In the case of haemorrhage by diape-desis, and more particularly in haemorrhage due to passive hyperaemia and alterations in the blood, the haemorrhage will cease when the cause is removed. If the vessel has been actually ruptured, then the stoppage will occur by thrombosis. The torn edge of the aperture will _itself induce thrombosis (Fig. 27, page 98), and as the blood is in motion the thrombus will be a white or mixed one. It often happens that as the current is too strong to allow of a thrombus readily forming at the aperture, the coagulation may begin in the blood outside, and the clot thus formed may materially assist in stopping the aperture. Thus in the case of a divided artery the contraction of the muscular coat causes narrowing of the calibre and a withdrawal of the vessel within its sheath. There is thus left a certain extent of empty sheath in which the blood may coagulate. This contraction of the muscular coat is brought about in the first instance by the stimulus of the agent which caused the rupture, and it will be kept up by the irritation of the blood on the exposed and torn structures. Other circumstances may favour the stilling of the haemorrhage. If the bleeding is considerable the anaemia, by weakening the heart and reducing the blood-pressure, will more readily allow of thrombosis. On the other hand, circumstances may be unfavourable to the stilling of the haemorrhage, more especially when they hinder contraction of arteries. Amongst these may be mentioned disease of the walls of arteries (atheroma, aneurysm), the application of warmth, and a longitudinal wound. In the last-mentioned case the contraction of the artery will cause the wound to gape, and the haemorrhage is with difficulty stopped unless the surgeon cuts the vessel right across.