902 OF SENSATION, AND THE ORGANS OF THE SENSES. 



labyrinth ; for there is, in most animals, a second aperture, the fenestra rotunda, 

 leading into the cochlea, and simply covered with a membrane. It is generally 

 supposed that, the labyrinth being filled with a nearly incompressible fluid, this 

 second aperture is necessary to allow the free vibration of that fluid; the mem- 

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

 ovalis is pushed in. It may, however, be easily shown by experiment, 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 in- 

 closed 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 the Invertebrated animals, but even of Frogs; in 

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

 rinth. 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 fenestra 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 communication 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. 



Fig. 223. 



A view of the labyrinth of the Left Side, laid open in its whole extent so as to show its Structure ; these 

 figures are all magnified: 1, the thickness of the outer covering of the cochlea; 2, 2, the scala vestibuli, or 

 upper layer of the lamina spiralis ; 3, 3, the scala tympani or lower layer of the lamina spiralis ; 4, the ha- 

 mulus cochleae ; 5, centre of the infundibulum ; 6, the foramen rotundum communicating with the tym- 

 panum ; 7, the thickness of the outer layer of the vestibule ; 8, the foramen rotundum ; 9, the fenestra ovalis ; 

 10, the orifice of the aqueduct of the vestibule; 11, the inferior semicircular canal; 12, the superior semi- 

 circular canal ; 13, the external semicircular canal ; 14, the ampulla of the inferior canal; 15, the ampulla of 

 the superior canal; 16, the common orifice of the superior and inferior canals; 17, the ampulla of the external 



