572 



HEARING. 



were suggested by the result of several ob- 

 servations which be made upon tlie communi- 

 cation of vibrations through the air from a 

 vibrating body to one placed even at a great 

 distance from it, and susceptible of under- 

 going vibrations. The effect is best seen by 

 using a thin membrane, such as very fine paper, 

 carefully stretched in a horizontal position over 

 the mouth of a glass, or of a small delft basin : 

 a thin layer of sand is spread on this, and a 

 glass thrown into vibration by a violin bow 

 is held at a little distance from it; that the 

 paper immediately begins to vibrate is shewn 

 by the motions excited in the sand, the par- 

 ticles of which arrange themselves into figures, 

 which are sometimes perfectly regular, and 

 which form with so much rapidity that the eye 

 can scarcely follow " the circumstances which 

 accompany the transformation of the thin layer 

 of sand into a greater or less number of lines 

 of repose." 



By a series of experiments to be hereafter 

 detailed, Savavt showed that the tympanic 

 membrane is capable of being thrown into 

 vibrations by the sonorous impulses from a 

 vibrating body communicated to it by the in- 

 terposed air How far the external ear and 

 auditory canal serve to increase these vibrations 

 of the tympanic membrane, he sought to ascer- 

 tain by the following experiments. He formed 

 a conical tube of pasteboard, with a very wide 

 mouth at its base, the opening at the smaller 

 end being closed by a thin paper stretched 

 over it and glued to the margins of the open- 

 ing. This tube is placed resting on its base, 

 the membrane being upwards and per- 

 fectly horizontal, so that a layer of sand may 

 be spread over it. When a vibrating glass is 

 brought near and parallel to the upper surface 

 of this membrane, it immediately begins to 

 vibrate, and the grains of sand are tossed about 

 but raised but very slightly from the surface. 

 If, however, the vibrating glass be placed near 

 the base or the wide and open extremity of the 

 tube, the vibrations of the membrane will be 

 found to be much more manifest, and the ex- 

 cursions of the grains of sand so considerable, 

 that they are often raised to a height of three 

 or four centimetres ; so that there is a manifest 

 difference in the influence produced upon the 

 membrane by the sonorous undulations excited 

 in the air according as they impinge upon the 

 external surface of the membrane or upon that 

 which is turned towards the interior of the tube. 

 This phenomenon, Savart adds, may depend 

 upon two causes, namely, upon the concen- 

 tration of the sonorous undulations by the tube, 

 or upon the communication of motion to the 

 parietes of the tube, which again would com- 

 municate it to the membrane. With a view 

 to ascertain which of these causes was the 

 effective one, a second conical tube, open at 

 both ends, was held with its narrow extremity 

 a little above and corresponding to the narrow 

 extremity of the former one, but so that there 

 was no contact between them. If now the 

 glass is made to vibrate successively at the 

 large orifices of the two tubes, it will be found 

 that when placed at the orifice of the tube to 



which the membrane is attached, the oscil- 

 lations of that membrane are considerably 

 greater than when the aerial undulations reach 

 it through the other tube. Whence it may be 

 inferred that in all probability the external ear 

 and auditory canal have, besides any influence 

 they may exert in modifying the movements 

 of the particles of the air, the additional function 

 of presenting a large surface to the aerial un- 

 dulations, consequently to enter into vibration 

 under their influence, and thus to contribute 

 to increase the excursions of the vibrating parts 

 of the membrane with which they are imme- 

 diately in contact; the auricle, by the variety 

 of the direction and the inclination of its sur- 

 faces to one another, can always present to the 

 air a certain number of parts, whose direction 

 is normal (at right angles with) to that of the 

 molecular movement of that Huid. 



We get a general notion of the value of this 

 external part of the auditory apparatus in col- 

 lecting and directing the sonorous undulations, 

 from the assistance often derived in hearing 

 from increasing the concavity of the external 

 ear by placing the hand behind it, so as to 

 draw it forward and shorten it by pressure at 

 its upper and lower part ; by the dulness of 

 hearing which it is said follows the loss of the 

 auricle, and from the fact, so stated, that the 

 seal and walrus are extremely dull of hearing. 

 As regards the loss of the auricle, it is said 

 by Kerner that this loss is followed by the 

 greatest dulness of hearing in those animals 

 in whom the osseous meatus is wanting. In a 

 cat, from whom the right ear was cut close to 

 the skull, after the wound had healed without 

 any stoppage of the meatus, there was a re- 

 markable disposition always to keep its head 

 turned so as to be ready to receive sounds with 

 the left ear, and this continued even after the 

 tympanic membrane of the opposite side had 

 been frequently perforated, that of the right side 

 remaining whole ; and when the left ear was 

 stopped (although the right tympanic mem- 

 brane was sound, and the only injury on that 

 side was the removal of the auricle,) a total deaf- 

 ness was manifested except to the loudest and 

 clearest sounds. 



The tympanum and its contents. We have 

 already stated that Savart had demonstrated 

 that the membrana lympani is thrown into 

 vibrations by undulations of the air excited by 

 a sonorous body. This he demonstrated ex- 

 perimentally upon the membrana tympani 

 itself. The temporal bone having been sepa- 

 rated, he sawed away the osseous meatus so 

 as to expose the membrane on a level with 

 the rest of the bone, and when it was suf- 

 ficiently dry, he covered it with a thin layer 

 of sand. A vibrating glass held parallel and 

 very near to the surface of the membrane 

 occasioned a slight movement of the grains of 

 sand ; but owing to the slight extent and the 

 shape of the membrane, it was impossible to 

 determine the existence of any nodal line. 

 In a second experiment, the cavity of the tym- 

 panum was opened, so as to expose the ossi- 

 cles of the ear and their muscles ; and it was 

 observed that when the internus mallei muscle 



