30 FIXATION 



After fixatives have acted, they must be washed out of the tissues 

 so as to prevent the formation of extrinsic artifacts by their incom- 

 patibiHty with fluids in which the tissue must subsequently be 

 placed. In some cases the fluids that must be used for other pur- 

 poses (for instance, the alcohols used for dehydration) themselves 

 act as solvents for a fixative; in other cases special methods of 

 washing out must be adopted. Since the methods of elimination 

 are specific to each fixative, they will be considered separately 

 in chapters 5 and 6. 



When a tissue has been fixed, one may sometimes wish to pre- 

 serve it indefinitely before embedding. This applies especially on 

 scientific expeditions, when there are usually no facilities for 

 embedding. The fixative itself may be unsuitable for indefinite 

 preservation, either because it is volatile or because it will eventu- 

 ally produce artifacts. In such cases a post-fixation preservative 

 may be used. Solutions of ^-hydroxybenzoic acid and its esters 

 are suitable. ^^^ These fluids are of an entirely different nature from 

 the preservatives mentioned at the beginning of this chapter, for 

 they are quite unsuited to the preservation of fresh tissues. 



