CHAPTER III 



FIXING AND HARDENING 



28. The Functions of Fixing Agents. The meaning of the 

 term " fixing " has been explained above (§ 2). Here is an 

 example showing the necessity of hxation. 



If a portion of living retina be placed in aqueous humour, serum, or 

 other so-called " indifferent " medium, or in any of the media used for 

 permanent preservation, it will be found that the rods and cones will not 

 preserve the appearance they have during life for more than a very 

 short time ; after a few minutes a series of changes begins to take place, 

 by which the outer segments of both rods and cones become split into 

 discs, and finally disintegrate so as to be altogether unrecognisable, 

 even if not totally destroyed. Further, in an equally short time the 

 nerve-flbres become varicose, and appear to be thickly studded with 

 spindle-shaped knots ; and other post-mortem changes rapidly occur. 

 If, however, a fresh piece of retina be treated with a strong solution of 

 osmic acid, the whole of the rods and cones will be found perfectly 

 preserved after twenty-four hours' time, and the nerve-fibres will be 

 found not to be varicose. After this preliminary hardening, portions 

 of the retina may be treated with water (which would be ruinous to the 

 structures of a fresh retina), they may even remain in water for days 

 without harm ; they may be stained, acidified, hardened, imbedded, 

 cut into sections, and mounted in either aqueous or resinous media 

 without suffering. 



Tliis example shows that one of the objects aimed at in fixing 

 is to impart to tissues the degree of hardening necessary to enable 

 them to offer such mechanical resistance to post-mortem change 

 and to the processes of after-treatment as not to suffer change of 

 form. Another important function of fixing is to render insoluble 

 elements of cells and tissues that would otherwise be more or less 

 dissolved out by the liquids employed in the after-treatment. A 

 third and highly important function of fixing agents consists in 

 producing optical differentiation in structures. By coagulating 

 the elements of tissues and cells, fixing agents alter their indices of 

 refraction, raising them in varying degrees. They do not act in 

 an equal degree on all the constituent elements of cells and tissues, 

 but raise the index of some more than that of others, thus pro- 

 ducing optical differentiation where there was little or none before. 



Compare the aspect of the epithelium of the tail of a living tadpole, 

 observed in water, with its aspect after the action of a little diluted 

 solution of P'lemming. In the living state the protoplasm of its cells 

 has a refractive index little superior to that of water, and consequently 

 so low an index of visibility that hardly any structure can be made out 

 in the object. But as soon as the protoplasm has been sulliciently 



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