The red corpuscles have a certain degree of elasticity which helps them to pass through capillaries which are smaller even than their own diameter; after escaping from these the corpuscles regain their original form. In the presence of carbon dioxide they lose this elasticity and become distorted or crenate. The influence of high altitudes in increasing the number of corpuscles is known, but unexplained. For instance, a residence of less than a month in the mountains will cause an increase of from 2,000,000 to 3,000,000 corpuscles per cubic millimeter. It has been surmised that under the influence of oxygen red corpuscle formation is stimulated to greater activity; in other words, that the red marrow becomes more active in the production of the hematoblasts. In general terms it may be said that the blood of a normal adult male contains 5,000,000 red corpuscles per cubic millimeter, and that of an adult female 4,500,000. These figures are, of course, approximate and variable. When the number is reduced to 3,000,000 by common consent the case will be regarded as oligocythemia, and when increased to 6,000,000 as one of polycythemia. The latter condition is most evident in cases of newly born infants. The excess rapidly diminishes during the first week of extrauterine life. It is to be explained by the loss of fluid suffered by the infant upon the establishment of respiration. The proportion of red cells also varies according to the nutrition of the individual, the season of the year, the altitude (as above), and climate, and varies during menstruation, pregnancy, lactation, and at the climacteric. With the loss of red cells the number is reduced in proportion to the hemoglobin, although the change in one respect is not exactly proportionate to that in the other. That the _colorless corpuscles_, or leukocytes, are not all of one kind has been recognized for nearly sixty years, and long ago they were divided into granular and nucleated cells. A vast impetus to the study of hemocytology was given by Ehrlich, in 1878, when he introduced the use of aniline dyes. The reader must be reminded that some of these, like eosin, are acid in reaction, and others, like methyl blue, are basic; while a third group has been supposed to be neutral in reaction, like a mixture of methyl blue and acid fuchsin; but it has been found that the so-called neutral dyes have really a slightly acid reaction. We may, therefore, divide the cells according to the reaction of the dyes with which they usually are distinguished into the acid and basic, or, more technically, into oxyphile, which includes neutrophile, and basophile.