One of the difficulties of writing an article on this subject is that of finding a suitable title, since one is not dealing with a system of the body in the usual sense of the term, but, instead, with a number of conditions which are related only in that an abnormality exists either in the formed elements or the clotting mechanism of the blood. Accordingly there is a great variation in the spectrum of possible physical abnormalities and, indeed, there may be a serious blood disorder with

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... symptoms nor signs. For this reason it is necessary to approach the subject in a manner somewhat different to that employed by other writers in the current series of articles, and to stress particularly the importance of both the history and the laboratory investigations.