Spontaneous Arrest Of Hemorrhage takes place in several ways, but depends upon several factors - the direction of the injury, whether transverse or parallel to the axis of the vessel; the size and nature of the vessel, artery, vein, or capillary; the force of the heart's action and the blood-pressure, and the amount of fibrin-forming substances present in the blood: (I) When a vessel is injured its walls contract and the lumen is diminished in size. The vessel also, being elastic, retracts within the surrounding tissues. ' (2) The blood, coming in contact with abnormal surroundings, coagulates just outside, then upon, and finally within the vessel; this latter being known as a thrombus. In this way the vessel becomes plugged and the bleeding ceases. Another factor is that, as a result of the escape of large amounts of blood, the heart becomes weaker, even to a point where syncope may result; following this the blood-pressure falls and is unable to displace the clot.

The results of hemorrhage vary not only according to the amount of blood lost, but also as to the rapidity; if occurring slowly, the blood-forming tissues have time to supply the loss. Then, too, the results depend upon the locality of the hemorrhage, an ounce or so may prove fatal in cerebral apoplexy. If the amount has been small, there will be no ill effects; if comparatively large, weakness and unconsciousness; if very large, death will result from cerebral anemia. When the blood collects within the tissue various changes take place. It undergoes coagulation, a condition in which fibrin factors acted upon by fibrin ferments, in the presence of calcium salts, form a solid body known as fibrin. The greater the amount of fibrin, the more difficult is it for the tissue to recover. The fluid elements are first taken up by absorption by the lymphatics. The corpuscular elements and the fibrin break up, hemoglobin is set free, and the particles are scattered through the tissue. The greater part will be slowly removed by the phagocytes, but some will remain. If the coagulation has been extensive the tissues may undergo a liquefaction necrosis, giving rise to a cyst.