I OHAPTEK i. 



tuting for it the preservative liquid or other reagents whiuli 

 it is desired to employ. Ulterior preparation comprehends 

 the processes sketched oat 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, whetlier 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 Ufe, but is fixed m 

 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. A\'ithout 

 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 matter 

 of indifference. If corrosive snbhmate (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 x^icric acid, have been used, the washing should be done 

 witli alcohol. The reason of this difference is that the iirst-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 by suljsequent 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 Ije 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 vet 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 



