The object of this book is to give accounts of the structure and life-histories of the numerous parasitic organisms that affect man more particularly. It is, therefore, inappropriate to devote much space to a consideration of technique, regarding which many volumes have already been written. Methods of procedure are largely matters of opinion, and the technique that gives brilliant results when used by one investigator may be a complete failure in the hands of another. In the present appendix, brief notes regarding certain relatively simple methods only can be given, because the number of fixatives in use is very great; there are also large numbers of stains as well as many modifications of them, while the methods of applying both fixatives and stains are, perhaps, still more numerous. There are so many, in fact, that confusion frequently arises from the multiplicity of choice presented to the worker. Those desiring more information on the subject of technique are advised to consult the treatises of Bolles Lee[1280] and of Langeron.[1281]
Fresh Material.
(a) Simple Examination.
Fluid Substances, such as Blood and Sputum.—A small quantity of the substance to be examined is taken on a sterile platinum loop and transferred to a perfectly clean glass slide. A clean cover-slip is gently lowered on to the drop, air bubbles being avoided. The preparation is luted with vaseline or paraffin and examined first with a low power and then with a high power objective. The light is cut down by partly closing the diaphragm of the substage of the microscope.
Skin Ulcers and Similar Sores.—Scrapings are made from the edge of the sore, mixed with sterile physiological salt solution, and prepared and examined as above.
Fæces.—A small portion of fæces, or flakes of mucus (which may be blood-stained) from the same, is removed on a sterile platinum loop, spread out thinly after dilution, if necessary, with physiological salt solution on a slide, covered and examined as before.
Alternatively, hanging drop preparations of blood, ulcerative tissue, or fæces, appropriately diluted if necessary with sodium citrate or physiological salt solution, may be made on a cover-slip, which is inverted over a slide with a well in it. The cover-slip is then luted and examined.
For the elucidation of the developmental processes of such organisms as trypanosomes, spirochætes and piroplasms, fresh preparations may be often kept under observation longer by the use of a thermostat, maintained at or near blood heat, in which the microscope is inserted.
(b) Intra vitam Staining of fresh Preparations.
Intra vitam staining is of service on some occasions, more particularly for the study of the nucleus and other chromatoid substances of the living organism. Two methods are in common use. In the first case, the stain, employed usually in very dilute solution, is mixed with the medium containing the organism. The latter takes up some of the stain, the amount of coloration depending on the organism concerned and on the stain employed.