STRUCTURE OF NERVE-FIBRES. 83 



LESSON XVIII. 



STRUCTURE OF NERVE-FIBRES. 



1. TEASE a piece of fresh nerve in saline solution (or by the method of 

 demidesiccation, afterwards mounting in salt solution), injuring the fibres as 

 little and obtaining them as long and straight as possible. Study the medul- 

 lated fibres, carefully noticing all the structures that are visible viz., nodes 

 of Ranvier, nuclei of primitive sheath, double contour of medullary sheath, 

 medullary segments, etc. Measure the diameter of half a dozen fibres. Draw 

 a short length of a fibre very exactly. 



2. Prepare a piece of a sympathetic nerve in the same way. Measure and 

 sketch as before. 



3. Separate (in dilute glycerine) into its fibres a small piece of nerve or 

 nerve-root that has been twenty-four hours in \ per cent, osmic acid. The 

 nerve should have been moderately stretched on a piece of cork by means of 

 glass pins before being placed in the acid. Keep the fibres as straight as 

 possible and only touch them near their ends with the needles. Sketch two 

 portions of a fibre under a high power, one showing a node of Ranvier and 

 the other a nucleus of the primitive sheath. Look for fibres of Remak. 

 Measure the length of the nerve-segments between the nodes of Ranvier. 



4. Mount in Canada balsam sections of a nerve which has been hardened 

 in picric acid. Stain with picro-carmine or hsematoxylin. The nerve should 

 be straitened out before being placed in the hardening solution. Examine 

 the sections first with a low and afterwards with a high power. Notice the 

 lamellar structure of the peririeurium, the varying size of the nerve-fibres, 

 the axis-cylinder in the centre of each fibre, etc. Measure the diameter of 

 five or six fibres, and sketch a small portion of one of the sections. 



Nerve-fibres are of two kinds, medullated and non-medullated. The 

 cerebro-spinal nerves and the white matter of the nerve-centres are 

 composed of medullated fibres ; the sympathetic and its branches is 

 chiefly made up of non-medullated. 



The medullated or white fibres are characterised, as their name 

 implies, by the presence of the so-called medullary sheath or white 

 substance. This is a layer of soft substance, physically of a fatty nature, 

 which encircles the essential part of a nerve-fibre, viz., the axis-cylinder. 

 Outside the medullary sheath is a delicate but tough homogeneous 

 membrane, the primitive sheath or nucleated sheath of Schwann, but this 

 is not present in all medullated fibres, being absent in those which are 

 within the nerve-centres. The primitive sheath is also known as the 

 neurolemma. 



