We are here concerned with varix as it occurs in the veins of the lower extremity. _Etiology._--Considerable difference of opinion exists as to the essential cause of varix. The weight of evidence is in favour of the view that, when dilatation is the predominant element, it results from a congenital deficiency in the number, size, and strength of the valves of the affected veins, and in an inherent weakness in the vessel walls. The _angioma racemosum venosum_ is probably also due to a congenital alteration in the structure of the vessels, and is allied to tumours of blood vessels. The view that varix is congenital in origin, as was first suggested by Virchow, is supported by the fact that in a large proportion of cases the condition is hereditary; not only may several members of the same family in succeeding generations suffer from varix, but it is often found that the same vein, or segment of a vein, is involved in all of them. The frequent occurrence of varix in youth is also an indication of its congenital origin. In the majority of cases it is only when some exciting factor comes into operation that the clinical phenomena associated with varix appear. The most common exciting cause is increased pressure within the veins, and this may be produced in a variety of ways. In certain diseases of the heart, lungs, and liver, for example, the venous pressure may be so raised as to cause a localised dilatation of such veins as are congenitally weak. The direct pressure of a tumour, or of the gravid uterus on the large venous trunks in the pelvis, may so obstruct the flow as to distend the veins of the lower extremity. It is a common experience in women that the signs of varix date from an antecedent pregnancy. The importance of the wearing of tight garters as a factor in the production of varicose veins has been exaggerated, although it must be admitted that this practice is calculated to aggravate the condition when it is once established. It has been proved experimentally that the backward pressure in the veins may be greatly increased by straining, a fact which helps to explain the frequency with which varicosity occurs in the lower limbs of athletes and of those whose occupation involves repeated and violent muscular efforts. There is reason to believe, moreover, that a sudden strain may, by rupturing the valves and so rendering them incompetent, induce varicosity independently of any congenital defect. Prolonged standing or walking, by allowing gravity to act on the column of blood in the veins of the lower limbs, is also an important determining factor in the production of varix. Thrombosis of the deep veins--in the leg, for example--may induce marked dilatation of the superficial veins, by throwing an increased amount of work upon them. This is to be looked upon rather as a compensatory hypertrophy of the superficial vessels than as a true varix.