90 MICROSCOPIC METHODS 



is sufficient to keep it in this reagent for a few hours. If the pieces 

 are thicker a longer exposure is necessary, and in such cases it is better 

 to change the alcohol at the end of the first twenty-four hours. The 

 tissue must be tough without being hard, and the necessary consistence, 

 as estimated by feeling with the fingers, can only be judged of after 

 some experience. If the tissues are not to be cut at once, they may be 

 preserved in 50 per cent spirit. 



(b) Formol-alcohol formalin 1, absolute alcohol 9. Fix for not more 

 than twenty-four hours ; then place in absolute alcohol if the tissue is 

 to be embedded at once, in 50 per cent spirit if it is to be kept for some 

 time. For small pieces of tissue fixation for twelve hours or even less is 

 sufficient. The method is a rapid and very satisfactory one. 



(c) Corrosive sublimate is an excellent fixing agent. It is best used 

 as a saturated solution in '75 per cent sodium chloride solution. Dis- 

 solve the sublimate in the salt solution by heat ; the separation of 

 crystals on cooling shows that the solution is saturated. For small 

 pieces of tissue ^ inch in thickness, twelve hours' immersion is sufficient. 

 If the pieces are larger, twenty-four hours is necessary. They should 

 then be tied up in a piece of gauze, and placed in a stream of running 

 water for from twelve to twenty-four hours, according to the size of the 

 pieces, to wash out the excess of sublimate. They are then placed for 

 twenty-four hours in each of the following strengths of methylated 

 spirit (free from naphtha l ) : 30 per cent, 60 per cent, and 90 per cent. 

 Finally they are placed in absolute alcohol for twenty-four hours and 

 are then ready to be prepared for cutting. 



If the tissue is very small, as in the case of minute pieces removed 

 for diagnosis, the stages may be all compressed into twenty-four hours. 

 In fact after fixation in corrosive the tissue may be transferred directly 

 to absolute alcohol, the perchloride of mercury being removed after the 

 sections are cut, as will be afterwards described. 



(d) Methylated Spirit. Small pieces of tissue may be placed in 

 methylated spirit, which is to be changed after the first day. In from 

 six to seven days they will be hardened. If the pieces are large, a 

 longer time is necessary. 



The Cutting of Sections. 1. By Means of ttte Freezing 

 Microtome. Pieces of tissue hardened by any of the above 

 methods must have all the alcohol removed from them by wash- 

 ing in running water for twenty-four hours. They are then 

 placed for from twelve to twenty-four hours (according to their 

 size) in a thick syrupy solution containing two parts of gum 

 arabic and one part of sugar. They are then cut on a freezing 

 microtome and placed for a few hours in a bowl of water so that 

 the gum and syrup may dissolve out. They are then stained or 

 they may be stored in methylated spirit. 



1 In Britain ordinary commercial methylated spirit has wood naphtha 

 added to it to discourage its being used as a beverage. The naphtha being 

 insoluble in water a milky fluid results from the dilution of the spirit. By 

 law, chemists can only sell 8 ounces of pure spirit at a time. Most patho- 

 logical laboratories are, however, licensed by the Excise to buy "industrial 

 spirit," which contains only one -nineteenth of wood naphtha. 



