the Mode in which it administers to the Perception of Sound. 373 



The tympanum of birds is provided with a single muscle only, 

 the effect of which, when contracted, is to relax the membrane, 

 i. e. to draw it inwards (Breschet, pp. 24<, 30). Hence the po- 

 sition of equilibrium of the auditory apparatus of birds (t. e. the 

 position which it assumes when not acted upon by any sound) 

 may be defined to be that in which it is placed when the muscle 

 or muscular fibres connected with the organ have produced their 

 utmost effect, by drawing in the membrana tympani to the full 

 extent which the fibrous band above mentioned will admit of; in 

 which position, of course, the membrane will be incapable of 

 being forced further inwards through the action of condensed 

 waves. 



If a rarefied wave be incident upon the organ when in this 

 position, the tendency would be of course to move the tympanal 

 membrane outwards ; and the membrane being convex outwards, 

 in order that such motion outwards may occur one of two things 

 must happen — namely, either the membrane must be elastic, or 

 else it must, when in the position of equilibrium, be somewhat 

 loosely stretched upon the cartilaginous spring of which we have 

 spoken. 



I have nowhere found any statement as to the elasticity or 

 inelasticity of the tympanal membrane of birds ; but for the sake 

 of perspicuity I shall assume, as seems most probable, that, like 

 the tympanal membrane in mammalia, it is inelastic, and conse- 

 quently that in the position of equilibrium the membrane rests 

 loosely on the cartilaginous spring which supports it. 



When the general apparatus is in equilibrium, we may suppose 

 that the cartilaginous spring which forms part of it will also be 

 in equilibrium. But when through the action of a rarefied wave 

 the membrane has been moved outwards, the elasticity of the 

 spring will immediately come into play, and will tend to bring 

 the membrane back to its original position — a contrast being pre- 

 sented in this respect in the apparatus in birds and in mammalia ; 

 for whereas in the latter case, when the membrane has been 

 moved outwards, the muscles of the tympanum are the essential 

 and only means of bringing back the organ to its original status, 

 there are in the former case two different and efficient agents for 

 producing the same result — to wit, the elasticity of the cartila- 

 ginous spring and the tympanal muscle. It may be observed, 

 however, that although the elasticity of the spring would in the 

 first instance tend to bring back the membrane in the manner 

 above described, there can be no doubt that, when the membrane 

 had reached the position in which its further motion inwards 

 would be stopped by the fibrous band above spoken of, it would 

 receive a sudden and complete check ; and this occurring at a 

 time when its velocity was a maximum, the membrane would 



