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PLATES XI-XVI) IT has been shown (Duguid, 1946) that thrombi in the coronary arteries become organised and form fibrous thickenings which are indistinguishable from atherosclerosis. The same occurs in the aorta, where mural thrombi are commoner than is generally supposed. I n the aorta such thrombi comprise not only the familiar masses of fibrin which form in relation to atherosclerotic ulcers, but also a variety of fine fibrinous encrustations many of which are too small to be seen by the naked eye. Although these encrustations occur with considerable frequency, even in young subjects, they are a form of lesion which does not appear to have been taken into account in the pathology of aortic disease. Nevertheless they become organised and are undoubtedly a source of intimal thickening. The larger mural thrombiThe transformation of mural thrombi is essentially the same in the aorta as in the coronary arteries, but the appearances produced are somewhat different and there are certain features which call for special description. These are best shown in some of the larger examples ( figs. 1-7).* Most aortic thrombi are composed mainly of fibrin, which when newly formed is loose in texture and somewhat ragged ( fig. l), but which later becomes more compact. At first the fibrin is exposed to the blood stream, but soon a layer of endothelium grows over its surface (fig. 2), and this is followed by the formation of a layer of fibrous tissue in the sub-endothelial zone ( fig. 3), making the thrombus appear as if it were a part of the intima. Two forms of change then follow, both of them tending to accentuate this effect. The first consists of a condensation of the fibrin whereby it comes to resemble fibrous tissue ; the second is a true organisation.

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