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.

HAEMORRHAGE IN SURGICAL OPERATIONS

The management of the haemorrhage which accompanies an operation includes
(a) preventive measures, and (b) the arrest of the bleeding.

#Prevention of Haemorrhage.#--Whenever possible, haemorrhage should be
controlled by _digital compression_ of the main artery supplying the
limb rather than by a tourniquet. If efficiently applied compression
reduces the immediate loss of blood to a minimum, and the bleeding from
small vessels that follows the removal of the tourniquet is avoided.
Further, the pressure of a tourniquet has been shown to be a material
factor in producing shock.

In selecting a point at which to apply digital compression, it is
essential that the vessel should be lying over a bone which will furnish
the necessary resistance. The common carotid, for example, is pressed
backward and medially against the transverse process (carotid tubercle)
of the sixth cervical vertebra; the temporal against the temporal
process (zygoma) in front of the ear; and the facial against the
mandible at the anterior edge of the masseter.

In the upper extremity, the subclavian is pressed against the first rib
by making pressure downwards and backwards in the hollow above the
clavicle; the axillary and brachial by pressing against the shaft of the
humerus.

In the lower extremity, the femoral is con