556 HEARING. 



sounds are transmitted to the internal organs of hearing in two 

 manners ; first, in the manner ordinarily understood, and secondly 

 by the solid part of the head. 



The perception that we have of the direction of sounds he sup- 

 poses to arise solely from the portion which is transmitted through 

 the solid parts of the head, and which, affecting the three semi- 

 circular canals, situated in planes at right angles with each other, 

 with different degrees of intensity according to the direction in 

 which the sound is transmitted, suggests to the mind the corre- 

 sponding direction. If the sound is transmitted in the plane of 

 either of the semicircular canals, the nervous matter in that canal 

 will be more strongly acted on than that in either of the other 

 two ; and if it be transmitted in any plane intermediate between 

 any two of the rectangular planes, the relative intensities in these 

 two canals corresponding therewith will vary with the direction of 

 the intermediate plane. The ordinary notion respecting our per- 

 ception of the direction of sound is that we compare the relative 

 intensity of the sensation in the two ears, and thus judge of its 

 direction. Were this true, a sound produced any where in the 

 prolongation of the mesial plane, whether before, behind, or 

 above the head, should suggest no difference of direction ; 

 whereas we know from experience the contrary to be the case. 

 The views of Professor Wheatstone were first announced in Mr. 

 Mayo's lectures at the College of Surgeons. 



As most of the lower invertebrate animals live in water, the percussions of 

 which must powerfully effect their surface, naturalists do not find a special 

 organ for hearing among them, till they ascend as high as the air-breathing 

 insects. In these it is more complicated than in any other invertebrata, for 

 many of them emit sounds intended to be heard by their kind. Some have hard 

 instruments for this purpose which they rub against each other, and thus the 

 male and female correspond in the dark for their amours. Insects have not only 

 the first elements of an ear, the auditory nerve and vestibule, but the rudi- 

 ments of twp semicircular canals : fluid exists within, and the vestibule has a 

 fenestra ovalis covered with a thin membrane. But in the aquatic invertebrata, 

 as the lobster, cray-fish, and crab, the nerve and vestibule with its fenestra ovalis 

 and membrane only are found. In those which have long tails and swim briskly, 

 like the two former, the membrane is delicate and vibratile, whereas in those 



