Chemicals Commonly Used in Fixing Solutions 5 



tin acidophilic. 3. Acetic acid compensates for both the latter defects. 



No hard and fast rule exists concerning the choice of a fixative; gen' 

 erally selection is determined by the type of investigation. If there is 

 any question as to future plans for a tissue, formalin is a safe choice; it 

 permits secondary or post-fixation. Some suggestions can be included. 

 If over-all anatomy of the tisstie is satisfactory, the rotitine fixatives can 

 be used: formalin, Siisa, Zenker, Helly, or Bouin. Special fixatives for 

 cell inckisions are Carnoy, Flemming, Champy, Helly, Schaudinn, 

 Regand, and others. For histochemistry the researcher is limited to 

 formalin, acetone, or ethyl alcohol. 



Most fixing solutions are named after the person originating them; 

 Zenker and Bouin are examples. If the same man originated more than 

 one combination of chemicals, additional means of designating them 

 have been used. Flemming's weak and strong sohuions, Allen's B3, Ely 

 series, etc. Susa fluid was named by Heidenhain after the first two let- 

 ters of two words: jwblimate and ^aure. 



Chemicals Commonly Used in Fixing Solutions 



Acetic Acid, CH^COOH 



Acetic acid is called glacial acetic acid because it is solid at tempera- 

 tures below 17°C. It can be considered one of the oldest fixatives on 

 record — in the 18th century vinegar (4-10% acetic acid content) was 

 used to preserve Hydra. In modern technics it is rarely used alone but is 

 a frequent component of fixing soliuions. Its efficient fixing action on 

 the nucleus and its rapid penetration make it an important part of good 

 fixatives. It fixes the nucleoproteins, but not the proteins of the cyto- 

 plasm. Acetic acid does not harden the tissue; actually it prevents some 

 of the hardening that may without it be induced by subsequent alcoholic 

 treatment. In some technics, however, acetic acid must be avoided be- 

 cause it dissolves out certain cell inclusions, such as Golgi and mito- 

 chondria. Some lipids are miscible with acetic acid, or actually soluble 

 in it. It neither fixes nor destroys carbohydrates. 



Acids in general cause swelling in tissues — in collagen in particular — 

 by breaking down some of the cross-linkages between protein molecules 

 and releasing lyophil radicles which associate Avith ^vater molecules. This 

 swelling in some cases may be a desirable property of acids, coimteract- 

 ing and preventing some of the shrinkage caused by the majority of 

 fixing chemicals. In order to curtail continued swelling after fixation 



