STErCTUEE OF MEDULLATED FIBEES, 



129 



ance, from tlie absence of the comparatively tough 

 membranous sheath. Although the name of axis- 

 cylinder would seem to imply that it has actually a 

 cylindrical figure, yet this is by no means certain ; 

 and whether naturally cylindrical or not, it certainly 

 often appears more or less flattened when subjected 

 to examination. In the fresh state, and under high 

 powers of the microscope, the axis-cylinder fre- 

 quently presents an appearance of longitudinal 

 striation, indicating a fibrillar structure (see fig. 89, 

 p. 134) ; indeed both at the origin and termination 

 of a nerve it may commonly be seen to separate 

 into excessively fine filaments or fibrils. These, the 

 ■primitive filyrilJm of Max Schultze, are regarded, 

 with the axis-cylinder, which they mainly compose, 

 as the essential part of the nerve : at all events, 

 it frequently happens that these form the only por- 

 tion of the nerve that remains at the peripheral 

 extremity. Minute varicosities are commonly found 

 on them (fig. 89, h), probably the result of post- 

 mortem change. These are not to be confounded 

 with the varicosities to be immediately mentioned 

 as being frequently met with in the medullary 

 sheath. Cross sections of the spinal cord, or of any 

 nerve trunk, which have been stained with carmine 

 and subsequently mounted in balsam or dammar 

 varnish, exhibit the sections of the axis-cylinders 

 as deeply stained dots in or near the centre of the 

 medullated fibres. 



Nodes of Ranvier. — It has been shown by 

 Eanvier,* that there constantly occur in 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 con- 

 stricted at these places. The breaks or nodes, as 

 they may conveniently be termed, divide the fibre 

 into a series of segments of nearly equal length. 

 The segmentation is readily made apparent by the 

 action of a solution of osmic acid, which leaves the 

 nodes (fig. ^2, r, e) almost colourless, while the 

 medullary sheath, or white substance of Schwann, 

 becomes stained of an wky black colour. In such 

 preparations also the primitive sheath of the fibre 

 often becomes visible, and within this, between it and 

 the white substance, a clear oval nucleus {c), with a 

 Fig. 82. — PoKTioNS iiF TWO Nerve-FibrEs Stained with Osmic Acid. 425 Diameters. 



R, R. Nodes of Ranvier, with axis-cylinder passing across, 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 here stained black. In A the 

 nodes are wider, and the intersegmental substance more apparent than in b. (From 

 a drawing by Mr. J. E. Neale). 



* Comptes Eendus, 1871 ; and Arch, de Physiologic, 1872. 



n 



