To obtain the color index, divide the percentage of hemoglobin by the percentage of corpuscles. The percentage of corpuscles is found by multiplying the first two figures of the red corpuscle count by two. This simple method holds good for all counts of 1,000,000 or more. Thus, a count of 2,500,000 is 50 per cent. of the normal. If, then, the hemoglobin has been estimated at 40 per cent., divide 40 (the percentage of hemoglobin) by 50 (the percentage of corpuscles). This gives 4/5, or 0.8, as the color index. IV. ENUMERATION OF LEUKOCYTES The normal number of leukocytes varies from 5000 to 10,000 per cubic millimeter of blood. The number is larger in robust individuals than in poorly nourished ones, and if disease be excluded, may be taken as an index of the individual's nutrition. Since it is well to have a definite standard, 7500 is generally adopted as the normal for the adult. With children the number is somewhat greater, counts of 12,000 and 15,000 being common in healthy children under twelve years of age. _Decrease in number of leukocytes_, or _leukopenia_, is not important. It is common in persons who are poorly nourished, although not actually sick. The infectious diseases in which leukocytosis is absent (p. 160) often cause a slight decrease of leukocytes. Chlorosis may {157} produce leukopenia, as also pernicious anemia, which usually gives it in contrast to the secondary anemias, which are frequently accompanied by leukocytosis. _Increase in number of leukocytes_ is common and of great importance. It may be considered under two heads: A. Increase of leukocytes due to chemotaxis and stimulation of the blood-making organs, or _leukocytosis_. B. Increase of leukocytes due to _leukemia_. The former may be classed as a _transient_, the latter, as a _permanent_, increase. A. LEUKOCYTOSIS This term has not acquired a definite meaning. By some it is applied to any increase in number of leukocytes; by others, it is restricted to increase of the polymorphonuclear neutrophilic variety. As has been indicated, it is here taken to mean any increase in number of leukocytes caused by chemotaxis and stimulation of the blood-producing structures; and includes every increase of leukocytes except that due to leukemia. By chemotaxis is meant that property of certain agents by which they attract or repel leukocytes--positive chemotaxis and negative chemotaxis respectively. An excellent illustration is the accumulation of leukocytes at the site of inflammation, owing to the positively chemotactic influence of bacteria and their products. A great many agents possess the power of attracting leukocytes into the general circulation. Among these are bacteria and many organic and inorganic poisons.