XI 



ELECTRICAL FISHES 



405 



rises integrally from the spinal medulla, and subsequently bifurcates, 

 by repeated branching, into as many branches and rarni as there 

 are nerves in the electrical organ, i.e. as are contained within its 

 plates and chambers. The total diameter of the entire nerve, 

 characterised in the fresh state by its peculiar silvery colour, is in 

 no way due to extraordinary size of the original primitive fibre, 

 but much more to the vigorous development of the connective- 

 tissue sheaths, with which the nerve is invested down to its finest 

 terminal ramifications. The disposition of these sheaths is best 



FIG. 264. Portion of T.S. from electrical nerve-fibre of Main ptcr tints. (Fritsch.) 



understood from transverse sections, such as Fig. 264, after Fritsch, 

 from the nerve-trunk. In the centre is the round section of the 

 axis-cylinder, O'OOS mm. in diameter, surrounded with a medullated 

 sheath from about 0'03 to 0'012 mm. in breadth. Externally to 

 this there is first a broad zone of reticulated connective tissue, 

 regarded by Fritsch as the analogue of the Henle-Schwaiin 

 sheath (the " inner sheath " of Bilharz). This occupies ^ of the 

 total diameter of the trunk, which is about 1/1 mm., and may, 

 with the nerve-fibre, easily be shelled out from the next, concentric 

 layers of connective tissue, which are richly vasculated. Seeing 

 that at every division of a nerve-fibre the total cross-section of 



