Permanent Microscopic Preparations. The principal object of a permanent prepa- 

 ration is, firstly, to preserve the appearar.ee of the living animal or tissue and, secondly, 

 to render apparent structures which are not visible during life. The first of these objects 

 is attained by fixation, the second by staining and clearing. 



By fixation is meant the rapid coagulation of protoplasm to prevent the decomposition 

 processes which normally follow immediately after death. The principle fixatives are 

 solutions of mercuric chloride, with or without the addition of acetic acid; and alcohol, 

 picric acid and chromic acid mixtures. Alcohol and formalin, although largely used for 

 preserving animals, are of loss value for the fixation of objects for the microscope, as they 

 often cause distortion. 



As a general rule, it is not possible to stain tissues until the fixative has been washed 

 out of them. The principal stains are haematoxylin, carmine, and eosin, used either in 

 aqueous or alcoholic solution. 



After having been stained the object has to be rendered transparent and mounted in 

 some medium which will permanently preserve its shape. The medium generally employed 

 is Canada balsam, a resin which is dissolved in xylol or benzol. As water will not mix with 

 xylol, it is necessary first to replace the water in the tissue by alcohol, this process being 

 called dehydration. The change from water to alcohol must be made gradually and is 

 effected by passing the tissue through graded strengths of alcohol from thirty per cent to 

 absolute. The object is now rendered transparent by being placed in xylol, etc., or some 

 essential oil, a process known as clearing. The object can then be mounted in Canada 

 balsam, which gradually hardens as the xylol evaporates. 



In the case of larger animals, it is impossible to make preparations in the above manner, 

 and in order to study the microscopic structure of their tissues it is necessary to cut the 

 latter into thin sections. For this purpose the tissue is embedded in paraffin wax. This 

 is done by transferring it from xylol into melted wax and then solidifying the latter. The 

 block of wax coiitaining the tissue is cut into thin sections by an instrument called a 

 microtome. The sections are fixed to slides, the wax dissolved out by xylol and this 

 replaced by alcohol. Afterwards the sections may be stained and mounted in the manner 

 described above. 



