50 PRACTICAL HISTOLOGY [VII 



a low power; if there are notches in it, it requires 

 re-grinding. 



2. Ice and Salt Freezing Microtome, a. Ice 



is broken into fragments about the size of walnuts ; salt 

 is sprinkled liberally over the ice; and the mixture 

 packed in the box of the microtome 1 . The plate of 

 the microtome remains below freezing-point for 2 to 

 3 hours. 



Clean the plate of the microtome and the grooves 

 in it. Place the piece of tissue (given you in gum) in 

 the centre of the plate. If there is much gum around 

 the tissue cut it away after it is frozen. 



Take the tripod frame which carries the razor, 

 and see that the razor is firmly fixed by the screws. 

 Then adjust the level of the razor by means of the 

 three screws supporting the frame in the following 

 manner : 



Screw up the front screw till \ to f of it is below 

 the frame. Adjust the back right-hand screw so that 

 the right-hand edge of the razor will just cut into the 

 frozen gum over the tissue ; adjust similarly the back 

 left-hand screw; repeat these two adjustments, so that 

 whatever part of the razor is used in cutting, sections 

 of nearly the same thickness will be obtained. 



Hold the frame firmly, but without unnecessary 

 pressure, and push it straight forward over the tissue ; 

 this should cut off a portion of the gum. Lift up the 

 frame or its anterior end, carry it back, turn down- 

 ward the front screw th of a revolution, push the 



;ing mixture made by pounding together equal parts of 

 great a lowering of temperature, and the 

 iy to cut. 



B R A R V 



