HEARING. 265 



that the cochlea is a kind of spiral harmonicon, vibrating in 

 different parts of its extent in unison with sounds of different 

 pitch ; and these appear to be probable suppositions ; but 

 neither the actions of the semicircular canals nor those of the 

 cochlea are understood in detail. 



The most distinct hearing is, beyond question, that derived 

 from sounds which enter by the external ear. We turn an 

 ear towards a sound which we wish to hear distinctly, and 

 hear very badly when the ears are stopped. The membrana 

 tympani obviously receives principally sounds entering by 

 the external ear; and if the vibrations of that membrane 

 are converted into a swinging movement of the ossicles, as 

 experiments seem to show, and are thus communicated to 

 the labyrinth, it is very plain that vibrations entering by 

 the external ear can be of no use in enabling the semi- 

 circular canals to determine the direction from which a sound 

 is coming. It would appear from these considerations that 

 direction, except in so far as it is determined by trying in 

 what position of the external ear a sound is heard loudest, is 

 appreciated by means of those vibrations which pass through 

 the bones of the skull ; and as bearing on such a supposition, 

 it may be mentioned that sounds heard when the ears are 

 thoroughly stopped are sometimes correctly judged as regards 

 their direction, and that fishes, which have no external ears, 

 or only minute pores to represent them, have very large 

 semicircular canals. It is also possible that sounds conveyed 

 to an ampulla, along the length of the semicircular canal to 

 which it belongs, may affect it more than others; but nothing 

 certain is known on the subject. 



It must not, however, be forgotten, that we are often 

 guided to the direction from which a sound comes by circum- 

 stances which have nothing to do with the ear, such as expec- 

 tation of sound from a particular quarter, or the direction of 

 the eyes of onlookers. So also the distance from which a 

 sound comes is judged of altogether by experience. The art 

 of the ventriloquist consists simply in correctly imitating the 

 effects produced by sounds at different distances, and in 

 stimulating the imagination, and directing the attention, so 

 as to make his hearers believe that a sound comes from a 

 particular quarter. The illusion would be destroyed if the 



