462 THE CIRCULATORY SYSTEM. 



THROMBOSIS. 



Thrombosis of the inferior vena cava has been many times recorded. 

 It is usually due to pressure on the vessel or may occur as an extension 

 of thrombi from the contributory veins. 1 



Thrombosis of the superior vena cava usually follows pressure from 

 without by tumors, aneurisms, or enlarged lymph nodes. 



Thrombosis of other large venous trunks may occur under various local 

 and general conditions (see below, thrombosis and phlebitis, and page 

 72).* 



DEGENERATION. ' 



Fatty degeneration and calcification may occur in the walls of the veins 

 under conditions similar to those in which these changes take place in 

 the arteries. 



INFLAMMATION. (Phlebitis.) 



Inflammation of the veins, phlebitis, may involve chiefly the external 

 layers periphlebitis ; or the internal endophlebitis ; or, as is very fre- 

 quently the case, the entire wall may be affected. Phlebitis may be due 

 to an infectious thrombus, to injuries, or to an infectious inflammation 

 of the surrounding tissues. Thrombosis of the vein, either primary or 

 secondary, is a very constant accompaniment of phlebitis. Thrombosis 

 and phlebitis are not infrequent in association with or following infec- 

 tious diseases, in cachectic conditions, and with rheumatism and gout. 



Acute Infective Phlebitis may follow a suppurative periphlebitis. The 

 outer layers of the vein wall are congested, swollen, infiltrated with serum 

 and pus. The inner coats may become infiltrated with pus ; they may 

 become necrotic and disintegrate. A thrombus is constantly formed under 

 these conditions, which may for a time stop the circulation and keep 

 the products of inflammation and degeneration and infectious material 

 from, mixing with the blood ; but the thrombus itself is prone to disinte- 

 gration, and thus the exudates and fragments of disintegrated thrombi or 

 tissue or bacteria may enter the circulation. 



On the other hand, owing to the presence of irritating or infectious 

 material within the vein and the formation of a thrombus, the inflamma- 

 tory process may be at the commencement an endophlebitis, but usually, 

 if the inflammation be at all severe, the entire wall of the vessel is event- 

 ually involved. 



Acute phlebitis may terminate in the absorption of the thrombus and 

 the return of the vein to its normal condition, or in the obliteration of 

 the vein. The pyogenic cocci, the typhoid bacillus, the pneumococcus, 

 and several other micro-organisms have been found in phlebitis. In the 



1 For a case of complete fibrous obstruction of superior and inferior vena cavac, see 

 Meyer, Mt. Sinai Hosp. Reports, vol. iii., 1903, p. 35. 



>J For a consideration of peripheral venous thrombosis in heart disease consult Welch, 

 Trans. Assn. Am. Phys., vol. xv., p. 441, 1900, bibliography. 



