In the Aves the distribution and anatomy of this process present some differences. The aorta is as usual most conspicuously the seat of change, but it is noteworthy that the dilatation or ampulla immediately above the aortic valves and from which the main vessels spring, is practically always free of lesions which are on the other hand most marked in the thoracic and abdominal sections. One’s attention is usually attracted to the aortic surface by its roughness although visibly there may be no plaques, but upon close inspection a mottled opacity may be detected. This all seems due in the few cases subjected to tissue section, to hyperplasia of endothelia, with or without fibre increase. The media may show muscular granularity or no change at all. At the stage when plaques are formed, fairly well outlined, firm but rather brittle, raised areas are detected, seated upon a distinctly opaque gray wall. The remainder of the vessel may be smooth and elastic but sometimes, in the Accipitres for instance, a general resistance to pressure and tension is found. Microscopically such a vessel will show a media the seat of ruptured muscle fibres, split-up or broken elastica and some debris, while the intima is covered with active and distinct fibrocellular exudate. I have for comparison divided the cases into those in which the superficial productive character was prominent and those seemingly entirely a degeneration of the media, that is with inactive intima. In mammals 77 per cent. of the cases were of the degenerative type while in birds 50 per cent. were of this kind. The exact importance of this difference is not easy to evaluate, but with the facts that the bird has a greater elastic supply for its large vessels and a greater wall to calibre ratio, it is interesting. The aorta alone was affected in half of the birds, the remainder showing lesions in the carotids and femorals. The tendency for the media to degenerate would lay the basis for concavities on the intimal surfaces after the type seen in Mönckeberg’s sclerosis. A number of cases of this variety have been encountered, but instead of being better developed in the vessels of extremities as in man they have presented more definite pictures in the aorta and pulmonary vessels. Examples will be quoted under the discussion of the various orders. [Illustration: FIG. 3.—ARTERIOSCLEROSIS, ATHEROMA AND ANEURYSMAL DILATATIONS IN THORACIC AORTA. WILD CAT (FELIS RUFFUS). ]