IX. ARTEFACTS IN MOUNTED SECTIONS. A mounted section, after passing through the various stages indicated above, must of necessity present some appearances that are the result of the technical methods employed. The number and degree of such artefacts will depend upon the character of the methods employed and the care exercised in their performance. The trained observer ignores the presence of artefacts as having nothing at all to do with the significance of the section itself; but to the beginner in microscopic work they often appear to be the most important thing in the preparation, and are given a pathologic interpretation. How frequently do we see students, undergraduates and postgraduates, take up a section and pick out a fold, wrinkle, tear, staining-defect, precipitate, dirt, etc., as pathologic features! It is necessary, therefore, for the student to acquaint himself with the nature of artefacts so that he may ignore them and not give them an incorrect interpretation. The most important artefacts are as follows:— 1. =Artefacts due to fixation= (mercuric, chromic, osmic, etc., precipitates; alterations in blood-pigment due to formol; loosening of cells from basement membrane due to contraction [kidney tubules, etc.: loosened endothelium in blood-vessels, particularly confusing to students]; destruction of red blood cells, as in alcohol fixation; poor staining due to over-fixation). 2. =Artefacts due to hardening= (contraction, desquamation of cells, etc.). 3. =Artefacts due to imbedding and cutting= (tears, holes, ragged edges, irregular thickness, knife-streaks, compression of soft structures, dislocation and tearing-out of firm tissues or material, wrinkles, folds, etc.). 4. =Artefacts due to poor staining= (uneven, spotted or streaked staining, overstaining, understaining, poor differentiation, precipitates, insufficient washing, fading, poor contrasts, defective staining due to presence of paraffin, dextrin, etc., in section). 5. =Artefacts due to poor mounting= (imperfect dehydration and clearing, cloudiness, milkiness or opacity of section; folds; wrinkles; turned-over edges; tears in section caused by striking it with balsam-dropper or needle; air-bubbles; lack of balsam). 6. =Dirt and foreign-material= (opaque and translucent dirt, above or below section; coloring-matter in balsam; ink; pigment from pencil; cotton-, silk, wool-, linen-, vegetable and paper-fibres, hairs, desquamated squamous epithelium, portions of insects, etc.) CHAPTER XXV. STAINS AND STAINING METHODS.—NUCLEAR AND PROTOPLASMIC STAINS.