THE VIBRATORY SENSE 



them, its direct relations with the sensitive centre 

 appear to facilitate the fulfilment of such a function. 



But fishes really hear in quite another way through 

 the action of the lateral line, placed exactly where it 

 is best fitted to perform its function. Considered in 

 detail, on the side along which it passes from end to 

 end, it consists, usually, of a series of tiny pores, placed 

 in a row, which can be seen by the naked eye or with 

 the help of a lens. Each one, when there are scales, 

 goes through the whole thickness of a scale, and the 

 scales, thus pierced, take on a peculiar appearance, in 

 several cases even acquiring certain additional char- 

 acteristics. When a species is described, these par- 

 ticular scales are given separate mention, sometimes 

 as regards their shape, always as regards number. 

 They are called " the scales of the lateral line ", and 

 counted, the total figure, which varies from one species 

 and one genus to another, being given. This figure 

 is a measure of their size. A species with small scales 

 has more of them, proportionately, than its neighbour 

 with larger scales. The lateral line, despite the sim- 

 plicity of its appearance, thus acquires its full 

 significance, and shows that its little holes possess a 

 value beyond dispute, that it is much more than a 

 mere ornament. 



Each of these holes is the outer orifice of a short, 

 narrow, and often oblique tube, which opens, below 

 the corresponding scale, into a longitudinal canal, 

 broader and continuous. This canal is an interior 

 part which runs along the side underneath the row of 

 perforated scales, putting out, one after another, below 

 each of the latter, the tubes which open to the surface. 

 It is accompanied, in turn, by a nerve called the 

 " lateral nerve " which, all along its length, gives off 

 branches to the series of tubes. Each of these ends 

 in microscopic sensory centres, the principal elements 

 of which, constructed on the same model as those of 

 the auditory system, consist of cells terminating in 

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