33 



THE FREEZING METHOD. 



73. This method is expeditious and of use in the rapid ex- 

 amination of tissues, and therefore especially serviceable in the 

 pathological laboratory and in clinical diagnoses. It may also be 

 used in cutting tissues that are too hard to be cut satisfactorily by 

 means of either the collodion or paraffin methods, and in the exami- 

 nation of tissues for substances (e. g., fats) which the solutions 

 necessary for the paraffin and celloidin methods dissolve out. Both 

 fresh and fixed tissue may be cut by means of the freezing micro- 

 tome and with or without any special mass such as is used in paraffin 

 or celloidin imbedding. Some histologists quite prefer the freezing 

 method to the paraffin or celloidin methods for general use. 



When no mass is employed the tissue is simply frozen and cut, 

 or, if it is fixed tissue, soaked well in water first and then frozen. 

 When extreme haste is not so essential it is better to first saturate 

 the tissue with some solution that does not crystallize on freezing, 

 but simply hardens, since the formation of the ice crystals is hurtful 

 to the tissue. Such are solutions of gum arable or sugar and anise- 

 seed oil, and they are spoken of as Congelation masses. 



74. Infiltration. Gum arabic or anise-seed oil may be used. 

 (a) Gum arabic. If the tissue has been fixed and is in alcohol re- 

 move the alcohol by soaking it for several hours to 1 day in water. 

 Remove to a thick solution of gum arabic in water, in which it may 

 remain for about 24 hours. It is then ready ta freeze and cut. 



(b) Anise-seed oil. For this method the tissue should be first 

 dehydrated ( 51).* W^hen dehydration is complete, transfer the 

 tissue to anise-seed oil, in which it may soak for 12 to 24 hours; it is 

 then ready to freeze and cut. It is particularly adapted for use with 

 tissue that has been stained in toto. 



75. Cutting. Place a drop of the solution of gum arabic 

 (or anise-seed oil) upon the object carrier of the freezing microtome 

 and turn on the carbonic acid (or ether) spray. When the mixture 

 begins to harden, place the object upon it in an abundance of the 

 solution and freeze it nearly solid. Covering with an inverted cup 

 hastens the freezing. An especially wedge-shaped knife is necessary 

 because of the hardness of the mass. 



*Anise-seed oil will, however, clear from 90% or even 82% alcohol; this 

 is sometimes of advantage. 



