14 Dr. Knox on the Theory of a Sixth Sense in Fishes, 



of the tubes and place it under a microscope of small powers, 

 we perceive that the nerve is distributed to the short extremity 

 of the tube by fibrils. Let a, Plate II. Fig; 18, represent 

 the tube filled with the gelatinous matter ; b the nerve, di- 

 viding into several small branches, which creep up to a short 

 distance perpendicularly on the sides of the tube. But if 

 the extremity of the tube be examined after cutting the nerve 

 across, then the distribution of the nerve may be well enough 

 understood by inspecting Fig. 19, in which the nerves seem to 

 proceed from the centre to the circumference like the spokes 

 of a wheel. 



When strong pressure is applied, the gelatinous fluid fill- 

 ing the tubes passes by narrow apertures into the canals of 

 the lacunar system ; but I consider this as by no means proving 

 a direct communication, for we never perceive any of the 

 gelatinous fluid naturally in these canals, nor scarcely any 

 thing else, as M. de Blainville very well remarks in his ac- 

 count of these organs. 



In the thorn-back, the arrangement of the tubular organs in 

 the snout is precisely as in the tope or shark ; but the tubes 

 of the lacunar system are much more developed and distinct. 

 In the specimen I last examined these tubes contained only 

 a few globules of air, and a small quantity of a mucous fluid. 



What are the functions of the tubular organs ? And for 

 what purpose have nerves been distributed to them in such 

 abundance ? Mr. Jacobson (a distinguished German anato- 

 mist) has replied to this question ; he considers them to be or- 

 gans of touch, almost active. 



If I mistake not Mr. Jacobson's opinions, (which have been 

 given to the public only through the medium of Dr. De 

 Blainville,) that gentleman views the tubular organs, though 

 terminating!: on extended smooth and flat surfaces, as organs 

 of touch ; against which opinion it might be argued, that the 

 peripheral terminations of these organs are but ill adapted to 

 exercise the sense of touch, which we find in almost all ani- 

 mals to be more or less connected with a prehensile and mus- 

 cular tissue, calculated to be extended and applied in some 

 way or other to the surfaces of bodies ; 9.d, That in many 

 fishes there are organs of touch of an entirely different form, 

 relative to whose functions no doubt can be entertained ; 

 lastly, That these peculiar tubular organs exist in certain fishes, 



