NER VE- TISSUE. 43 



the myeline, it is quickly stained red by the carmine. {Indicate a node and the incisures in 

 PI. VIII., Fig 2.) 



It is remarkable that nearly all the phenomena which have been laboriously made out by 

 the use of special reagents can be seen in a perfectly fresh or even in a living nerve-fibre. 



EFFECT OF OSMIC ACID. 



PREPARATION. Kill a frog ; dissect out a sciatic nerve. Tie a fine thread round each 

 end of the nerve, carefully preserving it from getting dry, and by means of the thread stretch 

 the nerve on a small piece of wood an ordinary wooden match with a slit at either end to 

 hold the thread answers admirably. Place the nerve and wood in a one per cent, osmic acid 

 solution for ten minutes. The effect of the osmic acid is to blacken the phosphorised fats 

 of the myeline and to ' fix ' its elements. The nerve is stretched in order that the nodes and 

 incisures may be made quite distinct. Remove it and wash it carefully in water, and then 

 place it, still on the stretch, in a test-tube with picrocarmine. In twenty-four hours the stain- 

 ing is complete. I find it to be very advantageous to leave it in picrocarmine for a fortnight 

 or longer. The effect of this is to soften the connective-tissue of the nerve, so that on teasing 

 it at the end of this time the single nerve-fibres are easily obtained. Tease a small piece 

 always tearing off a fibre in its long axis in a drop of glycerine. Cover. 



EXAMINATION (H). Observe the myeline, blackened and still retaining its double con- 

 tour. Trace a fibre and observe, at pretty regular intervals, the constrictions or nodes of 

 Ranvier, and between each two nodes a brightly stained nucleus imbedded in a trace of pro- 

 toplasm, will be found lying under the primitive sheath and partly indenting- the myeline. 

 (Indicate two nodes in PI. VIII., Fig. 5.) Study a node. Observe the absence of the myeline, 

 and that only the slightly red-coloured axis-cylinder passes from one segment of the nerve to 

 another. The axis-cylinder, therefore, is the only essential part of a nerve. Some excessively 

 delicate nerve-fibres will be seen. The delicate connective tissue supporting the nerve-fibres 

 is easily made out. (Indicate a node, t/te nucleus, and the incisures in PI. VIII., Figs. 3 and 4.) 



Amongst the medullated fibres may be detected the grey or sympathetic nerve-fibres. 

 They are distinguished by being delicate fibres, with a large oval nucleus here and there in 

 their course, and are devoid of myeline, and therefore do not possess a double contour. 



ACTION OF NITRATE OF SILVER. RANVIER'S CROSSES. 



PREPARATION. () Tease out roughly a small piece of a perfectly fresh sciatic nerve of 

 a frog in a quarter per cent, solution of silver nitrate on a slide, and leave it in this solution for 

 five minutes or so. Wash it with water and then tease it carefully in glycerine, cover and 

 expose it to the action of light until it becomes brown. 



(ft) The small intercostal nerves of a rat or rabbit may be excised and plunged entire into 

 silver nitrate solution and mounted entire in glycerine, and exposed to the action of light as 

 above. 



EXAMINATION (L and H). Observe a fibre and notice the dark brown crosses seen here 

 and there (PL VIII., Fig. 8). The long limb of the cross is produced by the silver pene- 

 trating to the axis-cylinder, and staining it, or an albuminous material covering it, for a short 

 distance ; whilst the transverse, more deeply stained, bar is due to the cement which unites the 

 so-called segments of a nerve one to another (PI. VIII., Fig. 9). Sometimes the longitudinal 

 limb of the cross may be marked transversely by lines (Frommann's lines). If a whole nerve 

 be used, a complete investment of endothelial cells will be found outside all. 



