Causation

Any solid material in the circulating blood may obstruct a blood-vessel which it finds too small to give it passage. It may be a foreign body, such as a parasitic animal or vegetable, or a piece of a tumour or a piece of cretaceous matter broken off from a degenerated valve or vessel. Rut the most frequent source of embolism is a pre-existing thrombus. Thrombi most readily become detached from the heart or veins, and in either case the thrombus is most dangerous when it has grown into the current so as to be exposed to the force of the blood. In a vein a thrombus which completely obstructs the vessel will not be readily carried away. The actual displacement of the thrombus will often take place in consequence of some compression or movement of the body affecting the part where it is seated. This is sometimes seen in the case of thrombosis in the uterine veins where the first considerable movement after delivery may displace the thrombus and lead to embolism of the lungs.

The embolus, whatever its source, will obstruct, for the most part, an artery or capillary. The only practical exception to this is the case of the portal vein, which in its ramifications in the liver has the distribution of an artery. The possibility of a proper venous embolism has been asserted. Foreign bodies, especially if heavy, may fall backwards in the venous system, but this is not an occurrence of any practical moment, if, indeed, it actually occurs in human pathology (Recklinghausen). A thrombus or other loose solid body in the circulating blood will usually in its course be caught at a place where an artery is dividing; it often rides on the bifurcation. It sometimes becomes broken against the projecting bifurcation, and its fragments may be dispersed to the smaller branches, producing numerous embolisms in them. The embolus, acting as a foreign body, will usually induce thrombosis on its surface, so that it may get covered in by an encapsuling thrombus. As the embolus may be derived from a thrombus, and may induce thrombosis afterwards, it may be difficult to distinguish the one process from the other.

The Diagnosis of embolism from thrombosis rests on a survey of the existing conditions. In veins and in the heart any existing coagulum must be a thrombus, as embolism does not occur in them. In arteries we may have either, but the situation and circumstances will often guide us. Thrombosis in arteries almost necessarily implies disease of their walls, and the thrombus is firmly adherent. The embolus, on the other hand, will often be found riding over a bifurcation and nonadherent, or there may be part adherent (the encapsuling thrombus) and part non-adherent. Assistance may often be obtained by finding a source of embolism, such as thrombosis, elsewhere. Again, there are some arteries much more prone to embolism than others. In the arteries of the lungs, intestines, kidneys, and spleen, thrombi are comparatively rare, but in those of the brain, in the coronary arteries of the heart, and in those of the extremities both thrombosis and embolism are common.

Phenomena Of Embolism

These phenomena vary greatly in different cases, the variations depending chiefly on the circumstances of the vessels in regard to anastomosing communications. In this respect we have all degrees of difference.

(A) In Arteries With Free Anastomosis

In these, embolism will at once cause increase of pressure on the proximal side of the plug, and this, with relaxation of the arteries, may be sufficient to restore the circulation by means of the anastomosing connections. At the seat of the obstruction thrombosis will occur on each side, and by the organization of the thrombus the artery will be permanently converted into a solid cord from the nearest proximal to the nearest distal branch, the circulation being completely carried on by the anastomosing branches. The arteries with freest anastomosis are, in general, those whose branches are most liable from their situations to temporary obstructions from external pressure or otherwise. Hence embolism is usually of triv sequence in the arteries of muscle, of the skin, of the intestine, of the circle of Willis in the brain (the common carotids are liable to pressure), and of acinous glands. A similar result will follow capillary embolism. As the capillaries are in the freest possible inter communication, embolism produces little disturbance of the circulation, unless, as sometimes happens, there be many obstructed simultaneously.

(B) In Arteries With Very Imperfect Anastomosis

Cohnheim has endeavoured to distinguish certain arteries of the body as being entirely devoid of anastomosing connections - they are like the branches of a tree, each division having no distal communications with its fellows. Such arteries he calls End arteries. It is impossible to carry out this distinction absolutely, inasmuch as nearly all arteries have at least fine communications, and all communicate by their capillaries. It may be said, however, that the following arteries have at least exceedingly imperfect anastomosis, and are practically end arteries, namely, the pulmonary, renal, and splenic arteries, the coronary artery of the heart, the central artery of the retina, and the nutrient arteries of the brain, to which may be added the portal vein. In addition, the superior mesenteric artery and the external iliac arteries have anastomosing communications, which, in relation to the size of these arteries, are small. In such arteries as those mentioned the results of embolism are not strictly uniform, but are nearly always serious. It may be added that the capillaries may to some extent take the place of anastomosing arteries, and the results of obstruction may be thus rendered less serious. Indeed, the area of tissue affected by the conditions to be now described is usually less than that of the distribution of the obstructed artery. The position of the plug is therefore, for the most part, outside the affected area, that is to say, proximal to it.