FIXING AND HARDENING. 27 



sections from them is, in the majority of cases,, no longer 

 necessary. But there are some exceptions. Such are, for 

 instance, the cases in which it is desired to cut very large 

 sections, such as sections of the entire human brain. 



The reagents employed for hardening are for the most 

 part of the same nature as those employed for fixing. J>ut 

 it does not follow that all fixing agents can be employed for 

 hardening. Corrosive sublimate, for instance, would be most 

 inappropriate as a hardening agent. 



34. The Practice of Hardening. Employ in general a rela- 

 tively large volume of hardening liquid, and change it very 

 frequently. If the volume of liquid be insufficient, its com- 

 position will soon became seriously altered by the diffusion 

 into it of the soluble substances of the tissues ; and the 

 result may be a macerating instead of a hardening liquid. 

 Further, as soon as, in consequence of this diffusion, the 

 liquid has acquired a composition similar in respect of the 

 proportions of colloids and crystalloids contained in it to 

 that of the liquids of the tissues, osmotic equilibrium will 

 become established, and diffusion will cease ; that is to say, 

 the hardening liquid will cease to penetrate. This means, 

 of course, maceration of internal parts. On the other hand, 

 it appears that a certain slight proportion of colloids in the 

 hardening liquid is favourable to the desired reaction, as it 

 gives a better consistency to tl^e tissues by preventing them 

 from becoming brittle. Hence the utility of employing 

 a certain proportion of hardening agent. 



Hardening had better be done in tall cylindrical vessels, 

 the objects being suspended by a thread, or muslin bag, or 

 otherwise, at the top of the liquid. This has the advantage 

 of allowing diffusion to take place as freely as possible, 

 whilst any precipitates that may form fall harmlessly to the 

 bottom ; or, they may be laid on a layer of cotton-wool, or 

 filter-paper, or spun glass. 



In general, begin hardening with a weak reagent, increasing 

 the strength gradually, as fast as the tissues acquire a con- 

 sistence that enables them to support a more energetic action 

 of the reagent. 



Let the objects be removed from the hardening fluid as 

 soon as they have acquired the desired consistency. 



