126 PRACTICAL HISTOLOGY. 



CHAPTER VII. 



NERVOUS TISSUE. 



Preparation 1. Medullated nerve-fibres. For 



the study of the medullated nerve-fibres a piece of 

 one of the ordinary nerves those of the limbs, for 

 example may be cut out from any recently-killed 

 animal. If the nerve be a large one, a thin strip only 

 should be used, preferably taken from the interior 

 after the piece has been torn longitudinally into two 

 halves by fine forceps. The strip is to be placed on 

 a slide in a little serum or salt solution, and care- 

 fully separated as finely as possible. This separation 

 must be effected, not by seizing the piece anywhere 

 and tearing it up at random, but by inserting tine 

 needles into it near one end and gently drawing them 

 asunder, so that the piece is split into two. Repeating 

 this process a number of times on the resulting pieces 

 the nerve will be eventually separated into very tine 

 bundles of fibres, together with a number of more or 

 less isolated fibres, which are still nearly straight 

 and uninjured, except near one end. The prepara- 

 tion may then be covered, and the general character 

 and appearance of the fibres investigated. To see 

 the nodes of Ranvier well, a tolerably large fibre, 

 free for a considerable part of its length, should be 

 chosen, and by moving the slide it should be care- 

 fully followed with an ordinary high power. It 

 will be found that at definite and not very close in- 

 tervals along the fibre the double-contoured medul- 

 lary sheath fails altogether, and the axis-cylinder 

 alone continues the nerve at these points. It is not 

 very easy to see the oval nucleus in the middle of 

 each segment in the fresh, unstained preparation. 



