Leukopenia has still another meaning in certain cases of severe anæmia, where it indicates a highly unfavourable prognosis. Ehrlich has described (_Charité Annalen_ 1888) a case of posthæmorrhagic anæmia ending fatally, where an extreme diminution of the leucocytes occurred. Exact figures shewed that the greater proportion (80%) of white blood corpuscles consisted of lymphocytes, whilst the polynuclears amounted to 14% (instead of 70-72% normally). Eosinophil cells and nucleated red blood corpuscles were entirely absent. Ehrlich explained these phenomena by a deficient activity of the bone-marrow, which found expression in the insufficient formation of red and white blood corpuscles. As the anatomical basis of this deficient activity, he conjectured that in this case the fatty marrow of the big long bones could not have been changed to blood forming red marrow, as is the rule in severe anæmias. In two cases the autopsy fully confirmed this diagnosis made during life.
The ~blood-platelets~ were first described by Hayem, later by Bizzozero, as a third formed element of normal blood. They are roundish or oval discs free from hæmoglobin. They are extremely unstable under mechanical, thermal, and chemical influences. Their size amounts to some 3 µ. Specially characteristic is their tendency, the result of their extraordinary stickiness, to run together into largish clumps, "grape clusters." This circumstance greatly facilitates the distinction of the blood platelets from the other formed elements, but renders their enumeration most difficult. The apparatus usually used for counting the blood corpuscles is, for this reason, deceptive; for the platelets rapidly cling to its walls and remain there. All early authors (_e.g._ Bizzozero) endeavoured to obviate this error by some particular diluting fluid; but a number of these elements still remained fastened to the walls of the capillary tube of the mixing apparatus.
Recently Brodie and Russell have recommended a new mixture in which the platelets remain quite isolated, and are stained at the same time. They allow the drop of blood as it comes from the puncture to enter a drop of the fluid, and then estimate the relative proportion of red blood corpuscles to platelets[36]. The prescription for their solution is as follows: