FIXING AGENTS. 13 



CHAPTER III. 

 FIXING. 



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

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

 here to insist on 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 preservation, it will 

 be found that the rods and cones will not preserve the appear- 

 ance 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. 



23. 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, 



