Venous thrombosis may occur from purely mechanical causes—­as, for example, when the wall of a vein is incised, or the vessel included in a ligature, or when it is bruised or crushed by a fragment of a broken bone or by a bandage too tightly applied.  Under these conditions thrombosis is essentially a reparative process, and has already been considered in relation to the repair of blood vessels.

In other cases thrombosis is associated with certain constitutional diseases—­gout, for example; the endothelium of the veins undergoing changes—­possibly the result of irritation by abnormal constituents in the blood—­which favour the formation of thrombi.

Under these various conditions the formation of a thrombus is not necessarily associated with the action of bacteria, although in any of them this additional factor may be present.

The most common cause of venous thrombosis, however, is inflammation of the wall of the vein—­phlebitis.

#Phlebitis.#—­Various forms of phlebitis are met with, but for practical purposes they may be divided into two groups—­one in which there is a tendency to the formation of a thrombus; the other in which the infective element predominates.

In surgical patients, the thrombotic form is almost invariably met with in the lower extremity, and usually occurs in those who are debilitated and anaemic, and who are confined to bed for prolonged periods—­for example, during the treatment of fractures of the leg or pelvis, or after such operations as herniotomy, prostatectomy, or appendectomy.

Clinical Features.—­The most typical example of this form of phlebitis is that so frequently met with in the great saphena vein, especially when it is varicose.  The onset of the attack is indicated by a sudden pain in the lower limb—­sometimes below, sometimes above the knee.  This initial pain may be associated with shivering or even with a rigor, and the temperature usually rises one or two degrees.  There is swelling and tenderness along the line of the affected vein, and the skin over it is a dull-red or purple colour.  The swollen vein may be felt as a firm cord, with bead-like enlargements in the position of the valves.  The patient experiences a feeling of stiffness and tightness throughout the limb.  There is often oedema of the leg and foot, especially when the limb is in the dependent position.  The acute symptoms pass off in a few days, but the swelling and tenderness of the vein and the oedema of the limb may last for many weeks.

When the deep veins—­iliac, femoral, popliteal—­are involved, there is great swelling of the whole limb, which is of a firm almost “wooden” consistence, and of a pale-white colour; the oedema may be so great that it is impossible to feel the affected vein until the swelling has subsided.  This is most often seen in puerperal women, and is known as phlegmasia alba dolens.

Treatment.—­The patient must be placed at absolute rest, with the foot of the bed raised on blocks 10 or 12 inches high, and the limb immobilised by sand-bags or splints.  It is necessary to avoid handling the parts, lest the clot be displaced and embolism occur.  To avoid frequent movement of the limb, the necessary dressings should be kept in position by means of a many-tailed rather than a roller bandage.