FIXING AND HARDENING. 23 



28. The Characters of the Usual Fixing Agents. No single 

 substance or chemical compound fulfils all that is required 

 of a good fixing agent ; hence it is that without exception 

 all the best fixing agents are mixtures. A good fixing agent 

 should first of all preserve all the elements it is desired to 

 fix. But that is not enough ; it should also give good 

 optical differentiation, and should have sufficient power of. 

 penetration to ensure that small pieces of tissue be equally 

 fixed by it throughout. Osmic acid, which is one of the 

 finest fixatives known, fulfils some of these conditions, but 

 not all of them. It kills rapidly, and preserves admirably 

 the elements of cytoplasm, but nuclei not so well. But the 

 optical differentiation that it gives, though sometimes good, 

 is often very inferior. For osmic acid, by coagulating in 

 nearly equal degrees alike spongioplasm (the plastin reticu- 

 lum) and hyaloplasm (the enchylema) and the chromatin of 

 nuclei, raises alike the refractive indices of all of them ; so 

 that if the fixing action have been in the least degree over- 

 done, the cells acquire a homogeneous aspect in which the 

 finer details are obscured by the general refractivity of the 

 whole. If now, instead of using it pure, it be used in com- 

 bination with acetic acid, a better differentiation is obtained ; 

 Tor acetic acid is properly a fixative only for a limited time, 

 whilst, if its operation be prolonged, it exercises a swelling 

 and solvent action on the elements of protoplasm. It there- 

 fore, whilst enhancing, or at all events not interfering with 

 the fixation of the chromatin, serves to facilitate penetration 

 and to counteract the excessive action of the osmic acid on 

 the protoplasm, so that the cells come out less homogeneous 

 and with more detail observable in them. A still better 

 effect is obtained if to the osmic acid there be added not only 

 acetic acid, but also chromic acid. For osmic acid has the 

 property of blackening tissues, thus rendering them opaque. 

 Chromic acid counteracts in a considerable degree this 

 blackening action. It also helps, probably, to bring out the 

 chromatin of nuclei, which is insufficiently fixed by the other 

 two ingredients, and perhaps also to counteract the exces- 

 sive coagulation of hyaloplasm by the osmic acid ; so that in 

 the result a much clearer picture is obtained. 



Such a mixture gives admirable results so far as preser- 

 vation and differentiation are concerned. But as regards 



