THE CIRCULATION OF CEREBRO-SPINAL SUBSTANCE. 565 



he says: "Since the nerve-filaments, like the neuroglia-fibers, 

 are very fine, and take, like them, an irregular course, it often 

 hecomes very difficult in a section to determine exactly which 

 is neuroglia and which are nervous elements." 



What is the role of the neuroglia and how is it functionally 

 related to the true nervous elements? Suggestive, in this con- 

 nection, are the following lines of Professor Foster's: "A 

 medullated nerve-fiber of the white matter of the spinal cord 

 resembles a medullated nerve-fiber of a nerve in being com- 

 posed of an axis-cylinder and a medulla; but it possesses no 

 primitive sheath or neurilemma. This is absent, and, indeed, 

 is not wanted; the tubular sheath of neuroglia affords, in the 

 spinal cord (and, as we shall see, in the central nervous system 

 generally), the support which in nerves is afforded by the 

 neurilemma." 62 This conclusively shows that for a certain dis- 

 tance, at least, the neuroglia-sheath and the myelin act as coats 

 for the one axis-cylinder: i.e., for the fibrils containing blood- 

 plasma. But we have seen that myelin is not the passive in- 

 sulating substance that it is now thought to be; if our views 

 are sound, it represents one of the two most important factors 

 of nerve-composition, and, indeed, the main source of nervous 

 energy. In modifying the accepted view concerning its func- 

 tions, however, we have eliminated its role as insulating layer, 

 leaving nothing but the neurilemma, or external, tubular in- 

 vesting sheath, for the protection and insulation of the "battery 

 elements/' as it were, the myelin and its oxidizing plasma. It 

 is, therefore, this protective and insulating sheath that the neu- 

 roglia replaces in the white substance of the cord and in "the 

 central nervous system generally": i.e., wherever the myelin 

 and its inclosed blood-plasma are present in the cerebro-spinal 

 axis. 



We have seen that, according to Barker, and as shown by 

 the researches of Flechsig and Berkley, the dendrites of neu- 

 rons are furnished with a thin layer of myelin nearly to their 

 termination; while we have shown, conclusively, we now be- 

 lieve that their central canal contains blood-plasma. We have 

 precisely, therefore, the structure of a nerve, minus its neu- 



All italics are our own. 



