416 H1STOLOGICAL TECHNIQUE. 



gum-arabic, and to moisten the plate with the same before freezing. 

 This solution freezes in less coarsely crystalline form than water or 

 salt solution. 



When the tissues are frozen, thin sections are removed with a 

 quick forward and slightly oblique stroke of the knife. The motion 

 is intermediate between that of a plane and a single stroke of a saw. 

 The sections are floated from the knife in a dish of water or normal 

 salt solution ; or they may be fixed in a 4 per cent, solution of for- 

 maldehyde. The frozen tissue must not be too hard. Should that 

 be the case, the upper surface may be moistened by means of a 

 camePs-hair brush, dipped in water or salt solution, or warmed 

 with the breath. 



2. Collodion-sections. The block upon which the embedded speci- 

 men is fastened is secured in the clamp of the microtome in such 

 a position that the sections will be made in the desired plane. 

 The knife is then adjusted on its carrier in an oblique position, so 

 that the greatest possible length of its edge will be utilized in cut- 

 ting. The upper surface of the knife is flooded with 80 per cent, 

 alcohol, and slices are removed with the knife until the desired 

 level of the specimen has been reached. Sections are then made 

 as thin as is compatible with obtaining complete sections from the 

 whole surface. The sections float in the 80 per cent, alcohol, with 

 which the knife should be kept flooded, and may be removed with 

 a camel's-hair brush. At no time should either the knife or the 

 specimen be allowed to dry. The sections may be kept indefinitely 

 in 80 per cent, alcohol, or they may be dropped into water if they 

 are to be used within a short time. 



After use, the knife should be carefully wiped, stropped, and placed 

 in its case. The microtome should be dried and the tracks moistened 

 with a little oil of sweet almonds or paraffin oil, to prevent rusting. 



3. Paraffin-sections. The knife should be fixed perpendicular to 

 the direction of cutting, its edge acting like that of a plane. Its 

 surfaces must be clean and dry ; adherent paraffin can be removed 

 with a cloth moistened with xylol. 



The paraffin-block containing the specimen to be cut is firmly 

 clamped with one of its narrow edges parallel to the edge of the 

 knife. The block is now raised and moderately thick slices re- 

 moved until the desired level is reached, when the thin sections 

 desired may be cut. It not infrequently happens that the sections 

 roll before the edge of the knife. This is probably due to the 



