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tilting for it the preservative liquid or other reagents which 

 it is desired to employ. Ulterior preparation comprehends 



the processes sketched out in 3 et seq. 



2. Preliminary Preparation. The first thing to be done with 

 any structure is to fix its histological elements. (This state- 

 ment applies equally to all classes of objects, whether it be 

 desired to cut them into sections or to treat them in any other 

 special way.) Two 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 re- 

 agents with which it may subsequently be treated. Without 

 good fixation it is impossible to get good stains or good 

 sections, or preparations good in any way. 



The structure having been duly fixed by one of the pro- 

 cesses 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 malt.-r 

 of indifference. If corrosive sublimate (for instance), or osmic ;t<-i<l. or 

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

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

 agents, such as picric acid, Lave been used, the washing should lie done 

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

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

 fixing) enter into a state of chemical combination with the elements of 

 tissues, rendering them insoluble in water; so that the hardening 

 induced by these agents is not removed hy subsequent treatment with 

 water. Picric acid, on the other hand, produces only a very slight 

 hardening of the tissues, so that the tissue elements are left in a state 

 in which they are obnoxious to all the hurtful effects of water. Alcohol 

 should therefore be taken to remove the picric acid and to effect the 

 necessary hardening at the same time. 



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 



