The blood _platelets_ or _plaques_ first described by Bizzozero, in 1882, have no small interest for physiologists and pathologists, but little for the practising surgeon. They number perhaps 5,000,000 per cubic millimeter and sustain a fairly constant ratio to the red cells. Their surgical interest is limited to the role which they may play in the formation of thrombus. The term _phagocytosis_ has to do in a general way with those leukocytes which act as scavengers by removing from the blood its noxious elements, presumably by a process of ingestion and digestion (see Chapter III). Examination and estimation of the various formed elements of the blood are very valuable to the surgeon in the study of the anemias, of acute inflammation when the presence of pus is suspected, in the presence of suspected cancer, and in the presence of such conditions as Hodgkin’s disease, the various disorders of the spleen, etc. The so-called _primary anemias_ include only the pernicious anemias and chlorosis; all others are designated as _secondary_. This distinction is not for convenience only, but serves a useful purpose. _Pernicious anemias_ produce a reduction both of the red corpuscles and the hemoglobin, the former usually in a greater degree than the latter, so that the _color index_ (see below) is usually plus. Many of the cells become nucleated and, in general, their size is increased. In chlorosis the reduction of the hemoglobin is relatively large and the color index is extremely low. In the secondary anemias the red cells and hemoglobin are reduced disproportionately, so that the color index is minus. There may or may not be a relative increase of leukocytes and of the nucleated red cells, but these latter are not so likely to be as large as those seen in primary anemias. The _color index_ is obtained by dividing the percentage of the hemoglobin present by the percentage of the red cells. Leukocytosis becomes pathological in conditions of acute inflammation where the neutrophiles (A) show the greatest relative increase. The degree of leukocytosis depends on two different factors: the intensity or the virulence _of the infection_, and the _vitality_ or resisting power _of the individual_. These vary within such wide limits that it is hard to predicate anything definite in a given case. In general the increase is supposed to be proportionate to the severity of the infection, though the greater the reactionary ability of the patient the larger the number of white cells. Where vitality is very low leukocytosis is less pronounced. It is possible to have toxemia to such a degree that the activity of the leukocytes seems to be destroyed. The following summary from Cabot puts things in very distinct form. Infection mild, vital reaction good--small leukocytosis. Infection less mild, vital reaction less good--moderate leukocytosis. Infection severe, vital reaction good--very marked leukocytosis. Infection severe, vital reaction poor--no leukocytosis.