2 CHAPTER I. 



fixing agent from the tissues, and substituting 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 

 statement 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 Jailing 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. Too much stress can hardly be laid on this point, 

 which is the most distinctive feature of modern histological 

 practice ; 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 washed 

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

 traces of the fixing reagent. 



The kind of liquid witli which washing out is done is not a matter of in- 

 difference. 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 picric acid in any form, has been 

 used, the washing must be 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 rerqoved by 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 soft 

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

 Alcohol must therefore be taken to remove the picric acid and to effect the 

 necessary hardening at the same time. Instructions for washing out are 

 given, when necessary, in the discussion of the different fixing agents in the 

 following parts of this work. 



These operations having been duly performed, two roads 

 become open. The object may be further prepared by what 



