Conditions which influence the Natural Arrest of Haemorrhage.—­The natural arrest of bleeding is favoured by tearing or crushing of the vessel walls, owing to the contraction and retraction of the coats and the tendency of blood to coagulate when in contact with damaged tissue.  Hence the primary haemorrhage following lacerated wounds is seldom copious.  The occurrence of syncope or of profound shock also helps to stop bleeding by reducing the force of the heart’s action.

On the other hand, there are conditions which retard the natural arrest.  When, for example, a vessel is only partly divided, the contraction and retraction of the muscular coat, instead of diminishing the calibre of the artery, causes the wound in the vessel to gape; by completing the division of the vessel under these circumstances the bleeding can often be arrested.  In certain situations, also, the arteries are so intimately connected with their sheaths, that when cut across they were unable to retract and contract—­for example, in the scalp, in the penis, and in bones—­and copious bleeding may take place from comparatively small vessels.  This inability of the vessels to contract and retract is met with also in inflamed and oedematous parts and in scar tissue.  Arteries divided in the substance of a muscle also sometimes bleed unduly.  Any increase in the force of the heart’s action, such as may result from exertion, excitement, or over-stimulation, also interferes with the natural arrest.  Lastly, in bleeders, there are conditions which interfere with the natural arrest of haemorrhage.

#Repair of a Vessel ligated in its Continuity.#—­When a ligature is applied to an artery it should be pulled sufficiently tight to occlude the lumen without causing rupture of its coats.  It often happens, however, that the compression causes rupture of the inner and middle coats, so that only the outer coat remains in the grasp of the ligature.  While this weakens the wall of the vessel, it has the advantage of hastening coagulation, by bringing the blood into contact with damaged tissue.  Whether the inner and middle coats are ruptured or not, blood coagulates both above and below the ligature, the proximal clot being longer and broader than that on the distal side.  In small arteries these clots extend as far as the nearest collateral branch, but in the larger trunks their length varies.  The permanent occlusion of those portions of the vessel occupied by clot is brought about by the formation of granulation tissue, and its replacement by cicatricial tissue, so that the occluded segment of the vessel is represented by a fibrous cord.  In this process the coagulum only plays a passive role by forming a scaffolding on which the granulation tissue is built up.  The ligature surrounding the vessel, and the elements of the clot, are ultimately absorbed.

#Repair of Veins.#—­The process of repair in veins is the same as that in arteries, but the thrombosed area may become canalised and the circulation through the vessel be re-established.