CHAP. XVIII.] USE OF THE TYMPANUM. 95 



brana tympani is unfavourable generally to the propagation of 

 sounds, especially of those of a low pitch. By rendering the mem- 

 brana tympani tense in one's own person, the correctness of this 

 statement may be readily ascertained. In blowing the nose forcibly 

 air is forced into the tympanum, and its membrane is rendered con- 

 vex and tense, and every one must have experienced the temporary 

 deafness which remains after such an effort. The tympanic mem- 

 brane may also be rendered tense by forced inspiration, the nose 

 and mouth being kept shut. Under these circumstances the tym- 

 panic cavity is exhausted, and the membrane rendered tense by the 

 pressure of the external air. 



In descending the diving bell, the membrane of the tympanum 

 is rendered painfully tense by the increased pressure of the air, and 

 the want of counter pressure from within. This tension may be so 

 great as even to cause rupture of the tympanic membrane in some 

 cases, but it may be obviated by acts of swallowing, during which 

 the external air is driven into the tympana along the Eustachian 

 tubes. During the tense state of the membrane hearing is im- 

 paired ; M. Colladon found that the voices of his companions and 

 of himself were not so distinctly heard. 



Dr. Wollaston performed many experiments upon the effects of 

 tension of the membrana tympani, and he found that deafness to 

 grave notes was always induced, which, as most ordinary sounds 

 are of a low pitch, is tantamount to a general deafness. Shrill 

 sounds, however, are best heard when the tympanic membrane is 

 tense. Miiller remarks, and we have frequently made the same 

 observation, that the dull rumbling sound of carriages passing over 

 a bridge, or of the firing of cannon, or of the beating of drums at a 

 distance, ceases to be heard immediately on the membrana tympani 

 becoming tense; while the treading of horses upon stone pavement, 

 the more shrill creaking of carriages, and the rattling of paper may 

 be distinctly heard. 



The object of the Eustachian tube is chiefly to allow the free in- 

 gress of air into the tympanic cavity in order to provide for the due 

 vibration of the membrana tympani and of the chain of bones. 

 It also, by permitting a free egress of air, renders the tympanum a 

 non-reciprocating cavity, and therefore obviates the production of 

 echoes in it, which would materially interfere with perfect hearing. 

 The importance of the Eustachian tube to the integrity of hearing 

 is well known to all practical men by the deafness which always 

 accompanies chronic or acute disease of the tonsils, or occlusion of 

 the canal of that tube from any other cause. 



