346 



SPECIAL HISTOLOGY. 



suitable reagents, at least in the thicker fibres of the nerves and of the 

 central organs, it comes readily into view, corresponding in its chemical 

 characters, in all essential particulars, with the sarcolemma of the mus- 

 cular fibres. In the finest fibres of the peripheral, as well as of the 

 central nervous system, the existence of this membrane has not yet been 



Fig. 138. 



demonstrated, and it must consequently for the present be left undecided 

 whether these fibres possess sheaths or not. 



Within the structureless sheath, lies the nerve-medulla, or pulp, 

 ("medullary sheath," Rosenthal and Purkinje, "white substance," 

 Schwann), (Fig. 137, 3, 6, Fig. 139, 3, 4, b) in the form of a cylin- 

 drical 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 



FiG. 137. Nerve-fibres, magnified 350 diameters. 1, from the Dog and Rabbit, in their 

 natural condition; a, fine; 6, of medium thickness; c, coarse fibre from the peripheral 

 nerves : 2, from the Frog, with the addition of serum ; a, drop of the contents expressed ; 6, 

 axis-cylinder within the drop, continued into the tube : 3, from the spinal cord of Man, re- 

 cent, with serum added; a, sheath; 6, medullary sheath with double contour; c, axis-cylin- 

 der : 4, double-contoured fibre from the fourth ventricle in Man ; the axis-cylinder, a, pro- 

 jecting and visible within the fibre : 5, two isolated axis-cylinders from the cord, one undu- 

 lated, the other of unequal thickness, with some medullary substance attached to it. 



FJG. 138. Nerve-tubes of Man, magnified 300 diameters: four fine, two of which are 

 varicose ; one of medium size with simple contours ; and four thick, two of which have 

 double contours, and two, grumous contents. 



