Both arteries and veins are liable to become more or less completely obstructed during life by the coagulation of blood within them. When this condition occurs it is described as "thrombosis", and the obstructing clot is spoken of as a "thrombus". In the present day, thrombosis in the horse is only of seldom occurrence. Formerly, when blood-letting was resorted to in every conceivable ailment, it was a common affection of the jugular vein, and frequently gave rise to permanent obstruction and obliteration of the vessel.
The causes of thrombosis are chiefly injuries in one form or another, such as wounds, severe contusion, and stretching; indeed anything which will excite inflammation in the vessel wall or diminish its vitality may determine the coagulation of blood within it. It is also a consequence of degenerative changes in the structure of the vessel, and of arrest of the circulation from aneurism (fig. 196), or any other cause which induces the blood to stagnate.