THE VIBRATORY SENSE 



the density of which differs only slightly from their 

 own, feels to the full and without any hindrance its 

 movements and any oscillations which occur in it. 

 There is no difficulty about the transmission, whereas 

 in the air, the vertebrates, whose body is weighed 

 down by the force of gravity, are incapable of feeling 

 such vibrations, or experience only the most intense 

 as impressions of touch. The fishes have the added 

 advantage of possessing a sensory apparatus, definitely 

 constructed for the purpose, which localizes, amplifies, 

 and makes clear the perception. This apparatus con- 

 sists of tiny pits, richly supplied with nerves, situated 

 in a straight line along each side of the body from 

 behind the head to the end of the tail. Thus set in 

 a series, close to one another, they look like a line 

 traced from one end to the other of each side of the 

 body; hence the name "lateral line" which has been 

 given them, an expression which conveys a fair idea 

 of their general appearance. 



The possession of this sensitive apparatus does not 

 interfere with that of genuine auditory organs placed, 

 as is customary, in the thickness of the lateral walls 

 of the skull. But these are very small, and comprise 

 only internal ears, of which the principal parts are 

 the semicircular canals. From the small dimensions 

 of the other parts of these ears, we may presume a 

 correlated reduction in functional efficiency. It seems 

 likely that hearing is not their principal purpose, but 

 rather that of orientation in space, of changes of 

 direction, of the equilibrium of the body in its medium, 

 in conformity with the predominance in them of the 

 semicircular canals. Certain anatomical dispositions, 

 moreover, corroborate this. In the carp family and 

 those related to it, the vesica natatoria is bound to the 

 internal ear by the intermediarv of a chain of ossicles; 

 and this air-bladder filled with gas being the first to 

 experience the effects of change of direction and 

 equilibrium, being also the organ which best feels 

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