2 INTRODUCTORY 



the attitude it normally had during life ; and second, the harden- 

 ing 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. Without good fixation it is impossible 

 to get good stains or good sections, or jpreparations good in any way. 



The structure, having been duly fixed by one of the processes 

 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. 



These operations having been duly performed, two roads 

 become open. The object may be further prepared by what 

 may be termed the ivet method, in which all subsequent operations 

 are performed by means of aqueous media. Or it may be further 

 prepared by the dehydration method, which usually 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, chielly because of its great superiority as 

 regards the preservation of tissues. For the presence of water 

 is the most important factor in the conditions that bring about 

 the decomposition of organic matter, and its complete removal 

 is the chief condition of permanent preservation. 



3. Preservation. Considered as a mere dehydrating agent, 

 alcohol fulfils its function fairly well (but see § 124). But considered 

 as a histological preservative agent, it is far less satisfactory. If 

 tissues be left in alcohol for only a few days before further 

 preparation, injurious effects will perhaps not be very disagreeably 

 evident. But it is otherwise if they are put away in it for many 

 weeks or months before the final preparation is carried out. The 

 dehydrating action of the alcohol being continuously prolonged, 

 the minute structure of tissues is sometimes considerably altered 

 by it ; they become overhard and shrink, and become brittle, and 

 their capacity for taking stains well becomes seriously diminished. 

 KuLTSCHiTZKY {Zeit. iviss. Mik., iv, 1887, p. 349) has proposed 

 to remedy this by putting up objects after fixation and washing 

 out with alcohol in ether, xylol, or toluol. Flemming {Arch. mik. 

 Anat., xxxvii, 1891, p. 685) advises putting up objects after 

 fixation in a mixture of alcohol, glycerin, and water, in about 

 equal parts, pointing out that objects thus preserved may be at 

 any moment either prepared for sectioning by treatment with 

 pure alcohol or softened for dissection or teasing by a little soaking 

 in water, and that they do not become so hard and brittle as alcohol 

 specimens, and retain their staining power much better. Lee 

 after extensive experience of this plan recommends it, and further 



