350 



MECHANICAL AND ACOUSTICAL SENSES 



The canals of the lateral-line system have generally been assumed to con- 

 tain either mucus (they were, it will be recalled, first thought of as a mucus- 

 secreting system) or a special fluid as found in the labyrinth, where the hair 

 cells are bathed in a viscous liquid that is particularly rich in potassium and 

 low in sodium when compared with tissue fluid. 



Values obtained in recent analyses, given in Table 1, show that the ionic 

 contents of the lateral -line fluids are different from ear endolymph. In 

 Scyliorhinus, for example, where the canal system is open to the sea, the 

 endolymph is indistinguishable from seawater, but in fish in which the canal 

 system is closed, such as Corphaenoides and Lota, the endolymph potassium 

 is of similar concentration to, or even more concentrated than, the tissue 



Table 1. Ionic contents of various endolymphs and seawater (mM/C) 



fluid potassium. The open canal system can probably be regarded therefore 

 as the primitive condition, and changes in the endolymph composition be- 

 came possible only when the canals were closed. Even in systems in which 

 the neuromasts are exposed to a low K + environment, a high K + micro- 

 environment appears to exist within the cupula, immediately adjacent to the 

 hair cell surface (Russell and Sellick 1976). 



Neurophysiology of lateral-line organs— The properties of the lateral- 

 line organ have been examined in a number of fish preparations by recording 

 from single filaments teased from lateral-line nerve bundles. Hoagland (1933) 

 carried out this type of study on teleosts and was the first to show that the 

 unstimulated lateral-line organ is spontaneously active, an important observa- 

 tion that has since been frequently confirmed. He was uncertain about the 

 significance of this resting discharge, but shortly afterwards a similar feature 

 was recorded by Lowenstein and Sand (1936) in the elasmobranch labyrinth 

 and was shown by them to provide the basis for directional responsiveness. 



