WITH THE MICROSCOPE. 1 I 3 



the solution, it may be soaked out of the formed material, 

 while that taken up by the nuclei (masses of germinal matter) is 

 retained, and may be decomposed by being exposed to light. In 

 this case the nuclei appear very dark and surrounded by a pale 

 brown formed material. His thinks that when the nuclei are 

 coloured, the precipitate of chloride of silver in the formed ma- 

 terial is re-dissolved and absorbed by the nuclei, in which it is 

 afterwards reduced by the action of the light 



200. Solutions of Chloride of Gold. Weak solutions of per- 

 chloride of gold have been much used of late years for colouring 

 nerve fibres, which after exposure to light exhibit a blue or violet 

 tinge. A solution containing from '2 to i per cent in distilled water 

 should be made. The tissue after having been soaked till it becomes 

 straw-coloured is to be washed, and then placed in very dilute acetic 

 acid containing i per cent, or less. The nerves become coloured in 

 the course of a few hours. By this plan Cohnheim professes to have 

 made out very fine nerve fibres, which, he says, pass from the plexuses 

 in the cornea to intervals between the cells of the conjunctival epithe- 

 lium, and after reaching the surface of the structure end in terminal 

 free extremities. I think, however, we should receive such statements 

 with the utmost caution, and although Professor Kolliker has accepted 

 the view, I cannot adopt it without much stronger evidence than has 

 yet been advanced in its favour. Many considerations make me 

 think it will turn out to be incorrect. Cohnheim's drawings alone 

 excite doubt in my mind concerning the accuracy of his observations, 

 and, at least in my hands, the mode of preparation recommended 

 has not afforded results nearly so satisfactory as I have obtained by 

 other methods of investigation. 



207. Solution of Osmic Acid (Os. O 4 ) has been strongly recom- 

 mended for demonstrating delicate nerve structures by MM. Schultze 

 and Roudneff, because it tinges the white substance of Schwann and 

 Myelin entering into the formation of various kinds of nerve fibres, of a 

 very dark colour or almost black. Other textures are neither coloured 

 so quickly nor so intensely, and often exhibit only a brownish tint. 

 So that by this substance nerve fibres ramifying in various textures 

 may be stained and thus distinguished from other elements of the 

 tissue. Solutions of various strengths may be employed, but one 

 part of osmic acid in i oo of water is stated to be strong enough to 

 produce the desired effect. These processes are capable of almost 

 endless modification. 



I have also tried this plan, but have gained nothing by its use. 

 I can show finer nerves clearly by other methods, which I could not 

 demonstrate either by the gold or osmic acid solutions. 



i 



