372 Mr. 11. Moon on the Structure of the Human Ear, and on 



The apparatus among mammalia is essentially the same in 

 character as in man. That of birds differs (so far as regards our 

 present purpose) in two features : — first, that the bones are in 

 part replaced by cartilage, and, as regards their mutual colloca- 

 tion, are somewhat differently arranged; secondly, that the 

 tympanal membrane is convex outwards, and not concave out- 

 wards as in the case of mammalia. 



The apparatus in birds may be described as consisting of the 

 labyrinth and of a single true bone (which from the correspond- 

 ence of its functions with those of the stapes in mammals may 

 be designated as a stapedal bone), connected with the upper part 

 of which and with the sides of the tympanal cavity is a cartila- 

 ginous appendage to which the tympanal membrane is attached, 

 and by which the membrane is supported in its convex (out- 

 wards) position as upon a bent spring. 



A reference to the principles unfolded in my former paper will 

 make it evident that the membrana tympani being convex out- 

 wards, its want of elasticity (even if it were inelastic) would 

 oppose no obstacle to the transmission to the sensorium of the 

 action of condensed waves ; so that, so far as this part of the ap- 

 paratus is concerned (whatever may be the case in man and in 

 mammals), birds might have perception of sound through the 

 agency of waves of condensation — an instrument of conveyance 

 which, as has been stated, is slower, and therefore less efficient 

 than is offered by waves of rarefaction. 



Any such effect as that just described is obviated by means of 

 a fibrous band stretching from the neighbourhood of the Eusta- 

 chian tube, and attached at its other extremity to the cartilagi- 

 nous appendage before spoken of; which band, for the purpose 

 we are now considering, may be regarded as replacing the laxa- 

 tores tympani in man and in mammalia. Respecting this band, 

 M. Breschet informs us that "Lorsqu'on la tiraille, on opere 

 la tension de la membrane du tympan"* ; that is, the effect of 

 the band, if it were attached to a muscle (which it is not), would 

 be, when the muscle was contracted, to draw the tympanal mem- 

 brane outwards; and its effect in the (actual) absence of any 

 muscle attached to it must be to resist any tendency to force the 

 membrane inwards ; that is, its effect is to counteract the only 

 effect capable of being exerted upon the membrane by a con- 

 densed wave. 



III. Having shown the manner in which the auditory appa- 

 ratus in birds is adapted to suppress the action upon it of con- 

 densed waves, it may be proper to point out the mode in which 

 rarefied waves operate upon it. 



* Recherches Anatomiques et Physiques sur VOrgane de V Audition chez 

 les Oiseaux (Paris, 1836), p. 24. 



