Of the methods given, the physician should select the one which best meets his needs. With any method, practice is essential to accuracy. The von Fleischl has long been the standard instrument, but has lately fallen into some disfavor. For accurate work the best instruments are the von Fleischl-Miescher and the Dare. They are, however, expensive, and it is doubtful whether they are enough more accurate than the Sahli instrument to justify the difference in cost. The latter is probably the most satisfactory for the practitioner, provided a {149} well-standardized color-tube is obtained. The specific gravity method is very useful when special instruments are not at hand. The Tallquist scale is so inexpensive and so convenient that it should be used by every physician at the bedside and in hurried office work; but it should not supersede the more accurate methods. II. ENUMERATION OF ERYTHROCYTES In health there are about 5,000,000 red corpuscles per cubic millimeter of blood. Normal variations are slight. The number is generally a little less--about 4,500,000--in women. _Increase of red corpuscles_, or _polycythemia_, is unimportant. There is a decided increase following change of residence from a lower to a higher altitude, averaging about 50,000 corpuscles for each 1000 feet, but frequently much greater. The increase, however, is not permanent. In a few months the erythrocytes return to nearly their original number. Three views are offered in explanation: (_a_) Concentration of the blood, owing to increased evaporation from the skin; (_b_) stagnation of corpuscles in the peripheral vessels, because of lowered blood-pressure; (_c_) new-formation of corpuscles, this giving a compensatory increase of aëration surface. Pathologically, polycythemia is uncommon. It may occur in: (_a_) concentration of the blood from severe watery diarrhea; (_b_) chronic heart disease, especially the congenital variety, with poor compensation and cyanosis; and (_c_) idiopathic polycythemia, which is considered to be an independent disease, and is characterized by cyanosis, blood counts of 7,000,000 to 10,000,000, hemoglobin 120 to 150 per cent., and a normal number of leukocytes. {150} _Decrease of red corpuscles_, or _oligocythemia_. Red corpuscles and hemoglobin are commonly decreased together, although usually not to the same extent. Oligocythemia occurs in all but the mildest symptomatic anemias. The blood count varies from near the normal in moderate cases down to 1,500,000 in very severe cases. There is always a decrease of red cells in chlorosis, but it is often slight, and is relatively less than the decrease of hemoglobin. Leukemia gives a decided oligocythemia, the average count being about 3,000,000. The greatest loss of red cells occurs in pernicious anemia, where counts below 1,000,000 are not uncommon. [Illustration: FIG. 69.--Thoma-Zeiss hemocytometer: _a_, Slide used in counting; _b_, sectional view; _d_, red pipet; _e_, white pipet.]