256 OF SENSATION, AND THE ORGANS OF THE SENSES. 



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 pos- 

 sessed 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 any thing more than the mere consciousness 

 of sonorous vibrations. There is a considerable 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 was 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 a similar 

 relative position, and with corresponding proportional intensity to that which 

 they possessed when issuing from the object. We have no reason to believe 

 any thing of this kind to be the purpose of the Ear; indeed it would be incon- 

 sistent with the laws of the propagation of sound. Sonorous vibrations having 

 the most various directions, and the most equal rate of succession, are trans- 

 mitted 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 direction 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. In our inquiries into this ill-understood sub- 

 ject, we shall commence with a brief survey of the comparative structure of 

 the organ. 



350. The essential part of an Organ of Hearing being obviously a nerve, 

 endowed with the peculiar property of receiving and transmitting sonorous 

 undulations, it is by no means indispensable that a special provision should 

 be made for this purpose ; since the Auditory nerve, if merely in contact 

 with the solid parts of the head, will be affected by the vibrations, in which 

 it is continually participating. Hence we must not imagine the sense to be 

 absent, wherever we cannot discover a special organ. It is among the highest 

 only of the Invertebrate animals, that any such special organ presents itself; 

 and then only in a very simple form. Thus in the Crustacea and Cephalopoda, 

 the ear consists of a small cavity excavated in the solid frame-work of the 

 head ; this cavity is lined with a membrane, on which the nerve is distributed ; 

 and it is filled with a watery fluid. In some instances, the cavity is com- 

 pletely shut in by its solid walls ; and the sonorous vibrations can then only be 

 communicated through these : but in the higher forms of this apparatus, there 

 is a small aperture covered with a membrane, upon which the external me- 

 dium can at once act. In tracing this most simple into the more complex 

 forms, it is at once seen, that the cavity corresponds with the vestibule of the 

 ear of higher animals, and its opening with thefenestra ovalis. In the lowest 

 Cyclostome Fishes, the organ is but little more complicated ; from the vestibule 

 proceeds a single annular passage, which may be considered as a semicircular 

 canal ; and the auditory nerve is distributed minutely upon its lining mem- 

 brane, as upon that of the vestibule itself. In species a little higher in the 

 scale, two such canals exist; these are present in the Lamprey. And in all the 

 rest of the class, three semicircular canals are found, holding the same direc- 

 tion in regard to each other as they do in Man. Within the vestibular sac of 

 Fishes are found calcareous concretions, which are pulverulent in the Carti- 



