The injury to the artery may be a subcutaneous one such as a tear by a
fragment of bone: much more commonly it is a punctured wound from a stab
or from a bullet.
The aneurysm usually forms soon after the injury is inflicted; the blood
slowly escapes into the surrounding tissues, gradually displacing and
condensing them, until they form a sac enclosing the effused blood.
Less frequently a traumatic aneurysm forms some considerable time after
the injury, from gradual stretching of the fibrous cicatrix by which the
wound in the wall of the artery has been closed. The gradual stretching
of this cicatrix results in condensation of the surrounding structures
which form the sac, on the inner aspect of which laminated clot is
deposited.
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