b 



24 CHAPTER III. 



do SO, first, because tliey themselves become combiised 

 with the tissues much more quickly than the slowly working 

 osmic acid, and secondly, because they themselves have only 

 an extremely weak acid reaction." Hence the function oi 

 the oi'ganic acid is to bring into play the precipitating power 

 of the other ingredients. 



In default of special I'easons to the contrary, fixing 

 mixtures may conveniently be made to contain from 1 to ■-> 

 per cent, of acetic acid. But for some purposes they should 

 be neutral, or even alkaline. See, for instance, Neurofihrils. 



I think the beginner should avoid such things as liquid oi 

 Flemming and similar mixtures. He may take, instead, 

 Bouin's picro-formol. 



Corrosive sublimate (acidified) is a good all-rnund fixative, 

 with excellent penetration, but is not quite so easy to 

 manage. 



Picric acid gives a fair though weak fixation, with very 

 good penetration, is easy to manage, and does not make 

 tissues brittle, which sublimate easily may do. Pure forniol 

 is not bad, and very easy to manage. 



Speaking generally, osmic acid, chromic acid, bichromates, ciloride 

 of platinum, and the majority of the compounds of the heavy metals, 

 are hindrances to staining ; whilst heat, alcohol, trichloracetic acid, 

 formol, corrosive sublimate, nitric acid, picric acid, and acetic acid, are 

 neutral, or eyen. favourahle, in this respect. 



31. The Practice of Fixation. — See that the structures are 

 perfectly living at the instant of fixation, otherwise j'ou will 

 only fix pathological states or post-mortem states. 



Fixation is generally performed by immersion of the 

 objects in the fixing liquid. In this case, everything should 

 be done to facilitate the raiJid 'peiietrdtion of the fixing acent. 

 To this end let the structures be divided into the smallest 

 portions that can conveniently be employed, and if entire 

 organs or organisms are to be fixed whole, let openings, as 

 large as possible, be first made in them. 



The penetration of rea.gents is greatly facilitated by heat. 

 You may warm the reagent and put it with the object to be 

 fixed in the paraffin stove, or you may even employ a fixing 

 agent heated to boiling-point (as boiling sublimate solution 

 for certain corals and Hydroids, or boiling absolute alcohol 



