Fibrin is produced from _fibrinogen_, a globulin which is held in solution under ordinary circumstances, which has certain peculiarities of its own. When the change occurs it is entirely consumed and none remains in the blood serum. Fibrinogen is split up by a peculiar ferment called _thrombin_ into what we ordinarily speak of as fibrin and a small amount of a soluble globulin, which remains in solution in the serum. Thrombin is not a normal constituent of the blood, but is formed when it escapes, as the result of the reaction between certain calcium salts and a nucleoproteid, which has been called _prothrombin_. The latter arises from the disintegration of the leukocytes, especially the polynuclear, and the blood plaques, after the blood leaves the bloodvessels. Calcium salts seem absolutely necessary for coagulation; hence the value of the administration of calcium chloride in certain cases previous to operation. Another essential feature seems to be the absolute integrity of the endothelial lining of the bloodvessels, although for this fact there is no satisfactory explanation. If a portion of a vein is removed from the body after double ligation its contained blood will not coagulate for a long time. Blood which is kept circulating through the lungs and heart alone soon loses its coagulability; hence the liver seems to be concerned in some way in maintaining it. Certain other substances also seem to retard coagulation, such as the albumoses of snake venom, and certain synthetic, colloid, proteid-like substances, which can be introduced very gradually. If, however, they are introduced rapidly, or in large quantities, thrombosis occurs promptly. We have much to learn about the coagulation of the blood, but the above facts are at least suggestive to the surgeon. [Illustration: PLATE II FIG. 1 Small Vein showing Diapedesis of Leukocytes. (Engelmann.) _a_, leukocyte escaping between endothelial cells; _b_, _c_, leukocytes escaped; _f_, leukocytes migrating toward centre of attraction. FIG. 2 Septic Thrombosis of Pulmonary Capillaries after Puerperal Septicemia, showing Rapidly Increasing Colonies of Streptococci. (Klebs.)]