In arterial haemorrhage 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 haemorrhage, 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 haemophilia.

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.

Incomplete Subcutaneous Rupture.—­In the majority of cases the rupture is incomplete—­the inner and middle coats being torn, while the outer remains intact.  The middle coat contracts and retracts, and the internal, because of its elasticity, curls up in the interior of the vessel, forming a valvular obstruction to the blood-flow.  In most cases this results in the formation of a thrombus which occludes the vessel.  In some cases the blood-pressure gradually distends the injured segment of the vessel wall and leads to the formation of an aneurysm.

The pulsation in the vessels beyond the seat of rupture is arrested—­for a time at least—­owing to the occlusion of the vessel, and the limb becomes cold and powerless.  The pulsation seldom returns within five or six weeks of the injury, if indeed it is not permanently arrested, but, as a rule, a collateral circulation is rapidly established, sufficient to nourish the parts beyond.  If the pulsation returns within a week of the injury, the presumption is that the occlusion was due to pressure from without—­for example, by haemorrhage into the sheath or the pressure of a fragment of bone.