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 fixation. 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-fibres 

 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 

 osroic 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. 



This 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 



