Page:Tropical Diseases.djvu/74

46 The mixture is placed in a Petri dish, the dried film is put in this and left from five to twenty minutes at about 37° C.; the slide should be placed on glass rods to prevent the deposit of a precipitate. After staining, the preparation is washed with water, treated with a solution of tannin, washed again for a few minutes, and dried. The preparations are permanent, but keep better if unmounted.

Ross's thick-film method.—For the detection of small numbers of malaria parasites in the blood, and especially for the crescentic form, Ross recommends the adoption of thick films dehæmoglobinized in water. The parasites remain behind, clinging to the cell stroma. A drop of blood the size of a pin's head is dabbed on to a clean cover-glass so that an evenly spread smear results, about 5-7 mm. in diameter. After the dehæmoglobinized film has been dried in the air, it is stained in the ordinary way. The parasites are apt, however, to be greatly distorted by the action of the water.

Staining the flagellated body.—A sheet of thick blotting paper, having rows of oblong holes (1 in. by in.) cut out in it, is prepared; it is slightly but sufficiently moistened with water, and laid smoothly on a sheet of window-glass.

A patient in whose blood the crescent form of the parasite abounds is selected. His finger is pricked and a droplet of blood, the size of a large pin's head, is expressed. A clean microscope slip is then breathed on once, and the droplet of blood immediately taken up by lightly touching it with the centre of the breathed-on surface of the slip. The blood is now rapidly and somewhat unevenly spread out with the needle so as to cover an area of about in. by in. The slip is immediately inverted over one of the blotting paper cells and pressed down sufficiently to secure thorough apposition of the slip to the paper, without, at the same time, bringing the blood into contact either with the moistened paper forming the wall, or with the glass forming the floor of what is now a very perfect moist chamber. The rest of the paper cells are rapidly covered with blood-charged slips prepared in the same way. In from a quarter to three-quarters of an hour the slips are removed and dried by gently warming them over the spirit-lamp blood surface away from the flame. When dry the films are fixed with absolute alcohol, a few drops being poured on each. After ten minutes the alcohol is dried off, and a few drops of weak acetic acid (10 to 20 per cent.) are laid on the films and left there long enough thoroughly to dissolve out all the hæmoglobin. The slides are then washed in water and dried. They may now be stained with various reagents. So far, I have obtained the best results from weak carbol-fuchsin (20 per