The importance of the death of white blood-corpuscles in the formation of thrombi is generally admitted, and is especially insisted upon by Weigert. According to the observations of Zahn, the nucleus of certain thrombi is the result of the death of these leucocytes and their accumulation upon an altered intima. The experiments of Naunyn, Köhler, and others show that a thrombus may be rapidly produced by the injection into the blood of fibrino-plastic substances, and of those through which free hæmoglobin is admitted into the circulation. The former may be expressed from a fresh blood-clot; the latter may be obtained by thawing frozen blood, or by injecting such material (bile-acids, for instance) into the circulating blood as rapidly destroys the red blood-corpuscles. Although Weigert lays special stress upon the destruction of white blood-corpuscles in the formation of the thrombus, it appears, from the experiments above referred to, that indirectly the destruction of the red corpuscles is also of importance. Although largely made up of fibrin, a thrombus also contains blood-corpuscles, both red and white, and the appearance of the mass is modified according to the variations in the relative proportions of these constituents. Zahn[20] divides thrombi, according to their color, into red, white or colorless, and mixed varieties. The red owes its color to a large number of red blood-corpuscles, while the white and mixed forms contain various proportions of white blood-corpuscles and fibrin and a diminished number {58} of red corpuscles. The cause of this difference in the color of thrombi is to be sought for in their method of origin. When blood clots slowly in a dish, the heavier red corpuscles settle to the bottom, and the lighter white corpuscles form a superficial layer. Stagnant blood clotting rapidly furnishes a uniformly red mass. The red thrombus, like the red clot, is the result of the rapid coagulation of stagnant blood. The white thrombus, on the contrary, largely composed of white blood-corpuscles, represents a constantly increasing deposition of these from flowing blood. The mixed thrombi arise from a combination of both conditions, and are usually white at the outset. Thrombi formed in the heart and larger arteries are usually white, those in the auricular appendages and on venous valves are mixed, while red thrombi are more common in arteries and veins, since the conditions favoring their origin are more frequently met in such vessels. [Footnote 20: _Virchow's Archiv_, 1875, lxxii. 85.]