SOME HARDENING AGENTS. 371 



have dogmatised upon observations of tissues profoundly altered 

 in course of preparation, and all the science of modern histology, 

 with its innumerable precautions and warnings, is one vast machin- 

 ery for avoiding similar errors in the future. 



When a tissue dies, its elements undergo change if left to them- 

 selves and in their mutual relations. The cells shrink, fibres swell, 

 objects become effaced, and others unduly emphasised ; hence the 

 precaution of " fixing," which is the first great step in microscopy. 

 Observations made on a soft tissue which has been left untouched 

 for twenty-four hours are mostly worthless. If, for instance, a 

 portion of the retina be left in water after removal from the 

 body, after a few minutes changes commence, the rods and cones 

 disintegrate, nerve-fibres become varicose and appear to be covered 

 with little lumps. If a fixing agent be employed, these elements 

 remain unchanged for any length of time. 



Dehydration is another precaution that experience has shown 

 to be necessary, the presence of water tending to assist the post- 

 mortem decomposition. 



It is well to remember that it is dangerous to generalise or 

 deduce theories from the observations of objects that have been 

 submitted to a great variety of re-agents in the course of their 

 preparation. For instance, if a section has been exposed to 

 repeated extremes of temperature and raised a number of times 

 to the boiling point — possibly frozen as well— soaked in baths of 

 spirit, subjected to the action of acids, and otherwise tampered 

 with, the facts deduced from the examination of such a specimen 

 are always open to the suspicion of having been created in the 

 process of preparation. The simpler, therefore, the treatment 

 has been, provided that post-mortem putrefactive changes have 

 been duly guarded against, the more reliable will be the observa- 

 tions based thereon. 



In Fixing, the action of the agent is, as we have said, to 

 coagulate the albuminoids, gelatin, and mucin present in the 

 tissue. The action of the agent varies. Thus, chromic acid, 

 osmic acid, and chloride of gold chemically combine with the 

 tissues ; and if you use them, you must subsequently stain with 

 haematoxylin or an anilin dye, except in the case of gold, which has 



