464 THE CIRCULATORY SYSTEM. 



TUMORS. 



Primary tumors of the veins are rare. Small leiomyomata have been 

 described in the saphenous and ulnar veins. A myo-sarcoma as large as 

 a man's fist has been described, situated in the dilated veuacava inferior. 

 The veins are not infrequently secondarily involved by sarcomata and 

 carcinomata, and sometimes by chondromata. The thin walls of the veins 

 offer comparatively little resistance to the encroachment of malignant 

 tumors, which thus gain access to the circulation and may form metas- 

 tases in various parts of the body. 



PARASITES. 



Echinococcus is sometimes found in the veins, having either devel- 

 oped there or perforated from without. 



Two species of distoma (liver fluke} occur in man. D. Jiepaiicum 

 occurs rarely and, while usually found in the bile ducts, may occur in 

 the vena cava. I), licematobium is very common in man in Egypt and in 

 other parts of Africa, and usually occurs in the portal vein or its branches, 

 and frequently in other veins. 



The Capillaries. 



The walls of the capillaries are so thin and so intimately connected 

 with the surrounding tissues that 'their lesions are studied most appro- 

 priately among the diseases of the several organs. Dilatation of the 

 new-formed capillaries in tumors, granulation tissue, etc., and fatty and 

 hyaline degeneration of their walls, may be mentioned here as readily 

 observed lesions occurring under a variety of conditions.- The changes 

 which we assume to occur in the walls of the smaller veins and capillaries 

 in exudative inflammation, by reason of which fluids and blood cells pass 

 through them, are not yet sufficiently understood to be described with 

 definiteuess. 



The Lymph- Vessels. 



General Characters of the Lymph-Vessels. 



The smaller lymph-vessels can hadly be treated as independent structures, since 

 their walls are so closely joined with the tissues through which they pass. It is per- 

 haps wiser to follow the suggestion of Adler and Meltzer 1 and call the spaces of the con- 

 nective and other tissues into which transudation from the smaller blood-vessels takes 

 place, not lymph radicles or "lymph capillaries" as is commonly done, but to consider 

 them tissue spaces from which the transudates of varying composition are gathered 

 into the lymph-vessels. But the larger lymph-vessels we find the seat of various more 

 or less independent lesions. 



1 Adler and Meltzer, Jour. Exp. Med., vol. i., p. 482, 1896. 



