of delicate sounds which pass into the 

 ear: the changes that could be produced on the membrana tympani by the muscles 

 of the malleus, being considered as incapable of answering that purpose. 



This slight sketch of the organ of hearing, and of the uses, as they are gene- 

 rally understood, of the different parts, will enable me to point out, with more clear- 

 ness, what parts of the theory appear defective, and what improvements may be 

 made on it. It is true that the membrana tympani is stretched and relaxed by the 

 action of the muscles of the malleus, but not for the purpose alleged in the com- 

 monly received theory. It is stretched, in order to bring the radiated muscle of 

 the membrane itself into a state capable of acting, and of giving those different 

 degrees of tension to the membrane which empower it to correspond with the 

 variety of external tremors: when the membrane is relaxed, the radiated muscle 

 cannot act with any effect, and external tremors make less accurate impressions. 

 The membrana tympani, with its tensor and radiated muscles, may be not unaptly 

 compared to a monochord, of which the membrana tympani is the string; the 

 tensor muscle the screw, giving the necessary tension to make the string perform 

 its proper scale of vibrations; and the radiated muscle acting on the membrane 

 like the moveable bridge of the monochord, adjusting it to the vibrations required 

 to be produced. The combined effects of the action of these muscles give the per- 

 ceptions of grave and acute tones; and, in proportion as their original conformation 

 is more or less perfect, so will their actions be, and consequently the perceptions 

 of sound which they communicate. 



This mode of subdividing the motions of the membrana tympani between 2 sets 

 of muscles, allotting a portion to each, is not peculiar to this part. A remarkable 



* This experiment was made by Mr. Clift, who superintends Mr. Hunter's collection, and who has 

 afforded me material assistance in the different parts of this investigation. — Orig. 



4D2 



