MAKING SECTIONS 73 



About half a milliliter of syrup is picked up with the pipette and placed on 

 the freezing table of the microtome. A small jet of carbon dioxide is then 

 turned on. Within a moment or two the gum will be seen to have congealed 

 and the carbon dioxide is turned off. The object to be sectioned is then (Fig. 

 54) placed on top of this congealed layer of gum and more gum poured over 

 the surface. Care must be taken that a layer of uncongealed gum lies between 

 the object and the congealed gum, or it may loosen. The carbon dioxide is 

 again turned on, and, as soon as the gum covering the object is seen to be 

 congealing, a little more syrup is poured on the surface so that the object 

 is thoroughly covered. It will be seen from the figure referred to that, as a 

 matter of convenience, the knife has not yet been placed in its holder while 

 these preliminary operations are going on. As soon as the knife has been in- 

 serted, an experimental cut is taken across the top of the material with the 

 knife, and the block is then shaved down until the specimen is reached. The 

 device which controls the thickness of the section to be cut is then set to what- 

 ever thickness has been decided upon. It is strongly recommended that no at- 

 tempt be made to cut sections much less than 20 microns in thickness by this 

 method, and often sections of 30 microns are sufficiently good for diagnostic 

 purposes. It is then only necessary to continue to pull the handle until the 

 object starts to cut, while observing the nature of the sections. If the sections 

 crumble under the action of the knife, while the gum melts instantly on con- 

 tact with it, it may be presumed that the block has not been frozen sufficiently 

 hard; the carbon dioxide may again be turned on for a few moments, and another 

 cut taken. It will only take a moment or two to establish the optimum con- 

 dition under which only slightly curled sections appear on the blade of the 

 knife. As the blade, however, is likely to have become soiled, it is now washed 

 with a drop of warm water to remove the dried gum and then used to cut as 

 many sections as are required. As each section is cut, it must be removed from 

 the blade of the knife to the dish of 70 per cent alcohol shown in the figure. 

 The majority of people working under pressure use their little finger (Fig. 55) 

 for the removal of the section, though a number of very competent technicians 

 prefer to use a brush for this purpose. As soon as the section has been removed, 

 it is transferred to the 70 per cent alcohol where the gum will soon be dissolved. 



Cutting without support is much more difficult and should be confined to 

 homogenous blocks of tissue. In this case the tissue, either fresh or fixed in 

 formaldehyde, is trimmed so that a straight edge will be opposed to the edge 

 of the knife. A drop of water is placed on the surface of the freezing table, 

 the gas is turned on, and the specimen is pressed firmly to the water while it 

 is frozen in place. The finger is then removed and the block watched until the 



