INTRODUCTORY 3 



suggests that the action of the liquid seems to be in many cases 

 much improved by addition of a little acetic acid (say 0-5 to 0-75 

 per cent,). (For Plants see § 1235.) 



Prolonged treatment of tissues in alcohol is to be avoided 

 whenever lipoid inclusions are being investigated. The higher 

 strengths of alcohol and water have the power of bringing into 

 solution many fatty substances which form the groundwork of 

 the cytoplasmic inclusions. In addition, prolonged immersion 

 in alcohol is fatal to success in most silver and gold impregnation 

 methods of the neurologist. The so-called " hardening " effect 

 of alcohol may be advantageous for rough work, but is better 

 carried out in formalin or chrome salts. 



For material that is intended only for section-cutting, it is 

 undoubtedly by far the best plan to clear (next §) and imbed at 

 once in paraffin. This affords an absolutely perfect preservation. 

 Cedar-wood oil is nearly, if not quite, as good as paraffin, so far 

 as the preservation of the tissues is concerned, but of course it 

 is not so handy for storage. 



4. Removal of Alcohol ; Clearing. The water having been 

 sufficiently removed, the alcohol is in its turn removed from 

 the tissues, and its place taken by some anhydrous substance, 

 generally an essential oil, which is miscible with the material 

 used for imbedding or mounting. This operation is generally 

 known as Clearing. It is very important that the passage 

 from the last alcohol to the clearing agent be a gradual 

 one. This is effected by placing the clearing medium under the 

 alcohol. A sufficient quantity of alcohol is placed in a tube (a 

 watch-glass will do, but tubes are generally better), and then with 

 a pipette a sufficient quantity of clearing medium is introduced 

 at the bottom of the alcohol. Or you may first put the clearing 

 medium into the tube, and then carefully pour the alcohol on to 

 the top of it. The two fluids mingle but slowly. The objects 

 to be cleared, being now carefully put into the supernatant 

 alcohol, float at the surface of separation of the two fluids, the 

 exchange of fluids takes place gradually, and the objects slowly 

 sink down into the lower layer. When they have sunk to the 

 bottom, the alcohol may be drawn off with a pipette, and after some 

 further lapse of time the objects will be found to be completely 

 penetrated by the clearing medium. (Read also §§ 124 et seq.) 



The clearing stage may be avoided by dioxan or N-butyl 

 alcohol imbedding (see § 127). 



This method of making the passage from one fluid to another apphes 

 to all cases in which objects have to be transferred from a hghter to a 

 denser fluid — for instance, from alcohol, or from water, to glycerin. 



This is a convenient stage for carrying out minute dissections, if any 

 such have to be done, a drop of clearing agent being a most helpful 

 medium for carrying out such dissections (see § 8). 



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