270 PATHOLOGY OF THE BLOOD AND CIRCULATION. 



Passive hypersemia is produced locally by thickening of 

 venous walls, external pressure on the veins of a part by a 

 tumor, gravid uterus, etc. Cardiac weakness produces a gene- 

 ral venous congestion finally, if severe enough, of all the tis- 

 sues of the body. 



The consequences of venous congestion are more important 

 than those of active congestion the blood-serum transudes in 

 greater quantities than can be removed by the lymphatics, and 

 the tissues become cedematous and swollen. In the internal 

 organs there may result pressure-atrophy of the true paren- 

 chyma-cells and an increase of the connective-tissue elements. 



FIG. 108. 



Section of white thrombus containing few leukocytes, a, granular masses ; &, granu- 

 lar threads of fibrin arranged in a network ; c. parallel arranged threads of fibrin. 

 X 200 (Zeigler). 



Local anaemia or ischaemia : This occurs where there is inter- 

 ference with the blood-supply of a tissue, and results in atrophy 

 or some degenerative change, or if long continued in necrosis. 

 It is produced by thrombosis or embolism, atheroma of the 

 vessel-walls, arterial spasm, or external pressure. 



Thrombosis is the coagulation of blood within the heart or 

 bloodvessels during life. 



The appearance and structure of a thrombus depend on the 

 rapidity with which it is formed ; if slowly, it is composed of 



