69 



LESSON XVII. 



STRUCTURE OF NERVE-FIBRES. 



1. TEASE a piece of fresh nerve in saline solution, injuring the fibres as little 

 and obtaining them as long and straight as possible. Study the medullated 

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

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

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

 a short length of a fibre very exactly. 



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

 and sketch as before. 



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

 nerve 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 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 Eanvier. 



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

 in picric acid and stained with picro-carmine. The nerve should have been 

 stretched 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 perineurium, the varying size of the nerve-fibres, 

 the axis cylinder in the centre of each fibre, &c. 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, chiefly 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 



