786 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



siderable intensity to solid bodies. n. Sonorous vibrations of solid bodies 

 are communicated with greater intensity to other solid bodies brought in con- 

 tact with them than to water; but with much greater intensity to water than 

 to atmospheric air. in. Sonorous vibrations are communicated from air to 

 water with great difficulty, this difficulty very much exceeding that with which 

 they are propagated from one part of the air to another; but their transition 

 from air to water is much facilitated by the intervention of a lax membrane 

 extended between them. IV. Sonorous vibrations are not only imparted from, 

 water to solid bodies with definite surfaces which are in contact with the 

 water, but are also returned with increased intensity by these bodies to the 

 water ; so that the sound is heard loudly in the vicinity of those bodies, in 

 situations where, if it had its origin in the conducting power of the water 

 alone, it would be faint. v. Sonorous undulations, propagated through 

 water, are partially reflected by the surfaces of solid bodies. vi. Thin mem- 

 branes conduct sound in water without any loss of its intensity, whether they 

 be tense or lax. vn. When sonorous vibrations are communicated from, 

 water to air inclosed in membranes or solid bodies, a considerable increase 

 in the intensity of the sound is produced, by the resonance of the air thus 

 circumscribed. vni. A body of air inclosed in a membrane, and surrounded 

 by water, also increases the intensity of the sound by resonance, when the 

 sonorous undulations are communicated to it by a solid body. 



644. Animals living in air are nearly always provided with an opening 

 into the vestibule, ihefenestra ovalis, covered by a thin membrane; and gen- 

 erally with a Tympanic apparatus also. The following experimental results 

 bear upon the manner in which the Ear of such animals is affected by sound: 

 ix. Sonorous undulations, in passing from air directly into water, suffer a 

 considerable diminution in their strength ; while on the contrary, if a tense 

 membrane exist between the air and the water, the sonorous undulations are 

 communicated from the former to the latter medium with great intensity.- 

 x. The sonorous vibrations are also communicated without any perceptible 

 loss of intensity from the air to the water, when, to the membrane forming the 

 medium of communication, there is attached a short solid body, which occu- 

 pies the greater part of its surface, and is alone in contact with the water. 

 xi. A small solid body, fixed in an opening by means of a border of mem- 

 brane, so as to be movable, communicates sonorous vibrations from air on 

 one side to water or the fluid of the labyrinth on the other, much better than 

 solid media not so constructed. But the propagation of sound to the fluid 

 is rendered much more perfect if the solid conductor, thus occupying the 

 opening, is by its other end fixed to the middle of the tense membrane which 

 has atmospheric air on both sides. The fact stated in ix is evidently one of 

 great importance in the physiology of hearing; and fully explains the nature 

 of the process in those animals which receive the sonorous vibrations through 

 air, but which have no tympanic apparatus. In x we have the elucidation 

 of the action of the fenestra ovalis, and of the movable plate of the stapes 

 which occupies it, in animals living in air but destitute of tympanic appa- 

 ratus ; this is naturally the case in many Amphibia; and it may happen as 

 the result of disease in the Human subject, In xi we have a very interest- 

 ing demonstration of the purpose and action of the tympanum, in the more 

 perfect forms of the auditory apparatus. We are now prepared to inquire, 

 in somewhat more of detail, into the actions of the different parts of this ap- 

 paratus; and it will be better to commence with those of the Middle and In- 

 terim/ /v/r, the accessory organs being afterwards considered. 



645. The J/n>//m>/m Tympani consists of three layers: an external one con- 

 tinuous with the cutis of the external meatus, and consisting of dermoid tis- 

 sue with a covering of epidermic cells ; an internal, which is extremely thin, 



