fixation(redirected from external fixation)
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name given by Sigmund Freud to a system of interpretation and therapeutic treatment of psychological disorders. Psychoanalysis began after Freud studied (1885–86) with the French neurologist J. M.
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a method of preserving the structure of animal and plant cells and tissues through treatment with chemical and physical agents. Fixation is used to obtain medicinal preparations.
Physical techniques include drying and freezing at low temperatures in a vacuum (lyophilization). Specimens may be treated chemically with such reagents as formaldehyde, alcohol, acetone, and osmium tetroxide. Many reagents are used in combination with other substances, such as potassium bichromate, mercuric chloride, and picric acid; such fixatives are usually named after the researchers who proposed them, for example, Zenker’s and Maximow’s fixatives and Flemming’s, Carnoy’s, and Bouin’s solutions. The choice of technique depends on the properties and size of the specimen and the purpose of the study. For example, alcohol fixation is adequate to preserve nucleic acids, but enzyme activity is completely preserved only after lyophilization. Fixation in aldehydes is recommended for electron microscopy. Double fixation is used for better preservation of tissue, that is, primary fixation with an aldehyde fixative and a second fixation with osmium tetroxide.
REFERENCESRoskin, G. I., and L. B. Levinson. Mikroskopicheskaia tekhnika, 3rd ed. Moscow, 1957.
Rukovodstvo po tsitologii, vol. 1. Moscow-Leningrad, 1965.
M. E. ASPIZ