20 CHAPTER ITI. 



tissues that would otherwise be more or less dissolved out 

 by the liquids employed in the after-treatment. A third 

 and highly important function of fixing-agents consists in 

 producing optical differentiation in structures. By coagu- 

 lating the elements of tissues and cellSj fixing agents alter 

 their indices of refraction, raising them in varying degrees. 

 They do not act in an equal degree on all the constituent 

 elements of cells and tissues, but raise the index of some more 

 than that of others, thus producing optical differentiation 

 where there was little or none before. Compare the aspect 

 of the epithelium of the tail of a living tadpole, observed in 

 water, with its aspect after the action of a little diluted 

 solution of Flemming. In the living state the protoplasm 

 of its cells has a refractive index little superior to that of 

 water, and consequently so low an index of visibility that 

 hardly any structure can be made oat in the object. But as 

 soon as the protoplasm has been sufficiently coagulated by 

 the reagent the refractive indices of some of its elements 

 will have been raised to above that of balsam, the chromatin 

 of the nuclei will be brought out, and other structures 

 be revealed where none was visible before. 



29. The Action of Fixing Agents consists in coagulating and 

 rendering insoluble certain of the constituents of tissues. 

 This is effected sometimes without any chemical action being 

 involved, as when alcohol is employed, which acts by simple 

 withdrawal of the water of the tissues. But in the majority 

 of cases the fixing agents enter into chemical combination 

 with certain of the elements of the tissues. The compounds 

 thus formed are sometimes iinstahle and sohMe, so that they 

 are removable by washing, as is the case with several of 

 those formed by picric acid. It is found in practice, how- 

 ever, that those formed by chromic acid and its salts, and 

 the salts of the heavy metals, as mercurj-, iron, platinum, 

 gold, and silver, are mostly insoluble. 



The insolubility of these bodies is an advantage in that 

 it ensures that the tissues shall not be robbed of their 

 essential constituents, nor deprived of their desired con- 

 sistency and optical differentiation, by the rengents 

 subsequently employed. It is also sometimes an advantage 

 in that certain of the compounds in question have the 



