290 Cc. U. ARIENS KAPPERS 
THE FORMATION OF THE MEDULLARY SHEATH 
The third point that might be mentioned in this discussion 
is the question as to why most axis-cylinders in the central 
nervous system get a medullary sheath, and why this medullary 
sheath is not present on the cell body and the dendrites.‘* 
If one were content here with a teleological explanation, it 
would be sufficient to say that the presence of a myelin sheath 
around the axis-cylinder probably has the function of insulating 
the current, and that an insulating sheath should not occur in 
places where this current proceeds from one neuron to another 
(dendrites, cell body, telodendria). And yet that would not 
bring us one step nearer to the solution of the question as to the 
way in which the process of myelin accumulation is effected by 
the axis-cylinder. 
Let us endeavor here also to trace the influence which may lead 
to the accumulation of myelin around the axon, and why it is 
not accumulated sheath-like or otherwise in the cell and the 
dendrite. 
That the primitive axis-cylinder itself is able to form myelin 
is proved most clearly in the central nervous system, where the 
cells of Ranvier (i.e., of the neurilemma) which may have to do 
with it in the peripheral nervous system, do not occur, and other 
adjacent (glia) cells are but seldom found provided with myelin 
granules.‘ 
48T do not refer here to the medullary sheath around the peripheral fiber 
of a sensory root, which is a dendrite anatomically and ontogenetically (it 
develops later than the central process). In the millions of neurons in the 
nervous system this is the only exception, which certainly requires explanation 
but at present need not disturb our reasoning concerning the central pathways. 
The peripheral nerve fibers—especially the sensory ones—do not seem to be the 
most adequate material to elucidate the questions involved here, since they 
seem to require more explanation instead of helping to elucidate these questions. 
Moreover the fact that spinal ganglion cells belonging to the sensory system of the 
skin receive stimuli from other neurones (of the sympathetic system—Dogiel) 
proves that nervous currents may also run toward their periphery. 
49 Vignal. Le développement des élements du systém nerveux cérébro-spinal. 
Masson, Paris, 1889. See also, Ariéns Kappers. Recherches sur le développe- 
ment des gaines dans le tube nerveux. Petrus Camper, Amsterdam, vol. 2, 
part 2, 1902. 
