eck is wounded, the effects of
respiration produce a rise and fall in the stream which may resemble
arterial pulsation.

In _capillary_ haemorrhage, red blood escapes from numerous points on the
surface of the wound in a steady ooze. This form of bleeding is serious
in those who are the subjects of haemophilia.

INJURIES OF ARTERIES

The following description of the injuries of arteries refers to the
larger, named trunks. The injuries of smaller, unnamed vessels are
included in the consideration of wounds and contusions.

#Contusion.#--An artery may be contused by a blow or crush, or by the
oblique impact of a bullet. The bruising of the vessel wall, especially
if it is diseased, may result in the formation of a thrombus which
occludes the lumen temporarily or even permanently, and in rare cases
may lead to gangrene of the limb beyond.

#Subcutaneous Rupture.#--An artery may be ruptured subcutaneously by a
blow or crush, or by a displaced fragment of bone. This injury has been
produced also during attempts to reduce dislocations, especially those
of old standing at the shoulder. It is most liable to occur when the
vessels are diseased. The rupture may be incomplete or complete.

_Incomplete Subcutaneous Rupture._--In the majority of cases the rupture
is incomplete--the inner and middle coats being torn, while the outer
remains intact. The middle coat contracts and retracts, and the
internal, because of its elasticity, curls up in the interior of the
vessel, forming a valvular obstruction to the blood-flow. In most cases
this results in the formation of a thrombus which occludes the vessel.
In some cases the blood-pressure gradually distends the injured segment
of the vessel wall and leads to the formation of an aneurysm.

The pulsation in the vessels beyond the seat of rupture is arrested--for
a time at least--owing to the occlusion of the vessel, and the limb
becomes cold and powerless. The pulsation seldom returns within five or
six weeks of the injury, if indeed it is not permanently arrested, but,
as a rule, a collateral circulation is rapidly established, sufficient
to nourish the parts beyond. If the pulsation returns within a week of
the injury, the presumption is that the occlusion was due to pressure
from without--for example, by haemorrhage into the sheath or the pressure
of a fragment of bone.

_Complete Subcutaneous Rupture._--When the rupture is complete, all the
coats of the vessel are torn and