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—Tumors of the mediastina possess more of a pathological than of a clinical interest. This is particularly true of new growths which invade the heart either primarily or secondarily. Until the researches of Virchow declared their real nature, mediastinal tumors were regarded for the most part as being carcinomatous. It is now known, however, that in the majority of instances they are sarcomatous or lymphosarcomatous. Very rarely the heart may be the seat of both cancer and sarcoma, but tumors in this situation are generally secondary. So slight are the symptoms they sometimes occasion, that their presence is not even suspected and is only discovered post-mortem. In all cases, where the probability of their existence is entertained, a positive diagnosis is extremely difficult. As a matter of pathological interest, and to illustrate the impossibility many times of differentiating new growths from valvular or other organic disease of the heart, we