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I. INTRODUCTORY. THE term " I'eriarteritis Nodosa " has been applied more or less promiscuously t o two entirely distinct diseases oi the smaller arteries, both of them extremely rare, and both arording problems of considerable interest and difficulty with regard to their histological characters and etiology. One of these conditions is probably a true pemhrteritis, commencing in and affecting chiefly the outer coat of the diseased vessel, though the media and intima may also be affected. For these cases it is desirable to retain the term (' periarteritis nodosa." To the other and still rarer group of cases, to which special attention will be given in this paper, the name '(Polyarteritis Acuta Nodosa," suggested by Ferrari, should be applied. This latter condition is chsracteriPed by the presence of small localised nodules situated upon the walls of the smaller and mediumsized arteries, these nodules having usually a widespread distribution throughout the organs and tissues, though in some cases they may be more or less restricted to the arteries of one or of a small group of the organs or tissues. These nodules are due to localised inflammatory, destructive, and proliferative changes occurring in the vessel wall, probably the most important part of the change being the complete destruction of localised areas in the muscular coat. This is accompanied or followed by the giving way of the internal elastic lamina and the inner and outer coats, in which there niay also be found well-marked inflammatory and proliferative changes, the first mentioned