cells, resulting in the formation of a layer of epithelioid cells among which giant cells are sometimes found; this form of tuberculous endoarteritis is not due to extension from without.] Similar changes are observed in tuberculous areas in the lungs, especially in the vessels which pass like bands through the caverns. In such the increase in the thickness of the walls leads to great narrowing and even closure of the lumen, whereby the entrance into the blood of the infectious agent is prevented. Later, the small, round cells and leukocytes disappear, and a fibrillated connective tissue forms in the former vascular channel. Occasionally, in the midst of this connective tissue a new elastic membrane appears, thinner than the original elastic membrane whose general course it imitates; it forms the inner bounds of the new, greatly narrowed, lumen. But the process is not necessarily at a standstill as yet. Inside of the new elastic layer connective tissue may again form, which then leads to complete obliteration of the lumen and occludes the vessels permanently (endarteritis obliterans). Obliterating endarteritis of this kind must not be confounded with the process of organization which occurs in occluding thrombi and which also eventually may obliterate the lumen by new fibrous tissue. This process is described in the General Part in connection with the consideration of Thrombosis. In addition to this indifferent form of arteritis, which, it is true, is observed principally in tuberculous and syphilitic granulation tissue, there are also true, specific forms, in which syphilitic or tuberculous nodules arise in the arterial walls. In gummous arteritis the adventitia especially is the seat of the nodules that more rarely develop in the media or reach to the intima, which becomes thick on account of newly formed fibrous and elastic tissue. At first, the areas are rounded aggregations of lymphocytes and epithelioid cells; soon a caseous necrosis |