SENSE OF HEARING. 



423 



rous vibrations. There is a considerable difference, however, between the 

 Eye and the Ear, in regard to the special purposes for which they are respect- 

 ively adapted. In the former we have seen, that the whole object of the in- 

 strument 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 a similar rela- 

 tive position, and with corresponding proportional intensity, to that 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 in- 

 consistent with the laws of the propagation of sound. Sonorous vibrations 

 having the most various directions, and the most equal rate 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 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. 



557. 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, 



[Fig. 180. 



General view of the external, middle, and internal ear, as seen in a prepared section through a, the 

 auditory canal, b. The tympanum or middle ear. c. Eustachian tube, leading to the pharynx, rf. Coch- 

 lea; and e. Semicircular canals and vestibule, seen on their exterior, as brought into view by dissecting 

 away the surrounding petrous bone. The styloid process projects below; and the inner surface of the 

 carotid canal is seen above the Eustachian tube. From Scarpa.] 



