CHAPTER III. 



FIXING AND HARDENING. 



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

 term " fixing " has been explained above ( 2). It remains 

 here to insist on the absolute necessity of the employment 

 of fixing agents, and to briefly illustrate this necessity. 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 unrecog- 

 nisable, 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 

 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. 



This example shows that one of the objects aimed at in 

 fixing is to impart to tissues the degree of hardening neces- 

 -;n-\ 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 func- 



