INTRODUCTORY. 



things are implied by the word " fixing " : first, the rapid killing of 

 the element, so that it may not have time to change the form it had 

 during life, but is fixed in death in the attitude it normally had 

 during life ; and second, the hardening of it to such a degree as may 

 enable it to resist without further change of form the action of the 

 reagents with which it may subsequently be treated. Without good 

 fixation it is impossible to get good stains or good sections, or prepara- 

 tions good in any way. 



The structure, having been duly fixed by one of the processes 

 described in the chapter on Fixing Agents, is, except in special cases, 

 washed in order to remove from the tissues as far as possible all 

 traces of the fixing reagent. 



The kind of liquid with which washing out is done is not a matter 

 of indifference. If corrosive sublimate (for instance), or osmic acid, or 

 a solution into which chromic acid or a chromate enters, have been used 

 for fixing, the washing may be done with water. But if certain other 

 agents, such as picric acid, have been used, the washing should be done 

 with alcohol. The reason for this difference is that the first -named 

 reagents (and, indeed, all the compounds of the heavy metals used for 

 fixing) cause certain of the elements of the tissues to become less reactive, 

 and partly or wholly insoluble in water. The combinations of picric 

 acid and the elements of the tissues, on the other hand, are generally 

 much more soluble in watery fluids. 



These operations having been duly performed, two roads become 

 open. The object may be further prepared by what may be termed 

 the wet method, in which all subsequent operations are performed 

 by means of aqueous media. Or it may be further prepared by the 

 dehydration method, which consists in treatment with successive 

 alcohols of gradually increasing strength, final dehydration with 

 absolute alcohol, imbibition with an essential oil or other so-called 

 clearing agent which serves to remove the alcohol, and lastly either 

 mounting at once in balsam or other resinous medium or imbedding 

 in paraffin for the purpose of making sections. The dehydration 

 method is the course which is generally preferred, chiefly because of 

 its great superiority as regards the preservation of tissues. For the 

 presence of water is the most important factor in the conditions that 

 bring about the decomposition of organic matter, and its complete 

 removal is the chief condition of permanent preservation. 



3. Dehydration. The further course of preparation by the 

 dehydration method is as follows : At the same time that the 

 superfluous fixing agent is being removed from the tissues, or as 

 soon as that is done, the water of the tissues must be removed. This is 



