In the fixation of tissue it is the aim to hold the structures as nearly as possible in the form they had during life, or at the moment when the fixing reagent acted. Exceptionally, an artefact may have a differentiating value; with this we are not concerned here. But when conditions exist which can be recognized as artefact, it is desirable to eliminate them, or to evaluate them.
At present 10 per cent. liquor formaldehydi, U. S. P., is the most extensively used fixative for whole brains. In dealing with unknown brain tissue so fixed, conclusions regarding deviations from the normal picture are based on our experience with formaldehyd fixed tissue. Certain conditions due to fixation rather than to antemortem influences are eliminated; for example, certain clouding of the pia-arachnoid is expected to appear after exposure to formaldehyd. In finer histologic work, detectable change in structure, due to fixation, is