2 INTRODUCTORY. 



feature of modern histological practice ; without good fixation 

 it is impossible to get good stains, or good sections, or prepa- 

 rations 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 with which washing out is done is not a matter of in- 

 difference. If corrosive sublimate (for instance) or osinic 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) appear to 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 subsequent treatment 

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

 hardening of the tissues, and does not appear to enter into any combination 

 whatever with their elements; as it is entirely removable by treating the 

 tissues with water or alcohol. If the removal be effected by means of water, 

 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. Instruc- 

 tions for washing out are given, when necessary, in the discussion of the 

 different fixing agents in the following parts of this work. 



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 necessary for two 

 reasons ; firstly, in the interest of preservation, the presence 

 of water being the condition of all others that most favours 

 post-mortem decomposition ; and secondly, because all water 

 must be removed in order to allow the tissues to be impreg- 

 nated with the imbedding material necessary for section- 

 cutting, or with the balsam with which they are to be finally 

 preserved. (The cases in which aqueous imbedding and pre- 

 serving media are employed are exceptional, and will be 

 treated of in the proper places.) The dehydration is performed 

 as follows : the objects are brought into weak alcohol, and 

 are then passed through successive alcohols of gradually in- 

 creased strength, remaining in each the time necessary for 

 complete saturation, and the last bath consisting of absolute 

 or at least very strong alcohol. 



In dealing with extremely delicate objects, it may be necessary to take 



