2O6 PRACTICAL HISTOLOGY. [XVII. 



Stain the preparation with picro-carmine, and note that this 

 reagent diffuses into the fibres at their cut ends and 

 at the nodes of Ranvier. It stains the axis-cylinder 

 red, and also the nerve-nuclei or corpuscles which 

 iie J ust un der the neurilemma. 



2. Nerve-Fibres in Osmic Acid (H). A nerve- 

 fibre is rapidly blackened by osmic acid, as can be 

 shown by applying a drop of i per cent, solution to 

 a fresh nerve, but for good permanent preparations 

 it is well to stain the nerve after the action of the 

 osmic acid. Place a small piece of nerve in a small 

 glass thimble along with 2 cc. of .5 per cent, osmic 

 acid. Cork the thimble, and after twenty-four hours 

 thoroughly wash the preparation ; tease it a little, 

 and place it for twenty-four hours in a solution of 

 picro-carmine. In fact, if it be left for days in this 

 dye it is better, as the fibres can then be more readily 

 dissociated. Tease a small fragment, and mount it 

 in glycerine acidulated with formic acid. 



(a.) Observe in each fibre the myelin stained black. 

 Search for a node of Ranvier, a narrow constriction, 

 and note .that the myelin is absent at the constriction, 

 although the axis-cylinder and neurilemma are pre- 

 sent (figs. 184, a, 185). Find the next node, and, 

 between the two adjoining nodes, the stretch of 

 nerve the internodal or interannular segment 

 (fig. 181). 



(6.) In the interannular segment, just under the 

 neurilemma, and lying in a slight depression of the 

 myelin (lig. 184, 6), a red-stained oval nucleus sur- 

 rounded by a small quantity of protoplasm, and 

 about midway between the two nodes (fig. 185, n). 

 The axis-cylinder stained red, and continuous 

 throughout the fibre. 



(c.) In the myelin what look like oblique slits 

 incisures running obliquely outwards from the 

 axis-cylinder to the neurilemma (fig. 185, i). They 

 correspond on each side, and break up the myelin 

 Fl Fibre 85 '~(Osmfc into a number of short lengths cylinder cones- 

 Acid), a. Axis- the bevelled end of one cylinder-cone fitting into the 



cylinder; r. Node -,11 n i i c Ji i v i 



of iianvier ; . in- oppositely bevelled end oi the next cylinder-cone. 



cleS^'w "prokt Many cones lie in an internode. 



piasn'; s. Neuri- (cZ.) Some of the fibres are broad, and others 



narrow, about half the breadth of the others. 

 (e.) Some fibres are not blackened by osmic acid at all. They 



