FIXING METHODS. 23 



of potassium, which interrupts the action of the macerating fluid. 

 Useful for macerating epithelia and involuntary and heart muscle. 



4. Hydrochloric acid, 20^ to 30% aqueous solution. Mace- 

 rates small pieces of tissue in twelve to twenty-four hours. The 

 pieces are then thoroughly washed in water. Useful for isolating 

 the uriniferous tubules and macerating glands. 



5. Nitric acid, 10% to 20% aqueous solution or made up with 

 normal salt solution. Macerates small pieces of tissue in twenty- 

 four to forty-eight hours. Wash thoroughly in water. Useful for 

 macerating involuntary and voluntary muscle. 



6. J. B. MacCallum ("Contributions to Medical Science," 

 Baltimore, 1900) recommends the following nitric acid mixture 

 for isolating heart-muscle fibers of embryos and adults : Nitric 

 acid, i part ; glycerin, 2 parts ; water, 2 parts. The hearts 

 remain in this fluid from eight hours to three days, according to 

 their size, and are then transferred to a 5 c / c aqueous solution of 

 glycerin. This method is especially useful for obtaining prepara- 

 tions showing the arrangement of the heart -muscle fibers. 



7. Nitric acid and chlorate of potassium (Schulze). Powder 

 the chlorate of potassium and add sufficient nitric acid to make a 

 thin paste. Embed the tissue to be macerated in this paste, in 

 which they remain from one to several hours. They are then 

 washed in water. Useful for isolating the branched, voluntary 

 muscle-fibers of the tongue of a frog. 



8. Concentrated sulphuric acid. Useful for isolating the corni- 

 fied cells of the epidermis, nails, and hair. 



FIXING METHODS. 



The fixing fluids most used for general purposes are the following : 

 Alcohol. Alcohol is frequently used as a fixing fluid. It is at 

 the same time a hardening fluid, as the water of the tissues is withdrawn 

 and their albumin coagulated. Small or thin pieces are put immediately 

 into absolute alcohol, in which they remain for from twelve to twenty- 

 four hours. The period required for fixation may be greatly shortened 

 by changing the absolute alcohol at the end of one or two hours. In 

 the case of larger pieces, a successive immersion in gradually increasing 

 strengths of alcohol (50%, 70%, 90%) is the method chosen. Pieces 

 i c.c. in size remain for twenty -four hours in each grade of alcohol, 

 larger pieces for a proportionately longer time. Alcohol used in this way 

 is a hardening fluid rather than a fixing fluid. 

 Carnoy's Acetic-alcohol Mixture. 



Glacial acetic acid I part. 



Absolute alcohol 3 parts. 



Fixes very rapidly. Pieces of i centimeter in thickness are fixed in 

 one-half hour to one hour. The after-treatment is with absolute alcohol, 

 which should be renewed at the end of twenty-four hours. 



Carnoy's Acetic Acid-alcohol-chloroform Mixture. 



Glacial acetic acid I part. 



Chloroform 3 parts. 



Absolute alcohol 6 " 



Fixes very rapidly, even larger pieces in from one-half to one hour. 

 The after-treatment is with absolute alcohol. 



