724 HISTOI^OGIC TECHNIC 



object the demonstration of certain structural features by the after ap- 

 plication of special staining methods. 



The action of the fixing fluids is in most cases dependent upon the 

 combination of the reagent with the chemical elements of which the 

 tissue consists; very elaborate compounds are thus formed. 



The distinction between a physical and a chemical combination of 

 a dye with particular tissue elements must be emphasized. 



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 glandular 

 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 

 of the nervous tissues with methylene blue and for the demonstration 

 of glycogeii in the hepatic cells, cartilage, etc. Alcohol causes consider- 

 able 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 partially avoided by the use of 'graded alcohol/ 

 viz., 67 per cent, alcohol for three to twelve hours, 82 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 graded alcohols may be derived from the 95 per cent, stock 

 supply by the following dilutions : 



95 per cent, alcohol, 1 part ; distilled water, 1 part=4S per cent. 

 95 per cent, alcohol, 3 parts; distilled water, 1 part=67 per cent. 

 95 per cent, alcohol, 5 parts; distilled water, 1 part=S2 per cent. 



For practical purposes these grades may be regarded as 50, 70 and 80 

 per cent, respectively; they may be derived also by the use of the fol- 

 lowing formula : 



.'<;."> c.c. !>">% alc. -f 235 c.c. aq. dist. - - 50% alc. 

 370 c.c. 95% alc. + 130 c.c. aq. dist. = 70% alc. 

 425 c.c. 95% alc. + 75 c.c. aq. dist. = 80% alc. 



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

 ciation which follows the use of the weaker strengths, may also be par- 



