NATURAL THEOLOGY. 49 



both of which changes may augment or facilitate 

 the still deeper action of the auditory nerves,'^ 



The benefit of the eustachian tube to the organ 

 may be made out upon pneumatic principles. Be- 

 hind the drum of the ear is a second cavity, or 

 barrel, called the tympanum. The eustachian 

 tube is a slender pipe, but sufficient for the pas- 

 sage of air, leading from this cavity into the back 

 part of the mouth. Now, it would not have done 

 to have had a vacuum in this cavity; for, in that 

 case, the pressure of the atmosphere from without 

 would have burst the membrane which covered it. 

 Nor would it have done to have filled the cavity 

 with lymph, or any other secretion ; which would 

 necessarily have obstructed, both the vibration of 

 the membrane and the play of the small bones. 

 Nor, lastly, would it have done to have occupied 

 the space with confined air, because the expansion 

 of that air by heat, or its contraction by cold, would 

 have distended or relaxed the covering membrane, 

 in a degree inconsistent with the purpose which it 

 was assigned to execute. The only remaining ex- 

 pedient, and that for which the eustachian tube 

 serves, is to open to this cavity a communication 



^^ It will be shown in the Appendix, that the fine apparatus con- 

 sisting of these bones, with their four minute muscles attached to 

 them, is not necessary to the sensation coming through the bones 

 of the head, as here described by our author: it is provided for the 

 more delicate vibrations of the erastic atmosphere, and is not foimd 

 except in animals that breathe the air. It will be also found, that 

 whilst these bones move with the slightest impulse of sound, they 

 regulate the impression, and protect the nerve. 



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