1 92 PRACTICAL MICROSCOPY. 



Pour off the spirit every day and replace it with fresh 

 until the excess of chromic acid has been abstracted, known 

 by the spirit remaining clear and colourless. 



Injected kidney, and, indeed, most other injected tissues, 

 must be hardened entirely in alcohol, as the chromic acid, 

 being a powerful oxidiser, would in all probability cause a 

 decomposition of the colouring matter. Kidney is often 

 injected with Prussian blue. In this case it is as well to add 

 a few drops of hydrochloric acid to the alcohol used in 

 hardening. 



The hardened tissue is now ready for cutting, and this 

 operation may be illustrated by employing the Rutherford 

 microtome, illustrated in Fig. 174. 



The substance when taken out of the spirit must be 

 placed on blotting paper, and allowed to become surface 

 dry. When this is effected it must be dipped into the gum 

 solution before mentioned, and, after removal, is to be again 

 laid upon the paper until dry. 



The microtome having been screwed to the table, and the 

 plug lowered by means of the screw, the melted mixture of 

 naphthalin and stearine is to be poured into the well. 

 When just about to set, the substance is inserted carefully, 

 and held in position until the bedding has sufficiently 

 solidified. After an hour or more, according to the tempera- 

 ture of the atmosphere, the substance will be in a fit state 

 for cutting, and may be performed in exactly the same 

 manner as described for vegetable stems. 



The section knife is best kept in order on an ordinary 

 razor-strop, with an occasional touch on a fine Washita oil- 

 stone. 



Fig. 179 shows a longitudinal section of the injected 

 kidney of a rat, a being the arterial trunk, b the venous 

 trunk, and c the glomerulus ; while Fig. 180 is a section of 

 human muscle showing Trichina spiralis in situ. 



