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



of the feuestra rotunda being made to bulge out, as that of the fenestra ovalis 

 is pushed in. It may be easily shown by experiment, however, as well as 

 by reference to comparative anatomy, that no such contrivance is necessary ; 

 for sonorous undulations may be excited in a non-elastic fluid, completely 

 inclosed within solid walls at every part, except where these are replaced by 

 the membrane through which the vibrations are propagated ; and this is 

 precisely the condition, not only of Invertebrated animals, but even of Frogs ; 

 in which last a tympanic apparatus exists, without a second orifice into the 

 labyrinth. Moreover, it is certain that the vibrations of the air in the cavity 

 of the tympanum must of themselves act upon the membrane of the feuestra 

 rotunda ; and this is perhaps the most direct manner in which the fluid in 

 the cochlea will be affected, although it will ultimately be thrown into much 

 more powerful action, by the transmission of vibrations from the vestibule. 

 For it has been satisfactorily determined by experiment (xn), that vibrations 

 are transmitted with very much greater intensity to water, when a tense 

 membrane, and a chain of insulated solid bodies capable of free movement, 

 are successively the conducting media, than when the media of communica- 

 tion between the vibrating air and the water are the same tense membrane, 

 air, and a second membrane ; or, to apply this fact to the organ of hearing, 

 the same vibrations of the air act upon the fluid of the labyrinth with much 

 greater intensity, through the medium of the chain of auditory bones and 

 the fenestra ovalis, than through the medium of the air of the tympanum 

 and the membrane closing the fenestra rotunda, which last, it is maintained 

 by Dr. Jago, has little if any influence in the transmission of sounds to the 

 internal Ear. The fenestra rotunda is not to be considered as having any 

 peculiar relation with the cochlea, since, in the Turtle tribe, the former exists 

 without the latter. 



647. It is obviously in the Labyrinth, as a whole, that the sonorous vibra- 

 tions are brought to bear upon the Auditory nerve spread out to receive 

 them. In regard to the special functions of particular parts of the labyrinth, 

 however, no certainty can be said to exist. The membrane which lines its 

 cavities not only contains a liquid (the endolympli), but is also separated 

 from the osseous wall by another collection of liquid, the perilymph, so that 

 it is suspended, as it were, in a liquid which bathes both its surfaces. In 

 the cavity of the Vestibule, which is subdivided by a membranous partition 

 into two, are round small masses of concretionary particles, collectively 

 named otoconia, or ear-powder ; these are obviously the rudiments of the 

 otoliths, or ear-stones, whose presence, in animals with a less perfect auditory 

 apparatus, seems needful to intensify the undulations. It is commonly sup- 

 posed that the Semicircular Canals have for their peculiar function, to re- 

 ceive the impressions by which we distinguish the direction of sounds ; and 

 it is certainly a powerful argument in support of this view, that, in almost 

 every instance in which these parts exist at all, they hold the same relative 

 positions as in Man, their three planes being nearly at right angles to one 

 another. The idea, however, must be regarded as a mere speculation, the 

 value of which cannot be decided without an increased knowledge of the 

 laws according to which sonorous vibrations are transmitted ; but it receives 

 a certain degree of confirmation from the curious movements witnessed by 

 M. Flourens after section of one or other of these canals ( 526). Regard- 

 ing the special function of the Cochlea, there is precisely the same uncertainty. 

 It has been surmised by M. Duges, that by the Cochlea we are especially 



1 Arid con li nurd l>y Loewenberg (Archives of Ophthalmology and Otology, vol. 

 iii, No. "2, p. 31, 1874), who attributes them to irritation, and not to paralysis of the 

 nerves. 



