VENA LAVA.
The veins which unite to form the inferior vena cava transmit the blood which is derived from the lower extremities and from the vis cera of the pelvis and of the abdomen.
1. Veins of the lower extremities.— The veins of the lower, like those of the upper extremities, are divided into a el, ep and a superficial set ; they are likewise provided with valves, which are most numerous in the veins of the former class.
Superficial veins of the lower extremities.. The small veins, which in great ninnbers exist in the subcutaneous cellular tissue of the dorsum of the foot, unite to form two prin cipal trunks termed saphenous. These are placed, the one on the inner, and the other on the posterior aspect of the leg.
Internal or long saphena vein. — This vein first exists as a distinct vessel a little below the ankle joint, in front of which it ascends and passes along the inner surface of the leg, at the distance of half an inch from the inner margin of the tibia. At the knee it lies be hind the internal condyle, superficial to the tendons of the sartorius, gracilis, and semi tendinosus muscles, and continuing to ascend on the internal and anterior surface of the thigh, gains the " saphenic opening" in the fascia luta, through which it passes backwards to end in the femoral vein, at the distance of about an inch below Poupart's ligament.
In this long course the saphena vein re ceives many cutaneous branch s, and also several communicating veins from the deep vessels. In the thigh, it is joined, near its termination, by a large branch, which comes from the back of the limb, and by two or three veins which are derived from the an terior and external surfaces of the thigh. These latter pursue a direction upwards and inwards, and thus cross anterior to the femoral artery, so that they are liable to be injured by the incisions made to expose that artery. The trunk of the saphena vein itself is seldom thus endangered, as it ascends on a plane internal to the artery. Shortly before its termination, the saphena vein is enveloped In the meshes of the eribriform fascia, where it is joined by three cutaneous veins from the parieties of the abdomen,— viz. the superficial
pubic, the superficial eireumflexa du, and the superfici l cpigastric veins. When, from any cause, the circulation through the inferior vena cava is obstructed, those veins become remarkably enlarged and tortuous.
The internal saphena vein is accompanied, from the ankle to the knee, by the internal saphena nerve. The valves, in its interior, vary from two to six sets.
Communicating branches pass between this vein and the deep veins of the leg and thigh ; they are most numerous in the leg, where they connect the saphena with the anterior and posterior deep tibial veins.
Posterior or external saphena vein. — The external saphena vein is formed by the union of several branches from the outer side of the foot, and ascends into the leg behind the outer ankle. Inclining inwards and upwards, it gains the centre of the back of the leg, passes superficial to the groove between the heads of the gastrocnemius muscle, and enters the poplitmal region. Opposite the line of flexion of the knee-joint, this vein passes through a small round opening in the popli toeal fascia, and joins the poplitaeal vein about its centre. This vessel, sometimes called the short saphena vein, is accompanied in part of its course by the posterior saphena nerve. It is usually furnished with two valves, which are found near the termination of the vessel. This vein communicates but rarely with the deep veins of the leg.
Deep veins of the lower extremity. — In the lower, as well as in the upper extremities, the deep system of veins is represented by the " venm comites " of the larger arteries. The satellite veins of the posterior tibial artery are derived from the deep plantar veins (external and internal). At the upper part of the leg the posterior tibial veins are joined by the satellite veins of the peroneal, and subse quently, at the lower edge of the poplitants muscle, by those of the anterior tibial artery. By the union of these is the trunk of the poplitzeal vein formed.