The Process Of Thrombosis

If a portion of blood inside a vessel be cut off from the circulation, then it will by and by coagulate just as it does outside the body, a leading factor in the coagulation being the disintegrating white corpuscles. In this way a Red thrombus is produced. But vessels are rarely so situated as to allow of a complete coagulation such as this; much more frequently the thrombus is I formed from the blood which is still moving, although, it may be, slowly, and it is of gradual growth. The formation of thrombi in the living vessels has been carefully studied by Zahn, whose experiments throw much light on the process.

The mesentery of a frog is exposed and subjected to microscopic •examination. A vessel of some size, an artery or vein, is chosen, and its wall in some way injured, as by twitching it slightly with the forceps, or placing a small crystal of common salt near it. Very soon white blood-corpuscles begin to adhere at the injured part. As the blood passes over it successive layers of white corpuscles adhere, and a growing clump of them is formed. Along with the white corpuscles a stray red one may be insinuated, or there may be several red ones. The clump" so formed, be it wholly white or partly mixed with red corpuscles, may be carried off', in which case a new one begins to form; but the clump may remain fixed and be continuously enlarged by successive depositions of corpuscles from the circulating blood. In course of time a change occurs in the appearance of the clump, the white corpuscles lose their individual outline to a great extent, and the clump gathers itself together into a grey granular mass in which neither by acetic acid nor by staining are the majority of the white corpuscles to be discovered. It has, indeed, very much the characters of fibrine which has been obtained by whipping the blood outside the body. The clump of white corpuscles, in fact, by the disintegration of the corpuscles and the attraction of the fibrinogen from the blood plasma has converted itself into a fibrinous coagulum. All the. white corpuscles, however, are not disintegrated, some are still recognizable in the mass. A similar mode of formation is observed when a vessel is injured by pricking with a needle, or by cutting it. The presence of foreign bodies, if they are rough on the surface, produces adhesion of white corpuscles and their conversion into thrombi in similar fashion. The thrombus formed in these various ways is called by Zahn the White thrombus.

Eberth and Schimmelbusch, from a very elaborate series of observations, have come to several very important conclusions. They assert that Zahn's observations are correct with the exception that it is the blood-plates and not the leucocytes which go to form the white thrombus. They do not regard the process as one of coagulation, but rather of conglutination; that is to say, the clumps of blood-plates run together and form white masses, in which no proper fibrine is present. When a foreign body, such as a thread, is introduced into the circulation then true fibrine is deposited, but not in the simple white thrombus. Leucocytes are often caught in the thrombi, in which they may be present in larger or smaller numbers. These observations are explained and amplified by the researches of Wooldridge and Lowit above referi'ed to. We may now say that an injury to the wall of a vessel causes the deposition of granules of solid matter from the blood, and that these granules have the characters of fibrine.

In actual pathological processes in the human subject the pure white thrombus is frequently seen, especially in the heart, but it is more frequently mixed. As a general rule the thrombus enlarges by fresh deposition, and it not infrequently happens that, as corpuscles accumulate, sufficient ferment is produced to allow of the coagulation of a layer of entire blood. Hence strata of red coagulum may alternate with strata of white. Again the thrombus after its formation may become compacted into a dense white structureless layer having a hyaline appearance, and composed of fibrine which adheres to the wall, and is so united that its boundary is indefinite; the fibrine is sometimes traversed by canals (channelled fibrine). This condition is often seen in the external layers of clot in aneurysms. We may thus •distinguish four forms of thrombus according to structure. (1) The red thrombus, composed of the entire blood; (2) the white thrombus; (3) the stratified and mixed thrombus; and (4) the hyaline thrombus.

The main cause of coagulation, then, is the contact of the blood with dead matter or altered living tissue. In the living body anything which interferes with the integrity of the vessel-wall or the endo cardium is likely to predispose to coagulation. Stagnation of the blood is often set down as a cause of coagulation, but this will act chiefly by altering the vessel-wall, and by keeping the white corpuscles removed from contact with the living tissue. As it is the endothelium of the vessels and endocardium which is in most immediate contact with the blood, it will be interferences with it that will conduce to coagulation.