894 OF SENSATION, AND THE ORGANS OF THE SENSES 



6. Sense of Hearing. 



897. In the Ear, as in the Eye, the impressions made upon the sensory nerve 

 are not at once produced by the body which originates the sensation ; but they 

 are propagated to it through a medium capable of transmitting them. Here 

 too, therefore, we take cognizance by the mind, not of the sonorous object, but 

 of the condition of the auditory nerve; and all the ideas we form of sounds, as 

 to their nature, intensity, direction, &c., must be based upon the changes which 

 they produce in it. The complex contrivances which we meet with in the organ 

 of Hearing among higher animals, are evidently intended to give them greater 

 power of discriminating sounds, than is possessed by the lower tribes ; in which 

 last it is reduced to a form so simple, that it may be questioned whether they 

 can be said to possess an organ of hearing, if by this term we imply anything 

 more than the mere consciousness of sonorous vibrations. There is a consider- 

 able difference, however, between the Eye and the Ear, in regard to the special 

 purposes for which they are respectively adapted. In the former we have seen, 

 that the whole object of the instrument is to direct the rays of light received 

 by it, in such a manner as to occasion them to fall upon the expansion of the 

 optic nerve in similar relative positions, and with corresponding proportional 

 intensities, to those which they possessed when issuing from the object. We 

 have no reason to believe anything of this kind to be the purpose of the Ear; 

 indeed, it would be inconsistent with the laws of the propagation of sound. 

 Sonorous vibrations having the most various directions, and the most unequal 

 rates of succession, are transmitted by all media without modification, however 

 numerous their lines of intersection ; and wherever these undulations fall upon 

 the auditory nerve, they must cause the sensation of corresponding sounds. Still 

 it is probable that some portions of the complex organ of hearing, in Man and 

 in the higher animals, are more adapted than others to receive impressions of a 

 particular character; and that thus we may be especially informed of the direc- 

 tion of a sound by one part of the organ, of its musical tone by another, and of 

 some other of its qualities by a third. 



898. The essential part of an Organ of Hearing is obviously a nerve, endowed 

 with the peculiar property of receiving sonorous undulations, and of transmitting 

 their effects to the Sensorium. This nerve is spread out over the surface of a 

 delicate membrane which lines the Vestibule and its prolongations ; and this 

 membrane incloses a fluid which is the medium whereby the sonorous vibra- 

 tions received through the external ear are communicated to the nerve. We 

 learn from an examination of the comparative structure of the auditory appara- 

 tus in the lower animals, and from the study of its development in the higher, 

 that the part which, being most constantly present and being also the earliest 

 in its development, may be considered as the most essential, is the simple Vesti- 

 bular cavity, which exists where there are no vestiges either of Semicircular 

 Canals, of Cochlea, or of Tympanic apparatus. Such a condition presents itself 

 in some of the higher Invertebrata and in the lowest Fishes ; but as we ascend 

 the Vertebrated series, we find the semicircular canals growing out (as it were) 

 of the Vestibule in Fishes, a tympanic apparatus superadded in Reptiles, and a 

 Cochlea first acquiring a more than rudimentary development in the class of 

 Birds, although only presenting in Mammalia that characteristic form from 

 which it derives its name. 1 Of the mode in which the ultimate subdivisions of 

 the Auditory nerve are distributed upon the lining membrane of the labyrinth, 

 it does not yet seem possible to give a certain account ; for although Wagner 



1 For a more detailed sketch of the Comparative Anatomy of the Organ of Hearing, see 

 the Author's " Princ. of Physiol., Gen. and Comp.," \\ 822-825, Am. Ed. 



