20 CHAPTEli II r. 



tion of fixing is to render insoluble elements of cells and tissues 

 that would otherwise be more or less dissolved out by the 

 liquids employed during the after-treatment. Compare in 

 this respect the aspect of sections of a piece of testis that 

 has been well fixed in liquid of Flemming and cut in paraffin, 

 with the aspect of paraffin sections of a piece of the same 

 testis that has not been fixed, or that has only been fixed by 

 some reagent inadequate for the purpose, such as alcohol or 

 picric acid. Jn the one case, plump, full, unshrunken cells, 

 free from vacuoles, fall of structure ; in the other, lean, 

 empty, shrunken cells, with foamy and vacuolated proto- 

 plasm, half their original structure lost, and that which 

 remains distorted ! Their appearance, compared with that 

 of living or well-preserved cells, suggests at once that much 

 must have been dissolved out of them. 



A third and highly important function of fixing agents 

 consists in producing optical differentiation in structures. 

 By coagulating the elements of tissues and cells, fixing agents 

 alter their indices of refraction, raising them in varying 

 degrees. They do not act in an equal degree on all the con- 

 stituent 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. Com- 

 pare 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 out 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 

 structure be revealed where none was visible before. 



The notion of fixing is distinct from that of hardening. 

 All fixing presupposes a certain degree of hardening, as 

 explained above. But it does not include the degree of 

 hardening necessary to give to soft tissues a consistency 

 which will allow them to be cut into thin sections without 

 imbedding. This is hardening proper (see 31). Of course, 

 if the stage of fixing be prolonged, with a view to procuring 



