Two forms of haemorrhage are to be distinguished, namely, by rapture (per rhexin), and by diapedesis (per diapedesiri). In the latter case the blood escapes through the walls without solution of their continuity. Haemorrhage by rupture may take place from any size or kind of vessel, and when it occurs there will be an escape of the whole blood. Haemorrhage by diapedesis occurs only in the finest vessels, mainly the capillaries; it is chiefly the red corpuscles which escape, and with them, of course, some fluid, but the fluid has not the constitution of the entire blood-plasma.

1. Causation

The escape of blood is produced for the most part by injury or disease of the walls of the heart or blood-vessels, or by an increase in the blood-pressure, or by both combined. Some would add to these, alteration in the constitution of the blood, but this will act in most cases by injuring the walls of the vessels. It is not possible to separate accurately in their causation haemorrhage by rupture and by diapedesis. It will be more useful to enumerate the various conditions which may lead in one way or other to haemorrhage.

Haemorrhage is most simply produced by Direct injury (traumatic haemorrhage), as in wounds, bruises, fracture of bones, etc. It may be produced by Reduction of pressure outside the vessels, so that the blood-pressure is relatively increased. In this way we may have haemorrhage as a result of reduction of the atmospheric pressure in cupping, in ascending hills, or in balloons. To a similar cause may be ascribed haemorrhage on sudden removal of tumours or exudations.

Venous obstruction is one of the most important causes of haemorrhage. A frequent example of this is afforded by haemorrhage from the stomach and intestine in consequence of obstruction of the portal vein by thrombosis or by cirrhosis of the liver. Thrombosis of the longitudinal sinus of the dura mater may lead to haemorrhage in the brain substance; the haemorrhage is usually capillary, but may be in the form of a large extravasation. Violent contraction of muscles is not a common cause, but in severe vomiting the obstruction of the veins by the contraction of the muscular coat of the stomach frequently produces ecchymosis of the mucous membrane and erosions, and sometimes leads to considerable haemorrhage. In all these cases the venous obstruction induces passive hyperaemia, and the haemorrhage is mostly by diapedesis.

Increase of arterial pressure is rarely in itself a cause of haemorrhage. It is probably the chief element in the causation of the small subpleural and subpericardial haemorrhages met with in death by asphyxia. (See under Organs of Respiration.) It is, on the other hand, a frequent contributing cause where there is existing disease of the vessels.

Disease of the walls of the heart or vessels is another cause of haemorrhage. In this category are included conditions of the heart due to disease of the coronary arteries (which see), aneurysm and atheroma of arteries, and varicose dilatation of veins. In all of these a rise in the blood-pressure is frequently the determining cause of the haemorrhage, and hypertrophy of the left ventricle, implying a more or less permanent elevation of the blood-pressure, forms a constant source of danger, where the vessels are diseased. New-formed vessels whether in inflammatory new-formations or in tumours, are prone to rupture, presumably on account of thinness of their walls. Obstruction of arteries and capillaries produces haemorrhage, as we have already seen, in the case of the haemorrhagic infarction. Fat-embolism, even when the obstruction is entirely capillary, may have this effect, and according to Busch and Recklinghausen the haemorrhages may be in the brain and heart as well as in the lungs.

Haemophilia or haemorrhagic diathesis is a condition in which haemorrhage is apt to occur from very trivial causes, and it is very difficult to get the haemorrhage stopped. It is an eminently hereditary condition, and is probably due to faulty construction of the vessels. Virchow has observed thinness and smallness of the aorta in one case, and .Blagden describes a case in which the arteries were thin and nearly transparent.

Blagden's description of the whole case certainly suggests defects in the arteries as the cause of the haemorrhage. The patient, when a boy, had a tooth extracted, and the bleeding lasted 21 days. At the age of 26 a slight wound of the forehead caused a long-continued and recurring haemorrhage. The divided vessel was ligatured in two places, but it gave way behind the ligature, and the haemorrhage returned. The coats of the vessel were observed to be very thin, like paper. This haemorrhage was ultimately stopped by the application of kali purum, which caused extensive sloughing and exfoliation of bone. The fatal haemorrhage resulted from the extraction of a tooth. There was profuse bleeding from the alveolus, which could not be controlled by stuffing the socket, by cold, or by the cautery. Then the carotid was ligatured, but the bleeding continued, and occurred also in the operation wound. The patient died a week after the extraction of the tooth. After death the coats of the temporal and some other branches of the external carotid were seen to be thin and nearly transparent.

Alteration of the blood is another cause of haemorrhage. It may be that the blood has been altered as regards one or more of its essential constituents, as in scurvy, anaemia, leukaemia, or that a morbid poison is present in the blood, as in typhus fever (petechiae in skin), small-pox, yellow fever, snake-bite, poisoning by phosphorus. Watson Cheyne and Russell have ascribed the haemorrhage in purpura to obstruction of capillaries by colonies of microbes. Weigert has described a similar condition in haemorrhagic small-pox.

Nervous influences may lead to haemorrhage. The physiological haemorrhage of menstruation is effected by means of the nervous system. The same applies to vicarious menstruation, in which, in consequence of interference with the ordinary process in the uterus haemorrhage occurs in other situations, as the nose, mouth, or lungs. Bloody-sweating or Haematidrosis is another instance of bleeding in consequence of nervous influences.