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 haemorrhage, or
later by the formation of a traumatic aneurysm.

_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.