XI MEDULLATED NERVE-FIBRES 461 



"primitive sheath" or "sheath of Schwann"). 2. The 

 central band becomes more opaque, and sometimes ap- 

 pears marked with fine longitudinal striae as if it were 

 composed of extremely fine fibrillse ; it is the neuraxon 

 or axis-cylinder. 3. Where the axis-cylinder traverses 

 one of the nodes the neurilemma is seen to embrace 

 it closely, but in the intervals between the nodes a 

 curdy-looking matter, which looks white by reflected light, 

 occupies the space between the neurilemma and the 

 axis-cylinder. This is the medulla (the "white 

 substance of Schwann ") largely composed of a complex 

 fatty substance often spoken of as myelin. If the 

 neurilemma of a fresh fibre is torn, the myelin 

 flows out and forms irregular lumps as if it were 

 viscous. The medulla sometimes breaks, by oblique 

 lines (Fig. 146, C. m'), extending from the axis-cylinder 

 to the neurilemma, into segments, the faces of which 

 are obliquely truncated and fit closely against one 

 another. These may be seen even in quite fresh and 

 living nerve-fibres. 4. The internodal nucleus is more 

 sharply defined ; and it will be seen to be attached to the 

 inner surface of the neurilemma. 



The essential part of each fibre, regarded as an 

 instrument for the transmission of that molecular dis- 

 turbance which is spoken of as a "nervous impulse," is 

 the axis-cylinder. This is suggested by the fact that 

 the axis-cylinder alone is apparently continuous through- 

 out each fibre, since it passes across each node ; but 

 all doubt is removed when we find that the axis-cylinder 

 alone provides the actual connection between the central 

 nervous system and the distant structures to or from 

 which the motor (efferent) or sensory (afferent) nerves 

 run. Thus, if we follow along the course of a motor 

 nerve, proceeding to its muscle, we find that it enters the 

 perimysium (with which the superficial layer of the peri- 

 neurium becomescontinuous), and divides in the perimysial 

 septa into smaller and smaller branches, each of which 



