In course of time the collateral branches of the vessel above and below the level of section enlarge and their inter-communication becomes more free, so that even when large trunks have been divided the vascular supply of the parts beyond may be completely restored. This is known as the development of the _collateral circulation_. _Imperfect Collateral Circulation._--While the development of the collateral circulation after the ligation or obstruction from other cause of a main arterial trunk may be sufficient to prevent gangrene of the limb, it may be insufficient for its adequate nourishment; it may be cold, bluish in colour, and there may be necrosis of the skin over bony points; this is notably the case in the lower extremity after ligation of the femoral or popliteal artery, when patches of skin may die over the prominence of the heel, the balls of the toes, the projecting base of the fifth metatarsal and the external malleolus. If, during the period of reaction, the blood-pressure rises considerably, the occluding clot at the divided end of the vessel may be washed away or the ligature displaced, permitting of fresh bleeding taking place--_reactionary_ or _intermediary hæmorrhage_ (p. 272). In the event of the wound becoming infected with pyogenic organisms, the occluding blood-clot or the young fibrous tissue may become disintegrated in the suppurative process, and the bleeding start afresh--_secondary hæmorrhage_ (p. 273). (b) If an artery is only _partly cut across_, the divided fibres of the tunica muscularis contract and those of the tunica externa retract, with the result that a more or less circular hole is formed in the wall of the vessel, from which free bleeding takes place, as the conditions are unfavourable for the formation of an occluding clot. Even if a clot does form, when the blood-pressure rises it is readily displaced, leading to reactionary hæmorrhage. Should the wound become infected, secondary hæmorrhage is specially liable to occur. A further risk attends this form of injury, in that the intra-vascular tension may in time lead to gradual stretching of the scar tissue which closes the gap in the vessel wall, with the result that a localised dilatation or diverticulum forms, constituting a _traumatic aneurysm_. (c) When the injury merely takes the form of a _puncture_ or _small incision_ a blood-clot forms between the edges, becomes organised, and is converted into cicatricial tissue which seals the aperture. Such wounds may also be followed by reactionary or secondary hæmorrhage, or later by the formation of a traumatic aneurysm.