CHAP. XVIII.] HEARING. 89 



form, varying from what appears to be little more than a mere car- 

 tilaginous lamella with a few irregularities upon its surface, enjoy- 

 ing scarcely any motion, to an elongated funnel-shaped ear- trumpet, 

 very moveable, and completely under the control of numerous large 

 muscles. Man and the quadrumana are at one extremity of this 

 scale ; the solipeds, the ruminants, and the bats at the other. 



That the auricle performs the office of an acoustic instrument 

 to collect and reinforce the sounds which fall upon it, cannot be 

 doubted in those cases in which it is large and fully developed, as 

 in the horse, ass, &c. These animals employ it as we might expect 

 such an instrument would be used; the open part is directed 

 towards the quarter whence the sound comes, and continues so 

 directed as long as the animal appears to listen. 



Savart's experiments illustrate the manner in which an instru- 

 ment like the external ear may contribute to the propagation of 

 sound to the internal ear. When a thin membrane is stretched in 

 a horizontal direction over the mouth of a glass or other hollow 

 vessel, it may be made to vibrate by holding near it a glass thrown 

 into vibration by passing a violin bow across it. The vibrations of 

 the paper are easily demonstrated by the movements of particles 

 of fine sand, or lycopodium powder strewed upon it. The sand 

 arranges itself into certain very definite figures, the shape of which 

 is determined by the position of the lines of repose, or nodal lines, 

 over which the sand accumulates. These phenomena may be shewn 

 in the membrana tympani itself, by scattering a little sand upon it, 

 the osseous meatus having been previously cut away. When the 

 vibrating glass is brought near to it, the movement of the particles 

 of sand affords sufficient evidence of the vibration excited in the 

 tympanic membrane, but owing to the slight extent of the mem- 

 brane it is impossible to determine the existence of any nodal line. 



Savart imitated the tympanic membrane, and the external audi- 

 tory apparatus by a hollow cone of paste-board ; across the nar- 

 row extremity some thin paper was stretched. When the vibrating 

 glass was brought near to the narrow end, movements of a slight 

 kind were excited in the paper, but when the glass was brought to 

 the wide extremity of the cone, much more extensive movements 

 were excited in the paper; although now the glass was much 

 more distant from the paper than previously. 



This result might have been due chiefly to one of two causes, 

 namely, either to the concentration of the sonorous undulations by 

 the walls of the cone, or to the excitation of vibrations in the walls 

 of the tube, which would be propagated directly to the paper ; and 



