FIXING AGENTS. 15 



CHAPTER III. 

 FIXING. 



21. The Necessity of Fixing. The meaning of the term 

 <c fixing " has been explained above ( 2). It only remains 

 here to insist 011 the absolute necessity of the employment of 

 fixing agents, and to briefly illustrate this necessity by a 

 single example. 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 preserva- 

 tion, 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 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. 



22. The Action of Fixing Agents consists in the coagulation 

 of certain of the constituents of tissues, of their albuminoids, 

 their gelatin, their mucin. Some fixing agents seem to have 

 further the property of combining chemically with the tissues, 



