8 9 



fixer known, but its penetrating power is weak, and it gives good 

 results only when very small pieces of tissue are taken ; furthermore, 

 the number of stains that give good results after its employment is 

 limited. On the other hand, such fluids as Miiller's fluid or Er- 

 licki's fluid should be avoided when the preservation of finer nuclear 

 structures is desired. Within these limits, the fixer that will 

 give the best results in a special case must be determined by experi- 

 mentation, or the experiments and results of others accepted and 

 their methods applied. 



(b) The time a fixer is allowed to act must be considered in 

 connection with the character of the fluid and the tissue. Often the 

 exact limitation of time is a matter of secondary importance ; in 

 other cases, however, its disregard affects the results seriously, and 

 as a general rule, there is a minimum and a maximum time and be- 

 tween them an optimum time that should be adhered to. 



(r) After the tissue has been in the fixing fluid a proper length 

 of time, it is necessary that it be washed thoroughly to remove the 

 fixer from it. This should be done by means of water or alcohol or 

 both. In general, fixers containing salts insoluble in alcohol, or but 

 slightly soluble, as osmic acid, chromic acid, potassium dichromate, 

 etc. , should be thoroughly washed in water. Fixers containing pic- 

 ric acid should always be removed by alcohol ; mercuric chlorid may 

 be washed out by either water or alcohol. 



Inadequate washing out of the fixer may either seriously affect 

 the cutting quality of the tissue if it is to be subsequently imbedded, 

 the ease with which it can be stained, or there may be formed pre- 

 cipitates in the tissue, giving illusory effects, distortions, or at least 

 a dirty appearance to the preparation. Time in properly washing 

 out a fixer is always well spent, as it is a matter for serious attention. 



20. Resume. In brief, then : In fixing, take relatively 

 large amounts of fluid and small pieces of tissue, choose the fixer 

 well with a view to the tissue and the results desired, regulate the 

 time carefully, and wash out thoroughly. 



HARDENING. 



21. Each fixer has also more or less of a hardening action 

 upon the tissue. Some fluids spoken of above as fixers are preemi- 

 nently hardeners, such as Miiller's fluid and Erlicki's fluid ; while 



