EMBOLISM. 271 



layers of fibrin containing blood- plates and leukocytes in its 

 meshes and is of a grayish-white color ; if rapidly, as the result 

 of the almost complete stoppage of the circulation, it is com- 

 posed of fibrin and all the elements of the blood, is soft and 

 red, resembling a post-mortem clot (Fig. 108). 



The cause of thrombosis is found either in alterations in 

 structure of the vessel-walls, in the composition of the blood 

 or the rapidity of its flow. If the circulation is arrested 

 at any point, coagulation ensues as the result of the altered 

 nutrition of the vessel-wall, producing structural changes 

 incompatible with preservation of the normal fluidity of the 

 blood. Inflammatory and degenerative changes in the vas- 

 cular walls, as well as alterations in the composition of the 

 blood, for instance after a prolonged illness like typhoid fever, 

 favor the same result. 



The fate of a thrombus may be organization, calcification 

 forming phleboliths, or fatty degeneration and liquefaction. 



Embolism is the process of plugging or stopping up of a 

 bloodvessel by foreign bodies of various description, carried 

 in the blood-stream and too large to pass through the vessel 

 at that point. 



The plug or embolus may be a portion of a thrombus ; of 

 a diseased heart-valve or of a tumor; masses of bacteria 

 and other parasites, and also pigment, as in malaria ; parti- 

 cles of fat ; and in wounds of large veins, air, which may 

 have gained entrance (Fig. 109). Its point of lodgement 

 depends on its source ; if from the systemic veins, it will be 

 arrested by branches of the pulmonary artery ; if from the 

 pulmonary veins or left side of heart, it will occlude some 

 systemic artery, most frequently in the spleen, kidneys, or 

 brain ; if from tributaries of the portal vein, it will be 

 arrested by branches of that vein within the liver. If the 

 obstructed artery is terminal i. e., has no free anastomoses 

 an infarct is the result. 



Infarcts may be anemic or hemorrhagic. The circulation 

 beyond the embolus is arrested a wedge-shaped anaemic area 

 is produced in which later occur coagulative necrosis and 

 caseation, the degenerated tissue being finally absorbed. 



In some cases, however, the affected wedge-shaped area is 



