SHEATH OF NERVE. 127 



Preparation 2, Osmic acid possesses the pro- 

 perty of staining fatty matters of an inky black 

 color. Since the medullary sheath of the nerves is 

 mainly composed of a phosphuretted fat, this becomes 

 accordingly stained by the acid, whilst the other 

 parts are left of a grayish tinge. Moreover, the 

 regular breaks in the fatty sheath the nodes of 

 Ranvier are by this means brought into promi- 

 nence, owing to the complete intermission of the 

 darkly stained medullary substances at those places. 

 At the same time the axis-cylinder can be distinctly 

 made out crossing the interval, and the primitive 

 sheath, or sheath of Schwann, can be often seen both 

 here and also near the centre of each of the segments 

 into which the fibre is divided, where it is bulged 

 out by the presence of the oval nucleus. The method 

 for preparing these osrnic preparations is as follows : 

 From an animal which has been quite recently killed, 

 a small nerve, not larger in diameter than an ordi- 

 nary thread, is chosen, and a piece about half an inch 

 long is cut out, but in doing so care must be taken 

 not to drag upon or injure the nerve more than is 

 absolutely necessary. The piece is placed for four 

 hours in a little covered glass pot contaiug a drop 

 or two of a 1 per cent, solution of osmic acid ; it is 

 then transferred to water for an hour, and finally 

 placed in a mixture of glycerine and water (equal 

 parts). It can either be teased in this at once or 

 left for a day or two or even longer ; in preparing 

 it, the same precautions must be used as were recom- 

 mended for the preparation of the fresh nerve (see 

 preceding paragraph). 



Preparation 3. Fibres of Remak. To see the 

 gray or non-medullated fibres, pieces of the sympa- 

 thetic nerve are taken from the neck of an animal 

 and prepared and examined both fresh and after 

 treatment with osmic acid, in the same manner as 

 the cerebro-spinal nerves. 



Mode of union of the nerve-fibres to form 

 the nervous cords. In the several teased prepara- 



