Microtome Sectioning of Material in Paraffin 47 



slicing and minimizes compression and wrinkling. At the moment of 

 contact the feed mechanism is disengaged and the only wear is on 

 sliding surfaces. High speed during the entire revolution makes 

 violent impact between the hardened steel j^awl and the much softer 

 ratchet wheel, and the pawl may skip a tooth, or strike the top of 

 a tooth. Excessive speed therefore produces excessive wear and is 

 inexcusable, unless the laboratory is lavishly financed. The use of 

 a motor drive, especial!) in the hands of the untrained "hired help" 

 that is sometimes used, is open to objections. 



A curved ribbon may be the result of one or more of the follow- 

 ing conditions: 



1. A chill spot on the knife; shift the knife laterally in its holder or replace 

 with a good knife. 



2. The upper and lower edges of the paraffin block are not parallel: trim 

 with a razor blade. 



3. The lower edge of the paraffin block is not parallel to the knife-edge; 

 adjust the object clamp. 



4. The piece of tissue is not centered laterally in the paraffin; trim the 

 unequal side. 



5. The piece of tissue is of irregular shape and bulk. In Fig. 6.4C the com- 

 paratively empty right side of the paraffin will compress more than the left 

 side, producing a curved ribbon (B) . This may be corrected by trimming the 

 upper face of the paraffin block (along the dotted line in Fig. 6.4C) . 



The method of straightening a slightly cur\ed ribbon on the 

 slide is described later. Handle the ribbon with a small brush. Do 

 not permit a needle or scalpel to touch the knife-edge. The slightest 

 contact will turn the fine cutting edge. For beginners a quill-shanked 

 brush is the safest implement. Lay the segments of ribbon, in the 

 order of removal from the knife, on clean, lintless black paper, and 

 keep in a cool dust-free place until you are ready to attach them to 

 slides. For handling long ribbons in serial order, cylindrical ribbon 

 holders are manufactured. Their operation is obvious from the 

 catalogue illustrations. The foregoing brief outline of the operation 

 of the rotary microtome should be supplemented by observing the 

 methods of experienced workers. Skill can be acquired only by ex- 

 perience with a wide range of subjects, and a thoughtful analysis of 

 failiues. 



The condition of the cells and tissues in the ribbon can be judged 

 with considerable accuracy. Examine the ribbon with a magnifier or 

 binocular microscope. 1 he paraffin should be firmly attached to 

 external surfaces and shovdd fill all wrinkles, folds, and visible 

 cavities. Inadequate infiltration may be one cause of separation of 



