cut end_, the tension of the
ligature is usually sufficient to rupture the inner and middle coats,
which curl up within the lumen, the outer coat alone being held in the
grasp of the ligature. An internal clot forms and, becoming organised,
permanently occludes the vessel as above described. The ligature and the
small portion of vessel beyond it are subsequently absorbed.

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 haemorrhage_ (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 haemorrhage_ (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 haemorrhage. Should the wound become infected,
secondary haemorrhage is specially liable to occur. A further risk
attends this form of injury, in that the intra-vascular tension