16 Animal Micrology 



KILLING, FIXING, AND HARDENING 



The first step in the preparation of tissues ordinarily is the 

 employment of some reagent which will kill the tissues and fix 

 their various components in the characteristic stages of their 

 activities. Such material may then be preserved indefinitely for 

 future use. 



It is customary to discriminate between killing, fixing, and 

 hardening, although the same reagent may fulfil all three require- 

 ments. Killing refers particularly to the destruction of the life 

 of the tissue, a process which may be either slow or instantaneous. 

 In slow killing it is usual to employ narcotics such as ether, 

 chloroform, chloral hydrate, chloretone, carbon dioxide, nicotin, 

 cocain, or weak alcohol. Ice is also used sometimes. Such 

 methods are of particular value with highly contractile animals 

 which are desired in the extended condition. Such forms are 

 narcotized completely or until they are unable to contract and 

 then frequently fixed and hardened in other or stronger fluids. 

 Where practicable, instantaneous killing and fixing is preferable 

 because tissues have then no time to undergo postmortem changes. 

 The same fluid ordinarily is employed for killing and fixing. 



The purpose of fixation is 



o) To preserve the actual form of tissue elements. 



6) To produce optical differences in structure, or so to affect 

 the tissues that such differences will be brought out through sub- 

 sequent treatment with stains or other reagents. 



To accomplish this the fixing agent must possess the following 

 qualities: 



1. It should kill the tissue so quickly that few structural 

 changes can occur. 



2. It should neither shrink nor distend the tissue. 



3. It should be a good preservative; that is, it must render the 

 tissue elements insoluble and prevent postmortem changes. 



4. It should penetrate all parts equally well. 



5. It should put the tissue in condition to take stains unless it 

 of itself produces sufficient optical differences in the various parts 

 of the tissue. 



