INTRODUCTORY. 3 



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 what may be termed the Dehy- 

 dration, 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. The 

 presence of water is the most important factor in the con- 

 ditions that bring about the decomposition of organic matter, 

 and its complete removal is the chief condition of permanent 

 preservation. It is of course not intended here to suggest 

 that wet methods of preparation should be altogether dis- 

 carded. They have great value, they are even indispensable 

 for special ends ; and all that is intended to be suggested is 

 that they should be regarded not as general, but as special 

 methods. 



3. Dehydration and Preservation. The further course of 

 preparation by the dehydration method is as follows : At 

 the same time that the superfluous fixing agent is being re- 

 moved from the tissues, or as soon ;is that is done, the water 

 of tJie tissue* 'inuxt he 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 im- 

 pregnated with the imbedding material necessjiry for section- 

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

 preserved. (The cases in which aqueous imbedding and 

 preserving media are employed are exceptional, and will be 

 treated of in the proper places.) This dehydration is per- 

 formed as follows : The objects are brought into weak 

 alcohol, and are then passed through successive alcohols of 

 gradually increased strength, remaining in each the time 

 necessary for complete saturation, and the last bath consist- 

 ing of absolute or at least very strong alcohol. 



