FIXING AND HARDENING. 29 



necessary ; by careful infiltration with paraffin or other good 

 infiltration mass, most soft objects can he satisfactorily cut 

 with no greater an amount of previous hardening than is 

 furnished by the usual passing of the tissues after fixing 

 through successive alcohols in order to prepare them for tin- 

 paraffin bath. 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 humnii brain. Such 

 an organ as this cannot be duly infiltrated with alcohol in a 

 few hours, and it is doubtful whether it can be duly infiltrated 

 with paraffin or any other imbedding mass in any reasonable 

 time. And certain organs that are either extremely delicate 

 or inaccessible, such as i-etina or cochlea, will require to be 

 specially hardened in order to give the best results. The 

 processes employed for hardening such specimens as these 

 will be described when treating of the organs in question. 



The reagents employed for hardening are for the most 

 part of the same nature as those employed for fixing. But 

 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. 



[The chapter on Hardening Agents- that lias appeared in 

 all former editions is xuj>/>r<'*Ked, its contents having been 

 incorporated with the chapters on Fixing Agents and on 

 Neurological Methods.] 



32. The Practice of Hardening Hints and Cautions. Employ 

 'in general a relatively large coliune of hardening liquid, nnd 

 change it very frequently. The exact proportions may be 

 made out by experiment for each reagent and each class of 

 objects. If the volume of liquid be insufficient its composi- 

 tion will soon become 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 estab- 

 lished, and diffusion will cease ; that is to say, the harden- 

 ing liquid will cease to penetrate. This means, of course, 

 maceration of internal parts. On the other hand, it appears 



