390 



SPECIAL HISTOLOGY. 



Valentin) (fig. 139, 1, 2, 3, 4, a) is an excessively delicate, flexible 

 but elastic, perfectly structureless and transparent membrane, 

 which, in quite unaltered nerve fibres, except in certain situa- 

 tions, is altogether invisible. But on the application of suitable 



reagents, at least in the 

 thicker fibres of the nerves 

 and of the central or- 

 gans, it comes readily in- 

 to view, corresponding, in 

 its chemical characters, 

 in all essential particulars 

 with the sarcolemma of 

 the muscular fibres. In 

 the finest fibres of the 

 peripheral, as well as of 

 the central nervous sys- 

 tem, the existence of this 

 membrane has not yet 

 been demonstrated, and 

 it must consequently, for 

 the present, be left un- 

 decided whether these 

 fibres possess sheaths or 

 not. 



Within the structure- 

 less sheath, lies the nerve-medulla, or pulp, (" medullary 

 sheath," Rosenthal and Purkinje, "white substance," Schwann), 

 (fig. 137, 3, b, fig. 139, 3, 4, b) in the form of a cylindrical 

 tube, closely and exactly surrounding the central fibre. In 

 the recent nerve- fibre this substance is perfectly homogeneous, 

 fluid, but viscid like a thick oil, and, according to the 

 light by which it may be viewed, transparent and clear, 



Fig. 137. Nerve-fibres, x 350 diara. 1, from the Dog and Rabbit, in their natural 

 condition; a, fine; b, of medium thickness; c, coarse fibre from the peripheral 

 nerves: 2, from the Frog, with the addition of serum; a, drop of the contents ex- 

 pressed ; b, axis-cylinder within the drop, continued into the tube : 3, from the spinal 

 cord of Man, recent, with serum added; a, sheath; b, medullary sheath with double 

 contour; c, axis-cylinder: 4, double-contoured fibre from the fourth ventricle in 

 Man; the axis-cylinder, a, projecting and visible within the fibre: 5, two isolated 

 axis-cylinders from the cord, one undulated, the other of unequal thickness, with 

 some medullarv substance attached to it. 



