642 TECHNIQUE 



the combination of the reagent with the chemical elements of which 

 the tissue consists ; very elaborate compounds are thus formed. 



The following reagents are recommended for general use. The 

 choice of a fixative is in great measure determined by the staining 

 method which is to be afterward applied. 



Alcohol, This is especially useful for the fixation of the glandu- 

 lar organs. Tissue may be placed directly in 95 per cent., or in 

 absolute alcohol. The fluid is to be changed in twenty-four hours, 

 and again in five to seven days. This method of fixation is desirable 

 for after staining the nervous tissues with inethylen blue and for 

 the demonstration of glycogen in the hepatic cells, cartilage, etc. 

 Alcohol causes considerable distortion of the internal architecture 

 of the cell by its rapid and forceful diffusion from the surface 

 toward the center of the tissue, the cytoplasmic granules often being 

 in this way forced to one side of the cell. This result may be par- 

 tially avoided by the use of "graded alcohol" viz., 67 * per cent, 

 alcohol for three to twelve hours, 82 f per cent, for twenty-four 

 hours, and finally 95 per cent, alcohol, which should be once changed 

 after a few days. Glycogen, however, is partially dissolved by the 

 action of the dilute alcohols. 



The distortion from the use of strong alcohol, as well as the dis- 

 sociation which follows the use of the weaker strengths, may also be 

 partially avoided by the addition of a little iodin to the stronger 

 alcohol, or by combination with acetic acid, thus : 



Glacial acetic acid 5 cc. 



95 per cent, alcohol 60 cc. 



Distilled water 35 cc. 



After fixation for three to twenty-four hours the tissues are 

 washed, and hardened by immersion for twenty-four hours in each 

 strength of " graded alcohol," and may be kept indefinitely in 95 

 per cent, alcohol. 



Tissues for fixation by this or any subsequent method are pref- 

 erably cut into small cubes : a size not exceeding 0.5 to 1 cm. is most 

 desirable. If larger pieces of tissue are necessarily used, the reagents 

 will each require increased time to insure complete penetration. 



Mercuric Chlorid. This salt is to be used in saturated aqueous 

 solution. As it dissolves with difficulty in cold water, the use of a 

 hot, normal saline solution hastens the operation. 



Mercuric chlorid is an excellent fixative for cytoplasm, but gives 



* 95 per cent, alcohol, 3 parts ; distilled water, 1 part. 

 1 95 per cent, alcohol, 5 parts ; distilled water, 1 part. 



