Although thrombosis is commonly a morbid process, it is not uniformly so. Its physiological significance is illustrated by the part it takes in the closure of the umbilical and uterine vessels, after childbirth. The surgeon makes use of it in his efforts to overcome certain of the ill effects of amputation, and to accomplish a cure of such local diseases as aneurism, where it is deemed important to diminish the supply of blood.
The thrombus being a blood-clot, it is composed, like the latter, of fibrin and blood-corpuscles. It is presumable that the fibrinous part of a thrombus owes its origin to the same conditions which determine the presence of fibrin in blood removed from the vessels during life or in that within the vessels after death.
According to A. Schmidt,18 the blood and other fluids, in which clotted fibrin makes its appearance, contain two generators, called fibrino-plastic and fibrinogenous. The former is considered to be paraglobulin, a substance contained mainly in the white blood-corpuscles, while the fibrinogenous generator is held in solution in the plasma of the blood. When these materials are acted upon by a third, the fibrin ferment, clotting takes place and fibrin is formed. It is thought that the ferment is intimately connected with the white blood-corpuscles, for with the microscope coagulation is seen to advance as these become destroyed, and where the leucocytes are most abundant, there coagulation advances most rapidly. The elements of clotted fibrin are always present in circulating blood, but Brücke has shown that blood remains fluid, under ordinary circumstances, because of its constant contact with the normal vascular wall.
18 Rollett, Hermann's Handbuch der Physiologie, Leipzig, 1880, iv. 1, 114.
The general causes of thrombosis are those which produce an abnormal condition of the endothelium, a rapid destruction of the white blood-corpuscles, or a stagnation of the blood. With the presence of one of these causes there is often conjoined another, and the conditions under which they are present are conveniently used in the classification of thrombi.
Although stagnation of the blood is often an important immediate cause of its coagulation, it is apparent, from the investigations of Durante19 and others, that stagnant blood clots in the living vessels only when their endothelium is in an abnormal condition. With the co-existence of abnormal endothelium and stagnant blood, thrombi form with greater frequency and become more voluminous in a given interval of time.
19 Wiener Medizinische Jahrbucher, 1871, 321.
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.
Zahn20 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 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.