42 PRACTICAL HISTOLOGY 



NERVE-FIBRES. 



There are two varieties of nerve-fibres : 



1. The medullated or white. 



2. The non-medullated or gray. 



MEDULLATED NERVE-FIBRES. 



Each nerve-fibre consists of a thin transparent structureless sheath, the primitive sheath, 

 with here and there a nucleus lying under it. Within this is the medullary sheath or white 

 substance of Schwann with a double contour forming the greater part of the nerve. It is 

 interrupted at intervals by regular constrictions, or Ranvier's nodes, where the white substance 

 is entirely absent. In the centre of the fibre is the axis-cylinder, a fine uniform cylinder 

 continuous throughout the whole length of the fibre. The proper view to take of the axis- 

 cylinder is that it is directly continuous at its central end with a process of a nerve-cell, and 

 is in fact one of the processes of a nerve-cell enormously elongated and pushed outwards 

 towards the periphery of the body, and covered in whole or part of its course with one or 

 more sheaths. This is the only view that explains the remarkable results of degeneration 

 in a nerve-fibre when it is separated from its so-called trophic nerve-centre (Waller) and 

 also explains the so-called union of nerve-fibres, which is in fact no union at all, but is due to a 

 pushing out or growth of the axial cylinder into the old sheaths of the degenerated nerve-fibres 

 from which the axis-cylinder and most of the white substance of Schwann, but not the nuclei, 

 have disappeared. 



FRESH NERVE-FIBRES. 



PREPARATION. Kill a frog and remove its sciatic nerve. With a pair of sharp scissors 

 cut off half an inch, and place it on a slide in a drop of salt solution. With a pair of needles 

 gently tease out one end of the nerve, and examine it from time to time with a low power, to 

 ascertain when it has been sufficiently teased. Cover. 



EXAMINATION (H). Observe the nerve-fibres of various sizes, some of them about the 

 breadth of a coloured blood-corpuscle, others larger or smaller. Note the double contour of 

 the white substance of Schwann which is highly refractive. This double contour exists in the 

 living nerve, and is not due to the action of reagents. This can easily be proved by examin- 

 ing a medullated nerve in the lung or tongue of a living frog or the lung of a newt. Select 

 the end of a fibre which has been broken across, and study the myeline exuding from its cut 

 end. The myeline, or white substance, collects into drops, with concentric markings. These 

 must not be mistaken for cells, from which they are easily distinguished by the absence of a 

 nucleus. (Indicate the myeline exuding from a nerve-fibre in PI. VIII., Fig. I.) 



In the centre of each fibre is a clear uniform band, the axis-cylinder. Trace the course of 

 a fibre carefully, and a Ranvier's node may be easily distinguished. The white substance is 

 interrupted here ; further, if the nerve-fibre be slightly on the stretch, the oblique incisures of 

 Schmidt and Lantermann, passing through the myeline, can also be made out. Irrigate the 

 preparation with picrocarmine, and continue the examination after twenty minutes. Observe 

 that a nucleus under the primitive sheath is stained red, the carmine passes in at the nodes of 

 Ranvier and also stains the axis-cylinder red. If an axis-cylinder happen to extend beyond 



