106 CELLO IDIN 



placed on the block holder and the block pressed firmly into 

 position. The whole is then placed in chloroform vapour or in 

 alcohol until the fresh celloidin has set. Celloidin blocks infiltrated 

 with paraffin can be attached with hard paraffin wax, provided 

 the wax is well heaped up around the sides of the block. When 

 mounted, the celloidin block is trimmed so that not more than a 

 few millimetres of celloidin are left around the margins of the 

 tissue. It is not essential to have a block with parallel edges and 

 an unnecessarily large margin of celloidin around the section will 

 make flattening difficult when the section is mounted finally on a 

 glass slide. A little more celloidin should be left on the edge of 

 the block, which first meets the knife edge, to accommodate 

 the manipulation of unrolling and flattening the section as it 

 is cut. 



194. Section Cutting. It is essential to use a sliding microtome. 

 Celloidin offers more resistance to the knife than paraffin wax and 

 hence the block and the block holder must be very rigidly mounted 

 in a heavily built carrier. The angle of adjustment of the knife 

 depends largely on the size of the block and the nature of the 

 tissue. The long axis of the knife must be inclined at an angle 

 between 45° and 70° to the edge of the block which first meets 

 the knife. The blade of the knife is generally inclined fairly 

 steeply to the upper surface of the block, otherwise the 

 cutting edge will tend to jump through and leave an incomplete 

 section. 



(a) Wet cutting method. For celloidin blocks stored in alcohol 

 and for parafiin-infiltrated blocks, the sections are more easily 

 cut with a knife wet with 70 per cent, alcohol. The knife must be 

 free from grease so that the alcohol will spread evenly over its 

 entire surface. The blade is kept flooded with alcohol during 

 the section cutting (some microtomes are provided with a 

 reservoir adjusted so that the alcohol falls on the blade in drops 

 at intervals). The upper surface of the block is also kept wet with 

 alcohol. 



The manipulation required in cutting the sections is not 

 difficult if the knife is sharp and the block is sufficiently rigid. 

 Begin by sliding the knife along towards the block until it just 

 cuts a few millimetres into the corner of the block. Sections 

 thinner than 15/i will generally commence to roll up. The motion 

 of the blade should then be stopped for a moment and the edge 

 of the section unrolled and pressed flat on to the surface of the wet 

 knife with a stiff camel hair brush dipped in alcohol. Then, if 

 the cutting is resumed and the brush is held lightly on top of the 

 section, the latter should slide evenly over the wet surface of the 

 blade. The section can be removed from the knife immediately 

 by fine forceps but generally it is better to slide each section 

 along the blade, arranging them in series if required. When there 



