434 



ON SENSATION, AND THE ORGANS OF THE SENSES. 



[Fig. 192. 



A view of the labyrinth of the Left Side, laid open in 1 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 

 hamulus 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 ; S, the foramen rotundum ; 9, the fenestra 

 ovalis; 10, the orifice of the aqueduct of the vestibule ; 11, the inferior semicircular canal; 12. the superior 

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

 ampulla of the superior canal ; l(i, the common orifice of the superior and inferior canals; 17, the ampulla 

 of the external canal.] 



commonly supposed that the Semicircular Canals have for their peculiar func- 

 tion, the reception of 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 position to each other 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 know- 

 ledge of the laws, according to which sonorous vibrations are transmitted. 

 Regarding the special function of the Cochlea, there is precisely the same un- 

 certainty. This part of the organ is peculiar in one respect, that the expan- 

 sion of the auditory nerve is here spread out (upon the lamina spiralis) in 

 closer proximity with the bone itself, than it is in any other part of the laby- 

 rinth ; so that the vibrations of the bone will be more directly communicated 

 to the nerve. It is not easy to see, however, what can be the peculiar object 

 of this disposition, in regard to the function of hearing. By M. Duges it is 

 surmised, that by the cochlea we are especially enabled to estimate the pitch 

 of sounds, particularly of the voice ; and he adduces, in support of this idea, 

 the fact, that the development of the cochlea follows a very similar proportion 

 with the compass of the voice. This is much the greatest in the Mammalia: 

 less in liirds ; and in Reptiles, which have little true vocal power, the cochlea 

 is reduced to its lowest form, disappearing entirely in the Amphibia. That 

 there should be an acoustic relation between the voice and ear of each species 

 of animal, cannot be regarded as improbable ; but the speculation of M. Duges 

 can at present only be received as a stimulus to further inquiry. 



567. We have now to consider the functions of the accessory parts, the 

 External Ear, and the Meatus. The Cartilage of the external ear may pro- 



