There are also eight thousand leucocytes or colorless cells in a cubic millimeter of blood, this giving a total number of four billion in the average adult, and these vary in character and in relative numbers (Fig.

12).

The most numerous of these are round and slightly larger than the red cells; they have a nucleus of peculiar shape and contain granules of a definite character.

These cells serve an important part in infectious diseases in devouring and destroying parasites.

They have power of active independent motion and somewhat resemble certain of the free living unicellular organisms.