STRUCTURE OF THE NERVE FIBRES. 153 



presence or absence of this sheath of Schwann. In some 

 instances the sheath is sufficiently thick to admit of measure- 

 ment, as, for ^example, in the solitary nerve fibres that run in 

 the mesentery of the frog ; or in the electrical organs of the 

 torpedo, where it attains a still greater thickness ;* or it may 

 even consist of a series of interlacing tubes, as in the nerves 

 supplying the electrical organ of the electrical eel (Malapte- 

 rurus), which are as thick as a knitting-needle, and still con- 

 tain only a single medullated primitive nerve fibre.f In these 

 instances the nuclei in the sheath are also much more distinct. 

 If the sheath be 'very thin, it only becomes visible in the fresh 

 state projecting for a short distance beyond the torn extremity 

 of the fibres. Destruction and removal of the nerve medulla, in 

 consequence of putrefaction or the action of reagents (concen- 

 trated acids, alcohol, and ether, which last dissolves the fat of 

 the medullary sheath), are in such cases the only means by 

 which the sheath of Schwann can be more distinctly demon- 

 strated. 



Just as a delicate sheath of Schwann, investing the medullary 

 portion of the nerve, is scarcely perceptible in the fresh state of 

 the nerve, so is it with great difficulty that an axis cylinder can 

 be distinguished within the fresh medullary sheath. The bright 

 lines which limit externally the highly refractive substance of 

 the latter material, and the scroll-like contour lines which are 

 occasioned by the gradually progressing coagulation of the nerve 

 medulla, usually prevent the difference in the refractive power 

 between the axis cylinder and the medulla from being observed. 

 On the other hand it is easy to isolate the axis cylinder, at all 

 events for short distances, in the medullated fibres of the cen- 

 tral organs, when the sheath of Schwann is deficient ; and it 

 may thus be demonstrated from an examination of perfectly 

 fresh specimens that in thick medullated fibres it is thick ; in 

 thin fibres, thin ; appearing in the form of a pale fibre with 

 the peculiarities above described. Moreover, it is possible in 

 the perfectly fresh thick medullated fibres of the central organs 



* See Hud. Wagner, Uber d.fein. Ban der Elect. Organes inn Zitterrochen, 

 1847, fig. 3, b, and woodcut 23 in this work. 



t Bilharz, Das Ekkt. Organ des Zitterwelses, p. 21. 



