MILIAEY TUBEBCULOSIS. 25 



general dissemination may be prevented. An old caseous deposit 

 is generally the starting-point of the blood infection. Tuberculosis 

 of the vessel walls ensues, not only at the site of invasion, but 

 also in other parts of the vascular system by direct settlement 

 of the organisms or by penetration of the vasa vasorum. General 

 dissemination throughout the body soon results. Bacilli which 

 have entered an artery are carried as emboli into the tissues 

 supplied by that vessel only ; but bacilli which enter a vein or 

 the thoracic duct may be carried into the general circulation and 

 reach all parts of the body. Unfortunately veins and the 

 thoracic duct are more likely to be perforated than are arteries, 

 since the walls of the latter are thicker. When the walls of the 

 thoracic duct are penetrated the general circulation is soon 

 invaded by way of the great veins into which the contents of 

 the duct are poured. It is noteworthy that the afferent lym- 

 phatics of the pleural and peritoneal membranes are not inter- 

 rupted by protective lymph-glands on their way to the thoracic 

 duct. 



There are several localities in the body where veins are 

 especially liable to become infected from tuberculous foci. The 

 pulmonary veins are involved more frequently than any other, the 

 infection spreading from tuberculous glands around the bifurca- 

 tion of the trachea or from the lung itself. The mesenteric veins 

 are liable to suffer by reason of their intimate relations to the 

 mesenteric glands. The lower part of the vena azygos major is 

 sometimes infected by tuberculous retroperitoneal glands, the 

 jugular veins by cervical glands, and the innominate veins by 

 glands in the superior mediastinum. 



Branches of the pulmonary artery may be directly invaded 

 in the lung, and even the main trunks of the aorta and pul- 

 monary artery have been infected from tuberculous mediastinal 

 glands. 



When bacilli enter the circulation by way of the systemic veins 

 or thoracic duct, they traverse the right cavities of the heart, and 

 are chiefly deposited in the lungs, the capillaries of which form 

 the first filter they meet; hence the most copious miliary deposit 



