MEDULLARY SHEATH. 



311 



of Ranvier when thus treated, the enlargement corresponding in situation to the annular con- 

 stricting 1 band of the node (Ranvier). 



It is not always easy to distinguish the axis-cylinder in the medullated fibres when they are 

 examined in the fresh condition, but it can generally be made manifest by staining the nerve 

 with carmine or ha3matoxylin. In a transverse section of a nerve thus stained the axis- 

 cylinders appear in the form of round or oval areas occupying the centre of the fibres (fig. 362), 

 but they are often much shrunken. The fibrils of the axis-cylinder have a tubular aspect in a 

 well prepared section, which is distinctly visible in photographs under a high power. 



Fig. 359. PORTIONS OF TWO NERVE-FIBRES STAINED WITH OSMIC ACID (FROM A ^ 



A YOUNG RABBIT). 425 DIAMETERS (E. A. S.). 



B, R. Nodes of Ranvier, with axis-cylinder passing through, a, Primitive 

 sheath of the nerve ; c, opposite the middle of the segment indicates the 

 nucleus and protoplasm lying between the primitive sheath and the medullary 

 sheath. In A the nodes are wider, and the intersegmental substance more 

 apparent than in B. (From a drawing by Mr. J. E. Neale.) 



Medullary Sheath. The myelin or substance of the 

 medullary sheath (which was termed the white substance by 

 Schwann on account of its presence being the chief cause of 

 the whiteness of the nerves), undergoes peculiar changes on 

 exposure of the nerve to the action of water and other fluids, so 

 that the outline of the fibre is often rendered uneven ; round 

 and irregular spots appear at various points, the medullary 

 sheath acquiring eventually a confusedly curdled aspect. 



The thickness of this sheath varies within wide limits, and 

 indeed this is the chief cause of the variation in size of the 

 medullated nerve-fibres, although the axis-cylinder may also 

 vary in diameter to a considerable extent. In some fibres, 

 the medullary substance forms an exceedingly thin layer, so as 

 to be scarcely distinguishable except by the darker outline 

 which it imparts to the fibre, or it may only occur in parts, 

 these alternating irregularly with other parts in which there is 

 a complete absence of white substance. Such fibres, which are 

 common in some parts of the sympathetic system (fig. 365, 

 , m\ may be regarded as transitional between the white 

 and the grey fibres. 



Nodes and iuternodes of Ranvier. It was shown by 

 Ranvier, that there constantly occur in the peripheral medullated 

 nerve-fibres, breaks in the continuity of the white substance, which 

 succeed one another at regular intervals along the course of the 

 nerves, and give the fibres the appearance of being constricted 

 at these places. These constrictions or nodes of Ranvier, as they 

 may conveniently be termed, divide the fibre into a series of 

 internodes of nearly equal length. The segmentation is readily 

 made apparent by the action of a solution of osmic acid, which 

 leaves the nodes (fig. 359, E, R ; fig. 360) almost colourless, 

 while the medullary sheath, or white substance of Schwann, 

 becomes stained of an inky black colour. 



The white substance of the medullary sheath is often found 

 to have shrunk somewhat in the neighbourhood of a node 

 (fig. 356, A), and it can then be seen that there is present, in 

 addition to it, a clear or finely granular stroma, which has become evident in 

 consequence of retraction of the fatty substance which normally pervaded it. 



The outer or membranous sheath of the fibre (neurolemma) appears to be continued 

 over the nodes, for when a medullated fibre is examined in water and the substance 



