The term _external hæmorrhage_ is employed when the blood escapes on the surface; when the bleeding takes place into the tissues or into a cavity it is spoken of as _internal_. The blood may infiltrate the connective tissue, constituting an _extravasation_ of blood; or it may collect in a space or cavity and form a _hæmatoma_. The coughing up of blood from the lungs is known as _hæmoptysis_; vomiting of blood from the stomach, as _hæmatemesis_; the passage of black-coloured stools due to the presence of blood altered by digestion, as _melæna_; and the passage of bloody urine, as _hæmaturia_. Hæmorrhage is known as arterial, venous, or capillary, according to the nature of the vessel from which it takes place. In _arterial_ hæmorrhage the blood is bright red in colour, and escapes from the cardiac end of the divided vessel in pulsating jets synchronously with the systole of the heart. In vascular parts--for example the face--both ends of a divided artery bleed freely. The blood flowing from an artery may be dark in colour if the respiration is impeded. When the heart's action is weak and the blood tension low the flow may appear to be continuous and not in jets. The blood from a divided artery at the bottom of a deep wound, escapes on the surface in a steady flow. _Venous_ bleeding is not pulsatile, but occurs in a continuous stream, which, although both ends of the vessel may bleed, is more copious from the distal end. The blood is dark red under ordinary conditions, but may be purplish, or even black, if the respiration is interfered with. When one of the large veins in the neck is wounded, the effects of respiration produce a rise and fall in the stream which may resemble arterial pulsation. In _capillary_ hæmorrhage, red blood escapes from numerous points on the surface of the wound in a steady ooze. This form of bleeding is serious in those who are the subjects of hæmophilia. INJURIES OF ARTERIES The following description of the injuries of arteries refers to the larger, named trunks. The injuries of smaller, unnamed vessels are included in the consideration of wounds and contusions. #Contusion.#--An artery may be contused by a blow or crush, or by the oblique impact of a bullet. The bruising of the vessel wall, especially if it is diseased, may result in the formation of a thrombus which occludes the lumen temporarily or even permanently, and in rare cases may lead to gangrene of the limb beyond. #Subcutaneous Rupture.#--An artery may be ruptured subcutaneously by a blow or crush, or by a displaced fragment of bone. This injury has been produced also during attempts to reduce dislocations, especially those of old standing at the shoulder. It is most liable to occur when the vessels are diseased. The rupture may be incomplete or complete.