This is not the place in which to go into any minute discussion of this subject nor further than should be of practical interest to the surgeon; nevertheless an examination of the blood by some common and routine procedure is so necessary in many surgical conditions that it is impossible to entirely avoid the subject in a work like this. I have accordingly condensed it and put the salient facts about leukocytes into the following table: _Classification of Leukocytes._ _Granular._ _Non-granular._ { A. With fine granules C. Hyaline. Transiti- {Oxyphile. (polynuclear) 60- onal (large mono- { 70 per cent. nuclear) 4-8 per {Neutrophile. cent. Normal. {The so-called B. With coarse {neutral stain granules (eosino- D. Lymphocytes (small {being slight- philes) 2-5 per mononuclear) 20-30 {ly acid. cent. per cent. { { E. Fine and coarse G. Atypical hyaline { granules (baso- (myelocytes). Patho- {Basophile. philes, mast- logical. { cells, etc.). { {Oxyphile. F. Atypical (myelo- { cytes). In normal blood by far the greater part of the leukocytes consists of A and D. _Lymphocytosis_ means a relatively high percentage of C and D. _Eosinophilia_ means an increase in the proportion of B. Basophile cells are not absolutely pathological, for they may be present in very small numbers in normal blood. The number of leukocytes in normal blood will average about 7000 to 10,000 per cubic millimeter, the percentage of each variety being given in the above table. Leukocytes are sometimes diminished in number; under diseased conditions they are often increased, and these are then included under the term leukocytosis. Variations occur daily and almost hourly under normal conditions. Increase naturally occurs after digestion, when the number of leukocytes may be almost doubled, the same being due principally to lymphocytes which are washed into the blood system from the lymph nodes by the flow of lymph or chyle. In starvation, however, the number may be remarkably reduced and in the case of the fasting man, Succi, the leukocytes were reduced at the end of the first week to 860 per cubic millimeter. The rather unusual condition of reduction of the number of corpuscles is called _leukopenia_. Leukocytosis is usually the rule in carcinoma, with increase in A and F; the more rapid the growth, the greater this increase. In sarcoma this is even more pronounced; when occurring without hyperinosis the probability of malignancy is greater. Non-malignant tumors produce no such changes.