XVII.] 



NERVE-FIBRES. 



205 



1. Medullated Nerve-Fibres (H). Select a small nerve, e.g., 

 the sciatic or one of its branches, of a frog. Cut out half an inch 

 of it, and place it on a dry slide, but add no fluid. Fix one end of 

 the thread by pressing on it with any thin blunt object, e.g., the 

 flat surface of a mounted needle, and fray out the opposite end in 

 a fan-shaped manner with a mounted needle, so as to isolate some 

 nerve-fibres. To prevent it from 

 drying, breathe on the specimen 

 from time to time. Add a drop 

 of normal saline, cover, and ex- 

 amine (fig. 183). 



(a.) Observe highly refractive 

 medullated nerve-fibres of vari- 

 able size, some as broad as a red 

 blood-corpuscle and others nar- 

 rower. Each fibre has a dmille 

 contour, i.e., two thin lines on 

 each side of the centre. The 

 double contour may be inter- 

 rupted here and there. The 

 double contour is due to the 

 white substance of Schwann, 

 medullary sheath, or myelin. 



(!>.} In the centre a clear 

 bright rod, the axis-cylinder. 



(c.) Outside the myelin is a 

 thin transparent sheath, primi- 

 tive sheath or neurilemma, 

 scarcely to be detected as such 

 unless the fibre is ruptured or 

 the sheath raised from the 

 myelin, or stretching as a funnel-shaped prolongation from the end 

 of a torn fibre. 



(d.) Selecting a fibre isolated for a considerable distance, trace 

 its outline, and observe at intervals slight constrictions nodes 

 of Ranvier where the myelin is absent. 



(e.) A small quantity of delicate connective tissue endoneurium 

 with, perhaps, a capillary and a few blood-corpuscles between the 

 fibres. 



(/.) The myelin tends to exude from the ruptured ends of the 

 fibres, and appears as highly refractive spherical droplets myelin 

 drops often with concentric markings, but there is no nucleus 

 in them (fig. 183, I). The myelin drops exude more rapidly in 

 a preparation irrigated with i per cent, acetic acid, or 2 per cent. 

 caustic soda, or even with distilled water alone. 



