312 



MEDULLARY SHEATH. 



of the medullary sheath exudes from the cut ends of the nerve-fibres, it is found 

 that the place of that which thus escapes is taken by the white substance from the 

 next internode ; and this substance may be seen to flow past the constrictions of 

 Ranvier without escaping at those points. Within the primitive sheath the inter- 

 nodes are united by an annular disc the " constricting band " of Ranvier the 

 nature of which is unknown, although like intercellular substance elsewhere, it 

 becomes stained by nitrate of silver. The last-named reagent stains also the axis- 

 cylinder in the neighbourhood of the node in the manner indicated by Frommann 

 (see page 310), so that the fibres after this treatment appear marked with little 

 crosses (fig. 381) ; the transverse limb of the cross being due to the ring of inter- 

 segmental substance, the longitudinal to the axis-cylinder. Many other fluids stain 

 the axis-cylinder at the nodes only, being prevented from reaching it elsewhere owing 

 to the presence of the fatty matter in the surrounding medullary sheath. 



The division of nerve-fibres always occurs at the site of a node of Eanvier (see 

 fig. 388, p. 333). 



Engelmann argues in favour of a discontinuity of the axis-cylinder (as well as of the 

 medullary sheath) at the nodes of Ranvier, basing his argument partly on the fact that the 



Fig. 360. NODES OP RANVIER FROM THE NERVE OP A PIGEON, 



TREATED WITH OSMIO ACID. (v. Gedoelst. ) 



The medullary sheath is stained black. The fibrils of the 

 axis-cylinder have enlargements at the middle of the node. In 

 A the constricting band is seen. 



degeneration which results above the section when a 

 nerve-fibre is cut, stops at the first node of Ranvier. partly 

 on appearances obtained after a certain method of treat- 

 ment with nitrate of silver, a dark deposit characteristic 

 of inter-cellular substance traversing, according to the 

 account given by him, the thickness of the axis-cylinder 

 at the nodes. He also points out that the axis-cylinders 

 very readily become broken across at the nodes. This view 

 has been in a measure confirmed by the researches of 

 G-edoelst, who has shown that there is a tendency at the 

 nodes of Ranvier for the fibrils of the axis-cylinder to 

 develop minute swellings or nodosities which closely 

 resemble those which are formed upon the achromatic 

 fibres which unite the two daughter-nuclei of a dividing 

 cell in most vegetable and some animal tissues {Zrll-plattc 

 of Strasburger) and which indicate the plane of separa- 

 tion between the two daughter-cells and of formation of 

 the cellulose or other membrane. Such a structure is 

 shown in the representations of a dividing ovum given by 

 Strasburger (see fig. 218, p. 188), and is superficially very similar to that figured by Gedoelst 

 upon the axis-cylinder at the nodes (fig. 360). But the identity of the two structures in spite 

 of this superficial resemblance must not be taken as proved. The axis-cylinder is not an 

 out-growth from a nucleus, nor can its segments be taken to represent nuclei. Neither do its 

 fibrils resemble in chemical characters so far as is known the achromatic fibrils of a 

 cell-nucleus. But in spite of these differences the correspondence between an inter-node 

 of a nerve and an elongated cell is so direct and obvious that a view which permits of the 

 presence of the nodes of Ranvier being explained as indicating a tendency of the fibre to 

 become divided across into cell-segments cannot be altogether disregarded. Ranvier, Boveri, 

 and others, have looked upon the medullary and primitive sheaths only as being thus segmented 

 up into cells, which they regard as wrapped around the axis-cylinder. But when first developed 

 the medullary sheath is a continuous and unsegmented layer, and appears to be laid down upon 

 the surface of the pale fibre which is becoming medullated, by the superficial protoplasm of that 

 fibre itself (see Development of nerves). 



The internodes or nerve-segments vary in length in different nerves ; in larger 

 nerve-fibres they may perhaps, speaking roughly, average about 1 mm. In the 

 nerves of young animals they are often much shorter than this, so that the growth 



