192 EXAMINATION AND PRESERVATION MEDIA. 



CHAPTER XIX. 



INDIFFERENT LIQUIDS, EXAMINATION AND PRESERVATION 



MEDIA. 



342. Introductory. I comprehend under this heading all the 

 media in which an object may be examined. The old distinc- 

 tion 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. 



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

 should be 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 

 osinic 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 instance), and 

 some are totally destroyed by its action if prolonged (for instance, red blood- 

 corpuscles). 



346. Theory of Indifferent Liquids. In order to render water 

 inoffensive to such tissues as these it must, firstly, have dis- 

 solved in it some substance that will give it a density equal to 

 that of the liquids of the tissue, so as to prevent the occurrence 



