Spinal Cord for Microscopical Examination. 29 



1 part, paraffin 5 parts. This mixture flows all round it and so 

 supports it. 



It is better than wax and oil when a machine is used, because 

 the latter shrinks so much when it cools that it becomes loose in the 

 box of the machine. 



The mixture is heated in a water-bath, and poured into the hole 

 of the machine, and the tissue, previously placed in absolute alcohol 

 for ten minutes, is placed in the composition by means of forceps, 

 and held there till it cools sufficiently to support it. 



When the composition has cooled, sections may be cut by moving 

 a razor across the top of the machine. It must be pushed obliquely 

 in a direction away from right to left, and the whole section must be 

 cut by one sweej) of the razor. A fiat hiife is never used. An 

 ordinary razor does very well, if its back be ground quite straight. It 

 is important that both surfaces of the blade should be concave. The 

 concavity of the surface which is below, when the razor is used in 

 the machine, permits of the face of the knife being kept closely 

 applied to the brass table on the top of the machine, while the con- 

 cavity in the upper surface permits of the presence of a pool of 

 spirit or other fluid for the slice of tissue to float over. The razor 

 must be kept wet with methylated spirit or absolute alcohol. The 

 finest sections are made by wetting the razor with absolute alcohol ; 

 but for ordinary purposes methylated spirit, or even water, answers 

 fairly well. The sections are transferred from the knife to spirit or 

 water, and any adherent paraffin is got rid of by gently moving them 

 round and round with a camel-hair pencil and pouring ofi" the fluid 

 with the floating paraffin. They are then washed in water and 

 placed for half an hour in a 1 per cent, solution of bichromate of 

 potassium in water, and are then ready for staining. 



Thirdly, with regard to the staining/ of the tissues. 



By staining the hrain and spinal cord the structure is rendered 

 clearer than if they are examined without being stained. I propose 

 only to deal with the staining by carmine. 



Various solutions of carmine have been proposed, and after very 

 many trials of the difi'erent solutions, it was found that a modifica- 

 tion of the fluid proposed by Dr. Beale was best. The fluid re- 

 commended by Dr. Beale has this disadvantage, with regard to the 

 staining of nerve tissue, however, in that it is much too strong, 

 and that it stains everything evenly. Thus, for instance, when a 

 cord is placed in it, the parts are all coloured alike, and con- 

 sequently the white siibsiance of Schivann, instead of presenting 

 an uncoloured ring round the coloured axial cylinder, is coloured 

 like the latter, and can with difficulty be difierentiated from it, gives 

 the appearance which would lead you to suppose the nerve fibril 

 was all of one composition. And the 7ierve cells, instead of stand- 

 ing boldly out, are fused, as it were, into the mass of the grey 



