The commoner intra vitam stains are pure, medicinal (zinc-free) methylene blue and neutral red, used in aqueous solutions. A solution of methylene blue of 1 per 1,000 of water may be tried, while neutral red in the proportion of 1 per 3,000 parts of water has proved of service.
The second method of vital colouring consists in placing a drop of 1 per cent. solution of methylene blue on a slide or cover-slip, slightly spreading it, and allowing it to dry. The living organism is then placed in a drop of saline on the prepared slide or cover-slip, which is then mounted and examined under the microscope. Progressive staining of the organism occurs and its internal structure can be seen. A similar procedure may be followed for neutral red. Intra vitam staining is useful for relatively large and easily deformed protozoa such as ciliates, as well as for amœbæ and flagellata of the gut.
When examining very actively motile organisms, it is sometimes useful to endeavour to restrict their movements by adding a little gum or gelatine to the medium.
(c) Examination by aid of the Paraboloid Condenser.
The use of one of the dark-ground illuminators (so-called ultra-microscopes) is of service for the detection of minute living organisms or of organisms present in small numbers only. The forms of paraboloid condenser manufactured by the firms of Zeiss and Leitz can be recommended. For details of their methods of employment, reference should be made to the leaflets of the firms supplying the said instruments. By the use of the paraboloid condenser, the finer details of certain stages of life-cycles, such as the formation of granules in spirochætes and treponemata, can be observed more readily than by using the ordinary substage of the microscope. The use of the paraboloid condenser for the detection of small numbers of living organisms renders it of value for rapid diagnostic purposes.
Stained Material.
Fuller accounts of the technique of fixed and stained material will be found in Bolles Lee and in Langeron, already mentioned.
Thin Films.—For the examination of blood-inhabiting Protozoa, it is necessary to make first thin films or smears of blood. There are many ways of doing this, and opinions differ as to their respective merits. A simple method is to take a straight surgical needle about 2 in. long, the eye of which has been removed, and a clean glass slide. The patient’s skin is pricked, and when the bead of blood reaches the size of a small pin’s head, the slide is applied to the surface of the blood, about 1/3 in. from the far (left-hand) end of the slide. The shaft of the needle is laid across the drop of blood, which spreads between the slide and the needle. The latter is drawn evenly along the slide towards the right. The film is dried by waving it in the air. The film should possess a straight edge parallel with that of the slide and should be as uniform and thin as possible. Another glass slide may be used as a spreader, or a cover-slip or thin glass rod may be employed.
Thick Films.—These are of service in detecting malarial parasites or trypanosomes, especially when the parasites are few. The method of Ross, or a modification thereof, has been much used. A small drop of fresh blood is spread evenly and quickly with a needle-point over a square area somewhat less than that of an ordinary square cover-glass. The blood is allowed to dry. The film is then carefully dehæmoglobinized in water in which there is a trace of acetic acid. The dehæmoglobinizing fluid is then carefully drained off and the film again dried. It is fixed in absolute alcohol and stained with Romanowsky’s solution. A cubic millimetre of blood divided into quarters may be thus dehæmoglobinized and stained. The parasites in such a cubic millimetre of blood may be counted. Such a procedure was followed by R. Ross and D. Thomson,[1282] in determining the periodic variation of the numbers of trypanosomes in the blood of a patient, as mentioned and figured on pp. [78] and 79.
For cytological details of various Protozoa, thin film preparations on cover-slips or slides are often useful. Cover-slip preparations are preferable, unless the organisms under investigation are extremely scanty. The medium containing the organisms, such as blood, lymph, intestinal contents, sputum, scrapings of ulcers, and urine, is spread thinly, either alone or diluted with a little physiological salt solution, on the cover-slip. Fixation while still wet is necessary. Various methods are employed.