VHi SECTION-CUTTING 137 



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

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



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

 and then to 70 per cent, for twenty-four hours, after which tliey may 

 be stained at once (jve- lielow), fir 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 deiahify 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, -g inch in thickness — into borax-carmine 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 tis.sues 

 once more in strong methylated spirit. 



c. Dehydrating. — Transfer from 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- 

 j^lete, 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 



