APPENDIX. 369 



Tissues to be cut with the freezing microtome are soak- 

 ed in gum ; if they have been kept in strong spirit, 

 they should be placed for a short time in weak spirit, 

 and for an hour or more in water, before being placed 

 in the gum ; it is best to let them soak in the gum for 

 a day ; but sections may be made, though probably 

 less good, after a very short period of soaking; in 

 this case the crystals of ice, which form, rapidly 

 blunt the razor. 



The gum solution is prepared by dissolving gum arabic 

 in warm water, and filtering through linen ; a rather 

 thin solution may be used to soak the tissue in, 

 a thicker one to surround the tissue on the micro- 

 tome. Since gum alone freezes into a hard mass, 

 a little sugar solution may with advantage be added 

 to it. 



The plate of the microtome, and the grooves in it, must 

 be quite clean ; a layer of gum is then spread over 

 it, care being taken that the gum fills up the grooves. 

 "When the tissue has become frozen on the plate of 

 the microtome, the gum around it should be bevelled 

 off so that the section to be cut is not much larger 

 than the piece of tissue. 



A hollow ground razor should not be used, since it 

 bends, and so makes a section, which, under the 

 microscope will be found to consist of bands of 

 unequal thickness. 



The proper rate to carry the razor through the tissue 

 varies with the temperature of the frozen mass ; 

 when it is a few degrees only below zero, the move- 

 ment may be the quickest possible ; when it is frozen 

 hard, it should be carried slowly through the tissue, 

 otherwise the sections are apt to curl or break up. 



L. 24 



