236 CHAPTER XIX. 



no medium is without action on tissues except the plasma 

 with which they are surrounded during the hfe of the 

 organism; and this plasma itself is only " indiiferent " 

 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 artificial. 



Water may be employed for the examination of structures 

 that ha.ve 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 destroyed by its action if prolonged (for 

 instance, red-blood corpuscles). 



In order to render it inoffensive to fresh tissues it must 

 have dissolved in it substances of similar diffusibility to 

 those of the liquids of the tissue, so as to prevent the occur- 

 rence of osmosis, to which process the destructive action of 

 pure water is mainly due. Now cell contents are a mixture 

 of colloids and crystalloids ; consequently, in order to reduce 

 osmotic processes to a minimum, it is necessary that the 

 examination medium contain a due proportion of both 

 crystalloids and colloids. By adding, for instance, white of 

 egg to salt solution this end may be in some measure 

 attained ; and, as a matter of fact, the liquids recommended 

 as "indifferent" are generally found to contain both 

 crystalloids and colloids. Liquids thus composed, in which 

 tissue-elements are in osmotic equilibrium — that is, neither 

 swell nor shrink — are said to be isotonic to the tissues ; 

 whilst those in which they shrink are called hypertonic, and 

 those in which they swell hypotonic. Solutions of common 

 salt, in different concentrations, form the base of the most 

 commonly employed isotonic liquids. For marine Inverte- 

 brates, sea-water is generally isotonic. 



402. Salt Solution (" normal salt solution," " physiological 

 salt solution"). — 0"75 per cent, sodium chloride in water. 

 Caenoy recommends the addition of a trace of osmic acid. 



Ringer's solution, much used in physiology, consists of 

 sodium chloride 08 parts, calcium chloride 0"02, potassium 

 chloride 0'02, sodium bicarbonate 0'02 and water 100 (with 

 or without 0"1 dextrose). 



