In it no structural changes in the vessel walls can be demonstrated, but severe bleeding takes place as the result of very slight injuries. In such individuals the hemorrhage resulting from the extraction of a tooth may be very dangerous and at times fatal. This condition is generally hereditary and is transmitted by the mother; as a rule the male children manifest the disease, but do not transmit it. On the other hand, the females pass it on to the males, but do not themselves manifest the disease. The most important change, and perhaps the only constant one, to be found in the blood in hemophilia is its lessened coagulability. This may be due to an excessive development of antithrombin, that substance in the blood which prevents coagulation within the vessels under normal conditions. It may be, however, that there is a lack of thrombokinase, the substance which brings about the coagulation of the blood under abnormal conditions. In some bleeders the amount of calcium present in the blood is decreased. Hemorrhage in such individuals has been successfully treated by the injection of normal blood-serum from man or horse. This probably supplies the necessary amount of thrombokinase.

Hemorrhage by rhexis may be caused by: (I) Increased blood-pressure, particularly in those cases in which, the bloodvessel walls being diseased, their elasticity is diminished. (2) Disease of the vessels, in which the walls become so weak that they are unable to withstand the normal pressure. (3) Traumatism, injury of some form sufficient to cause a lesion of the vessel wall.

Hemorrhage by diapedesis may follow in the course of (I) certain diathetic diseases, as scurvy, purpura hemorrhagica, leukemia, hemophilia, etc.; (2) in severe inflammations; (3) in severe hyperemia, either active or passive; (4) in certain forms of poisoning, particularly that by snake-bite; (5) alterations of innervation; (6) in hemophilia.