vin SECTION-CUTTING 137 



piece, place in water under a tap, and wash thoroughly for a quarter 

 of an hour or more, until the corrosive sublimate is removed. 



After washing, transfer to 50 per cent, alcohol for a few hours, 

 and then to 70 per cent, for twenty-four hours, after which the pieces 

 may be stained at once (see below), or transferred to strong methylated 

 spirit (90-93 per cent.), in which they may be kept until wanted. This 

 completes the process of hardening : it is done gradually, by alcohols of 

 increasing strength, in order to avoid shrinkage. 



In order to decalcify such tissues as bone, from which the lime-salts 

 must be extracted before cutting into sections, place a small piece for a 

 few days in 70 per cent, alcohol to which 2 per cent, of strong nitric 

 acid has been added : then wash thoroughly, transfer to alcohol, and 

 stain. 



b. Staining. Place the organs, cut into convenient sizes for im- 

 bedding i.e. not more than inch long, and, in the case of such 

 organs as the liver, \ inch in thickness into borax-carmine or hoema- 

 toxylin for one or two days or even more. They will become stained 

 throughout, and the difficulty of staining the sections after cutting will 

 thus be avoided. After staining, place them in weak alcohol (50-70 

 per cent.), slightly acidulated with hydrochloric acid : if a watch-glass 

 or some such vessel is used, it is sufficient to dip the end of a glass rod 

 into acid and stir it round in the alcohol. The effect of the acid is to 

 remove much of the colour from the protoplasm of the cells, leaving 

 the nuclei brightly tinted. After half-an-hour or less in the acid alcohol, 

 place the tissues once more in strong methylated spirit. 



c. Dehydrating. Transfer from the methylated spirit to absolute 

 alcohol, which must be kept in a stoppered or tightly corked bottle, as 

 it will otherwise deteriorate by absorption of water from the air. It has 

 the effect of withdrawing the last traces of water from the tissues, an abso- 

 lutely necessary step in order that they may be permeated with paraffin. 



d. Imbedding. Transfer the objects from absolute alcohol to turpen- 

 tine. This fluid acts as an intermediary between alcohol and paraffin, 

 being freely miscible with both : it gradually replaces the alcohol in the 

 tissues, rendering them transparent. If they are not transparent in the 

 course of an hour or so, the process of dehydration has not been com- 

 plete, and they must be returned to absolute alcohol. In the mean- 

 time, melt some paraffin over the water-bath, using various mixtures 

 of hard and soft according to the season : in a cold room in winter soft 

 paraffin will be hard enough ; in the height of summer hard paraffin 



