Appx. XXI.

Specimens for laboratory examination.

       An unbroken pustule should be selected and the surface sterilized
by rubbing with cotton wool moistened in absolute alcohol.

       The pus is obtained by plunging the point of a sterile capillary
pipette into the centre of the pustule and gently aspirating a portion
of the contents into the capillary stem by means of the teat fitted to
the end of the pipette. The end is then sealed off in the flame.

BLOOD FILMS FOR MALARIAL PARASITES, LEISHMANIA, BLOOD
DISEASES, ETC.

       21. The ordinary method.— The blood should be spread on a clean
microscope slide in as thin and even a film as possible. This is best
attained in the following manner:—A small drop of blood, obtained
by pricking the dorsal aspect of the terminal phalanx of the finger,
is touched lightly with the end of the slide. The slide, with the drop
of blood thus transferred, is placed on a horizontal surface and with
the edge of a second slide, which is held at an angle of 45 degrees, the
drop is distributed in an even film over the surface. A film with its
edges parallel to the edges of the slide can be obtained by using as a
spreader a slide broken so as to give a narrow end. Blood films should
be protected from flies and dust. The film may be stained by Leish-
man's, Giemsa's or other appropriate stain.

          22. The thick film method.— There are several varieties of this
method, one of the best being as follows:—

          Four large drops of blood are placed at the corners of a small
square, 1/2 in. by 1/2 in., near the centre of the slide. With
a round needle or glass rod they are then pooled so that the
blood covers the 1/2 in. square thickly and evenly: " pud-
dling " must be avoided.

          The slides are now laid flat on the table, are covered with a
Petri dish and are allowed to dry completely. This is the
most important point of the whole process. A thick film- may
appear to be dry in half an hour, but the leucocytes have
not yet emigrated to and become adherent to the slide.
At least two hours at room temperature, or an hour in the
37°C. incubator, is required: otherwise the film gets washed
away during subsequent manipulations.

          Lay the perfectly dry thick film,—surface upwards,—flat on
a 'staining rack. Flood the slide very gently with the
following mixed solution:—

            Glacial acetic acid: 25 per cent. in
distilled water.... 4 parts

            Tartaric acid crystalline: 2 per cent. in
distilled water.... 1 part

          This solution dehaemoglobinizes the film, and the process should
be watched. An ordinary thick film will he completely
dehaemoglobinized in 5 to 10 minutes, but films with
thicker patches may require a little longer.

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