92 MICROSCOPIC METHODS. 



embedding the tissue in solid paraffin and cutting with some 

 of the more delicate microtomes of which, for pathological 

 purposes, the small Cambridge rocker is by far the best. For 

 bacteriological purposes embedding in celloidin is not advisable, 

 as the celloidin takes on the aniline dyes which are used for 

 staining bacteria, and is apt thus to spoil the .preparation, and 

 besides thinner sections can be obtained by the paraffin method. 

 The Fixation and Hardening of Tissues. — The following 

 are amongst the best methods for bacteriological purposes : — • 



(a) Absolute alcohol may be used for the double purpose of fixing and 

 hardening. If the piece of tissue is not more than \ inch in thickness it is 

 sufficient to keep it in this reagent for one or two days. 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 jDer 

 cent spirit. 



(b) Formol-alcohol — formalin i, absolute alcohol g. 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. 



(t) Corrosive sublimate is an excellent fixative agent. It is best used as 

 a saturated solution in .75 per cent sodium chloride solution. Dissolve the 

 sublimate in the salt solution by heat ; the separation of crystals on cooling 

 shows that the solution is saturated. For small pieces of tis.sue \ 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 ') : 30 per cent, 60 per cent, and go 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. 



^ In Britain ordinary commercial methylated spirit has wood naphtha added to it 

 to discourage its being used as a bevetage. 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 pathological laboratories are, however, 

 licensed by the Excise to buy pure spirit in large quantities. 



