THROMBOSIS AND EMBOLISM. Under normal cir cumstances the blood within a blood-vessel remains fluid so long as the lining endothelium is living, but if from injury, or extension of inflammation from outside or circulatory failure from weakness of heart action the nutrition of the endothelium in any vessel is impaired, coagulation of blood or thrombosis occurs. Since the veins have thinner walls than arteries and the blood flow is slow in them owing to their relative greater diameter spontaneous throm bosis is far commoner in veins than in arteries.
A thrombus is always most extensive in that direction in which the blood pressure is least ; hence with reference to the heart, the greater part of an arterial thrombus is distal, a venous thrombus is proximal. On the proximal and distal sides respectively throm bosis extends at least as far as the nearest lateral branch. The effect of thrombosis depends upon the seat of blockage and its ex tent as well as on the character of the vessels affected and on the question whether the clot is aseptic or septic. In advanced old age when heart action is feeble thrombosis is apt to occur in the cerebral and meningeal veins leading to local death of brain mat ter (cerebral softening). When the formation of clot once begins the process is liable to extension partly because the primary cause is central, partly because every portion of clot is foreign material upon which more clot will be deposited. Hence in old age cerebral thrombosis in time—it may be a week or more—ends fatally.
In another condition in which the wall of arteries is the seat of advanced calcareous degeneration (atheroma) even a fairly normal degree of heart action is unable to prevent extensive clotting in arteries of distal parts (e.g., leg) and collateral circulation being unattainable gangrene on the part occurs. Where the vessel is larger and particularly where it is the seat of aneurism (q.v.) this same process of thrombus-formation is conservative, for the de posited blood clot strengthens the weakened vessel wall and some times even brings about complete filling up of a saccular aneurism.
Where the thrombus is septic the whole series of changes is dominated by the local presence of bacteria ; in most cases a local abscess forms and the character of the morbid condition is de termined by that fact (see ABSCESS ; INFLAMMATION ; PATHOL OGY). In "white leg," a condition usually met with in women after
childbirth, in which thrombosis extends from the uterine sinuses into the iliac veins and thence into the femoral vein the seriousness of the condition is infinitely greater if the clot be septic.
The cohesion of a thrombus varies widely and is much reduced where suppurative changes are involved. Moreover the thrombus is rarely attached to the vessel wall over its whole extent ; usually the greater portion is non-adherent and simply occupies the lumen of the vessel. Hence variations in posture, sudden movement, even such variations as are occasioned by cardiac action are apt to detach portions of thrombus. Such detached portions are then carried in the blood stream (embolism) to distant parts. Where they will become lodged and what will happen at the seat of their lodgement depends upon their size, the site of their parent throm bus and above all on their aseptic or septic character.
If the parent thrombus be formed in the systemic veins or the right side of the heart (particularly the auricular appendix) the embolus will be carried to the lungs and may be large enough to block a main pulmonary artery and cause sudden death. Many cases of sudden death after childbirth or surgical operation are due to such "pulmonary embolism." If smaller the clot may be arrested in the substance of the lung itself leading to localized ob literation of the corresponding amount of pulmonary tissue. This will give rise to a variable degree of distress, but recovery is not impossible particularly if the clot be aseptic. If septic it occasions local abscess or gangrene and empyema (q.v.). If the parent thrombus be in the left side of the heart (q.v.), left auricular ap pendix or on a diseased mitral or aortic valve (see HEART, DIS EASES OF) the embolus is carried into one of the systemic arter ies and the effect depends upon whether the blocked vessel is an "end artery" or has anastomotic branches (see ARTERIES). If an end artery, e.g., the middle cerebral or the arteries of kidney or spleen, local death of the part normally supplied by the blocked artery occurs and an "infarct" results ; if the blocked artery has free anastomoses, e.g., superior mesenteric artery, practically no ill effects follow unless the embolus be septic.