CHAPTER XX. 



EXAMINATION AND PRESERVATION MEDIA. 



387. Introductory. I comprehend under this heading all 

 the media in which an object may be examined. The old 

 distinction of " indifferent '' liquids, and those which have 

 some action on tissues, appears to be misleading more than 

 helpful ; inasmuch as it is now well understood that 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 all is 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 evidently artificial. 



It does not appear necessary to create a separate group for 

 mounting* media, as all preservative media may be used for 

 mounting, though the only media that will afford an abso- 

 lutely sure preservation of soft tissues are the resinous ones. 



For directions as to making permanent mounts in fluid 

 media see the early sections of Chap. XXI. 



Watery Media. 



388. Water. To preserve it from mould, a lump of thymol or camphor 

 should he kept in the supply. Water may be employed without inconveni- 

 ence, and sometimes (on account of its low index of refraction) with great 

 advantage for the examination of all structures that have been fixed with 

 osmic or chromic acid, or some salt of the heavy metals ; but it is by no 

 means applicable to the examination of fresh tissues, that is, tissues that 

 have not been so fixed. It is important that the beginner should bear in 

 mind that water is very far from being an "indifferent" liquid; many 

 tissue elements are greatly changed by it (nerve-end structures, for in- 

 stance), and some are totally destroyed by its action if prolonged (for 

 instance, red blood-corpuscles). 



