Tissue Fixation: The technique of using FIXATIVES in the preparation of cytologic, histologic, or pathologic specimens for the purpose of maintaining the existing form and structure of all the constituent elements.Fixatives: Agents employed in the preparation of histologic or pathologic specimens for the purpose of maintaining the existing form and structure of all of the constituent elements. Great numbers of different agents are used; some are also decalcifying and hardening agents. They must quickly kill and coagulate living tissue.Formaldehyde: A highly reactive aldehyde gas formed by oxidation or incomplete combustion of hydrocarbons. In solution, it has a wide range of uses: in the manufacture of resins and textiles, as a disinfectant, and as a laboratory fixative or preservative. Formaldehyde solution (formalin) is considered a hazardous compound, and its vapor toxic. (From Reynolds, Martindale The Extra Pharmacopoeia, 30th ed, p717)Paraffin Embedding: The infiltrating of tissue specimens with paraffin, as a supporting substance, to prepare for sectioning with a microtome.Nitrogen Fixation: The process in certain BACTERIA; FUNGI; and CYANOBACTERIA converting free atmospheric NITROGEN to biologically usable forms of nitrogen, such as AMMONIA; NITRATES; and amino compounds.Fixation, Ocular: The positioning and accommodation of eyes that allows the image to be brought into place on the FOVEA CENTRALIS of each eye.Fracture Fixation: The use of metallic devices inserted into or through bone to hold a fracture in a set position and alignment while it heals.Complement Fixation Tests: Serologic tests based on inactivation of complement by the antigen-antibody complex (stage 1). Binding of free complement can be visualized by addition of a second antigen-antibody system such as red cells and appropriate red cell antibody (hemolysin) requiring complement for its completion (stage 2). Failure of the red cells to lyse indicates that a specific antigen-antibody reaction has taken place in stage 1. If red cells lyse, free complement is present indicating no antigen-antibody reaction occurred in stage 1.Bone Screws: Specialized devices used in ORTHOPEDIC SURGERY to repair bone fractures.Bone Plates: Implantable fracture fixation devices attached to bone fragments with screws to bridge the fracture gap and shield the fracture site from stress as bone heals. (UMDNS, 1999)