Thrombi are also characterized by consistency and relative absence of moisture. A thrombus is brittle and dry as compared with a clot. In distinguishing between the two, difficulty arises only in the case of a thrombus which may have formed within a few hours before death. Post-mortem clots are moist, elastic, readily withdrawn from blood-vessels, and have a smooth and lustrous surface. Their color is either red, gray, grayish-yellow, or yellow, and is very often mixed. The lighter colors are due to causes which favor the precipitation of red blood-corpuscles before actual clotting takes place, or which occasion an increase of the white blood-corpuscles in fibrin. The thrombus becomes adherent to the vessel wall within a few hours, after its formation, in the case of the red thrombus, and at once, in the case of the white variety. A clot is never adherent, although it may seem so from its entanglement between the trabeculæ and tendons of the heart and the cavernous framework of venous sinuses. Such apparent adhesions are easily recognized by the smooth, shining, intact intima which is disclosed after the removal of a clot.
The thrombus not only tends to become enlarged by further depositions of material from the blood, but it also tends to become diminished in size from the contractile properties of its fibrinous constituent. Moisture is forced from the thrombus in consequence of this shrinkage, and its dryness is increased by subsequent absorption through the wall to which it adheres.
The changes eventually taking place in the thrombus are known as organization, calcification, and softening.
Organization is the transformation of the thrombus into a mass of fibrous tissue. This is accomplished, according to the researches of Baumgarten,21 by an outgrowth of endothelium from the intima of the vessel, the thrombus being absorbed as the growth of tissue advances. In the case of a thrombus due to the ligation of a vessel, a granulation-tissue also makes its way into the thrombus between the ruptured coats, and the new-formed fibrous tissue which replaces the thrombus becomes vascularized through this granulation-tissue. The vascularization of thrombi surrounded by unbroken walls is most likely to result from the extension into the thickened intima of new-formed branches of the vasa vasorum. Cohnheim claims that the organization of the thrombus may take place solely through the entrance of migratory cells, without any active participation of elements of the vascular wall. The canal is thus obstructed or obliterated by a fibrous tissue, which is pigmented or not, as the pre-existing thrombus contained red blood-corpuscles or not. These, when present, become transformed into granular or crystalline hæmatoidin, which may remain as a permanent constituent of the new-formed tissue.
21 Die sogenannte Organisation der Thrombus, Leipzig, 1877.
Even when the thrombus is completely obstructing at the outset, it is not necessary that a total obliteration of the vessel should result from its organization. It not rarely happens, either before or after the thrombus has yielded to the fibrous growth, in consequence of the shrinkage of the fibrin of the thrombus or of the contraction of the fibrous tissue replacing it, that gaps arise which become communicating canals. Through these the blood flows, and the vessel thus becomes only obstructed, not obliterated. The sieve-like tissue thus formed is spoken of as the result of a cavernous or sinus-like transformation of the thrombus. The length of time necessary for the removal of the thrombus and its replacement by fibrous tissue varies considerably. A vascularized granulation-tissue may be present within a week, and in the course of a month the thrombus may have been wholly removed, or a period of months may elapse and the thrombus and granulation-tissue still be present side by side.
The calcification of a thrombus takes place when the latter becomes impregnated with salts of calcium and magnesium. The condition may be present in thrombi which are exposed to a rapidly-flowing arterial stream, as well as in those which lie in venous pockets outside the course of the direct current of blood. The well-known phlebolites are examples of the latter variety. A calcified thrombus may be intimately united to the vascular wall, the results of calcification and organization being associated. Calcification and, in particular, organization represent favorable events in the history of thrombosis, as through their occurrence the process comes to an end, and disturbances, either local or remote, are prevented.
The softening of the thrombus, on the contrary, is always a source of danger. This is partly due to the nature of the products of the softening, whether bland or septic, and partly to the mechanical disturbances produced by the transfer of portions of the softened thrombus to remote parts of the body. All thrombi may become softened. When the process of organization advances normally, the softened parts are absorbed as rapidly as the formation of vascularized fibrous tissue progresses. If this formation is checked or stopped, the process of disintegration still continues. White corpuscles undergo fatty degeneration; red corpuscles give up their coloring matter and become converted, like the fibrin, into granules, and there results a granular detritus. This is present as a viscid, semi-fluid material, either red, gray, or yellow, according to the color of the thrombus. This simple softening is to be regarded as essentially chemical in character, and begins at the oldest portion of the thrombus and advances toward the periphery. Its products are capable of absorption without the production of serious disturbances, and are usually prevented from direct entrance into the blood-vessel containing the thrombus by the continuation of the latter from new coagulation or deposition upon its surface. The thrombus is thus extended as the softening progresses.
When the thrombus is comparatively free from red blood-corpuscles, the softened product, in consequence of its yellowish color, opacity, and viscidity, resembles pus. The so-called encysted abscesses projecting into the cavity of the heart, from its wall, are parietal and globular thrombi, in the interior of which softening has occurred. This form of softening is called simple or bland, as it is free from any evidence of local suppuration, inflammation, or general constitutional disturbance attributable to an absorption of poisonous material.
Septic softening is accompanied by general evidences of a blood-poisoning, and by the local phenomena of purulent inflammation. A suppurative thrombo-phlebitis or arteritis, occurs; that is, an acute inflammation of the wall of the vessel, corresponding in its origin to the seat of the thrombus, and characterized by the formation of pus. In the earliest stage the softened thrombus need not present products differing in appearance from those occurring in simple softening, but their effect is manifested by a rapidly-advancing inflammation of the vascular wall and by the evidence of septicæmia. Inoculation with such material produces a group of symptoms classified under the head of blood-poisoning.