HISTOLOGICAL METHODS. 163 



Melt the paraffin mixture in a water bath, at as low a 

 temperature as possible, pour it into a paper cone or paper 

 saucer, or into the well of a microtome. With forceps, dip 

 the tissue into the paraffin, at once remove it, and, when 

 the layer of paraffin has cooled, replace it finally in the 

 paraffin. By this means the overheating of the tissue will 

 be avoided. Tissues to be imbedded in paraffin should be 

 as dry as possible, otherwise the supporting substance will 

 not cling to the tissue. The water may be removed from 

 the surface of the tissue by immersing it for a short time in 

 spirit, and then allowing it to dry. 



The great fault of paraffin is, that its slices get in the 

 way, and often spoil the slice of the tissue while it is being 

 made ; moreover, it is difficult to get rid of. The sections 

 require to be picked out from its debris, or this must be 

 washed away from them. This implies considerable loss of 

 time. 



301. Imbedding in Gum. a. This method, which we 

 owe to Briicke, is a very valuable one. The solution of 

 gum should be as thick as possible. It is made by pouring 

 cold water on " picked " (clean} gum-arabic, and stirring it 

 occasionally. It is extremely useful for imbedding the lung, 

 and other tissues of irregular configuration, where an inter- 

 stitial and not merely an external support is necessary. 

 Any alcohol in the tissue must always be carefully removed 

 by immersion in water for twenty-four hours or so previous 

 to placing them in the gum. 



b. For imbedding the lung, a small piece that has been 

 previously hardened by chromic acid in the usual way, and 

 from which all spirit has been removed, is placed for thirty- 

 six or forty-eight hours in a thick solution of gum. It is 

 then transferred to methylated spirit, in which it soon be- 

 comes sufficiently hard to permit of easy section. 



c. For the retina, cochlea, small intestine, and other 

 parts with delicate tissues needing careful support, the 

 following method is used : Place the tissue in a cone of 

 bibulous paper containing thick solution of gum for six to 

 twenty-four hours. Then set the cone with its contents in 

 methylated spirit for forty-eight hours or so, to remove the 



