=Differential Leukocyte Count.=--It seems to be now quite clearly demonstrated that the mere establishment of a certain degree of leukocytosis does not furnish the surgeon a reliable guide for determining the presence of pus, it being an index of reaction rather than of actual severity of any particular kind of infection. A much more reliable guide is found in the proportion of polynuclear cells to the total number of leukocytes counted, _i. e._, by what may be called a differential count. In order to make this reliable, the normal ratio should first be determined. This is put at a point between 68 and 80 per cent. by various writers. As Gibson (_Annals of Surgery_, April, 1906) says, 75 per cent. may be considered the best working average. This average should be maintained as the total number of leukocytes increases, or else there is a disproportion which becomes significant. With a moderate leukocytosis there is a notable increase in polynuclear cells, and it may be estimated that there is either a severe form of lesion or less resistance to absorption, or both. [Illustration: PLATE I _Fig. I._ _Fig. II._ _Fig. III._ _Fig. IV._ _Fig. V._ _Fig. VI._ _Fig. VII._ _Fig. VIII._ _DRAWN BY J. N. Z. CHASE_] PLATE I. BLOOD. (Ehrlich triple stain.) (Prepared by DR. I. P. LYON.) Fig. I. TYPES OF LEUCOCYTES. _a._ Polymorphonuclear Neutrophile. _b._ Polymorphonuclear Eosinophile. _c._ Myelocyte (Neutrophilic). _d._ Eosinophilic Myelocyte. _e._ Large Lymphocyte (large Mononuclear). _f._ Small Lymphocyte (small Mononuclear). Fig. II. NORMAL BLOOD. Field contains one neutrophile. Reds are normal. Fig. III. ANÆMIA, POST-OPERATIVE (secondary). The reds are fewer than normal, and are deficient in hæmoglobin and somewhat irregular in form. One normoblast is seen in the field, and two neutrophiles and one small lymphocyte, showing a marked post-hæmorrhagic anæmia, with leucocytosis. Fig. IV. LEUCOCYTOSIS, INFLAMMATORY. The reds are normal. A marked leucocytosis is shown, with five neutrophiles and one small lymphocyte. This illustration may also serve the purpose of showing the leucocytosis of malignant tumor. Fig. V. TRICHINOSIS. A marked leucocytosis is shown, consisting of an eosinophilia. Fig. VI. LYMPHATIC LEUKÆMIA. Slight anæmia. A large relative and absolute increase of the lymphocytes (chiefly the small lymphocytes) is shown. Fig. VII. SPLENO-MYELOGENOUS LEUKÆMIA. The reds show a secondary anæmia. Two normoblasts are shown. The leucocytosis is massive. Twenty leucocytes are shown, consisting of nine neutrophiles, seven myelocytes, two small lymphocytes, one eosinophile (polymorphonuclear) and one eosinophilic myelocyte. Note the polymorphous condition of the leucocytes, _i. e._, their variations from the typical in size and form. Fig. VIII. VARIETIES OF RED CORPUSCLES.