218 EXAMINATION AND PRESERVATION MEDIA 



It will be seen that cedar oil has nearly the index of crown 

 glass (this is true of the oil in the thick state to which it is brought 

 by exposure to the air — not of the new, thin oil, which is less 

 highly refractive) ; it therefore clears to about the same extent 

 as Canada balsam. Clove oil has a much higher index, and 

 therefore clears more than balsam ; cinnamon oil higher still. 

 Turpentine and bergamot oil have much lower indices, and 

 therefore clear less. Note methyl benzoate as a thin substitute 

 for cedar- wood oil for cover slip fresh mounts. 



WATERY MEDIA* 



440. Isotonic and ' ' Indifferent ' ' Liquids. The old distinction 

 of " Indifferent " liquids, and those which have some action on 

 tissues, appears to be misleading more than helpful ; for no 

 medium is without action on tissues except the plasma with 

 which they are surrounded during the life of the organism ; and 

 this plasma itself is only " indifferent " whilst in situ ; as 

 soon as a portion of tissue is dissected out and transferred to a 

 slide in a portion of plasma the conditions become artificial. 



Water may be employed for the examination of structures that 

 have been -well fixed ; but this is by no means applicable to the 

 examination of fresh tissues. It is very far from being an 

 " indifferent " liquid ; many tissue elements are greatly changed 

 by it (nerve-end structures, for instance), and some are totally 



* These are again coming into use in cytology, especially for mounting 

 fat stains, etc. 



