Vll] SECTION CUTTING AND IMBEDDING 57 



this, do not put the left hand over the razor, but adjust the 

 carrier from the right-hand side. Screw up firmly the carrier. 

 Push the guard of the catch back as far as it will go, and move 

 the handle until the first section is obtained, then adjust the 

 guard so that the catch turns the milled wheel 8 to 10 teeth. 

 Each forward movement of the handle now gives a section which 

 is flat and adheres to the preceding section. When 3 or 4 are 

 obtained, lift them up with a forceps, so that the ribbon of 

 sections runs free of the razor. 



The chief causes of bad sections are (1) over- hardness of the 

 tissue ; this may be normal to the tissue, or produced by warming 

 in agents such as xylol, or to overheating in the warm bath ; 

 (2) imperfect imbedding ; this may be due to a lack of sufficient 

 penetration of the paraffin, to crystals of paraffin having been 

 formed, or to the presence of small bubbles of air. The surface 

 of the block to be cut should be examined with a lens after it has 

 been shaped, if patches of crystals or bubbles are visible it is 

 best to re-imbed ; (3) bluntness or notches in the razor. 



Orienting the imbedded tissue. It is often requisite to cut the 

 tissue in a particular plane. This can be done approximately in the 

 following way : the paraffin over the tissue is cut in the desired 

 plane, the opposite surface of the paraffin is fused to the holder, and 

 whilst the junction is still soft, the holder is slid along a horizontal 

 groove in wood or metal against a vertical piece of glass at the end of 

 the groove, and gently pressed against it until the paraffin at the 

 junction has become hard. For more delicate adjustment, a special 

 holder should be used, in which the part carrying the paraffin can be 

 fixed in any position. 



8. Cutting sections in paraffin melting at 58 C. Paraffin 

 of high melting point allows thinner sections to be cut than 

 paraffin of low melting point. On this account paraffin melting 

 at 58 G. is sometimes used. But sections of the paraffin melting 

 at 58 C. are apt to curl and do not adhere to a ribbon at ordinary 

 room temperatures. They form a ribbon, however, if the block 

 is coated with paraffin of a low melting point. 



Imbed a piece of tissue in paraffin melting at 58 C., shape the 

 block, and fix it in the usual way to the carrier of the microtome. 



