FIXATION. 



1. Fixation is one of the fundamental processes in the ex- 

 amination of plant and animal tissues. A fixer may be defined as 

 a fluid (or gas) into which the living or at least very fresh tissue is 

 placed in order to preserve the structure of its elements as nearly as 

 possible as in life. Living tissue when allowed to die and remain 

 undisturbed, gradually loses the structural features it had in life 

 and undergoes disintegration and decay. Fixation depends upon 

 physico-chemical processes wherein the chemical constituents of the 

 tissue are thrown down in situ by being rendered insoluble in some 

 form or represented by substitution products; the whole being at- 

 tended by as little distortion as possible. It should be appreciated 

 that the chemical constituents of cell protoplasm and of the tissues 

 are numerous and diverse in their chemical and physical properties, 

 so that a universal or ideal fixer not only does not exist but is logically 

 inconceivable. The bulk of protoplasm and the tissues is protein 

 and the basis of fixation in general is the precipitation or coagulation 

 of these chemical substances. It should be remembered however 

 that fats (lipoids*) are a constant though variable component of 

 cytoplasm; that carbohydrates (glucose, glycogen, etc.) are usually 

 present in small amounts, and that the products of cell activity such 

 as secretion-granules, zymogen, etc., may be quite distinctive in 

 their physico-chemical properties. Within the cell a certain "antag- 

 onism" exists between nucleus and cytoplasm, the former oxidative, 

 the latter reducing, requiring often somewhat different fixation 

 conditions. 



For the best results, the fixer should be chosen with a view to 

 the preservation of some particular part or constituent, though a 

 number of general fixers are very serviceable for routine work. 

 Rational fixation will depend upon a detailed knowledge of the 

 chemical and physical properties of the constituents that it is de- 

 sired to preserve and their interaction with the chemicals of the 

 fixer. In many respects, rational fixation still awaits further knowl- 

 edge of the physics and chemistry of fixation. 



2. The chemicals of most service as fixers are: (1) osmic 

 acid (osmium tetroxid), (2) platinic chlorid, (3) picric acid (tri- 



*Lipoid, while not a good chemical term is one that is quite useful in histology, 

 to include fats, fatty acids, phosphatids, cholesterol, etc., substances that have 

 the same solvents and which are found associated in protoplasm. 



