CHAPTER VII. 13 



NERVE. 



Medullated Nerve. Fresh.* Tease in normal saline a piece of 

 the sciatic or the dorsal cutaneous nerve of a frog, cover and recognise. 



(L) the cylindrical fibres colourless and transparent. (//) 

 Single out a fibre, note its smooth appearance and double 

 outline. Find a node of JRanvier, a constricted interruption of 

 the medullary sheath, which occurs at long intervals (1 mm ) on 

 the fibre, very soon the sheath begins to lose its smooth appear- 

 ance, through the swelling of the myeline, and exhibits 

 corrugations. Just at the commencement of this change, at 

 short intervals, the myeline exhibits slight inflections, or 

 actual oblique slits, indicating a subdivision into shorter 

 segments. These are the " incisures." The medullary sheath 

 appears to be made up of short segments (Schmidt- Lantermanri), 

 the ends of which fit into each other. The semi-fluid myeline 

 in the living condition, does not show these planes of cleavage, 

 and hardening reagents do not always bring them into view, 

 or with the same appearance. 



Osmic nerve. Tease in glycerin some nerve which has been in 

 ^ i'- c - osmic acid solution for 24 hours, well washed with water, and 

 .stained in picrocarmine for several days, and then placed in glycerin. 



(L) The myeline of the fibres is stained black, and there 

 is connective tissue between them stained pink. (H) The 

 nerve fibres are not all of the same size, and the smaller 

 ones, owing to the thinness of their medullary coat, appear- 

 only faintly stained. Find a node in an isolated fibre, note 



1 Nerves in a living condition. See tongue of frog, pg. G.'>. 



