906 ANATOMY [VASCULAR axillary. They receive in their course many small branches from the muscles. The axillary vein also receives the cephalic and basilic veins. Thus, a single large trunk con veys away all the blood that has been circulating through the upper limb. This large vein passes as the subclavian (ein behind the clavicle, and reaches the lower part of the aide of the neck, where it is joined by the large veins that return the blood from the head and neck. The veins that return the blood from the inner and outer parts of the head and neck are called the external and internal jugular veins. The external is the smaller, and may commonly be seen beneath the skin on the side of the neck. It returns the blood that has been circulating on the outer part of the head, and must be regarded as a superficial vein. The internal jugular returns the blood that has been circulating on the face, in the brain, and cranial blood-sinuses, and in the deeper parts of the neck. It accompanies the carotid artery, and must thus be regarded as a deep vein. By the junction of the jugular and sub clavian veins at the root of the neck a large brachio-cephakc vein on each side is formed; these gradually converge, join, and form a single trunk, the superior vena cava, which, after a short course, enters the upper part of the right auricle of the heart. The veins corresponding to the inter costal arteries, which run between the ribs, do not open directly into either the superior or inferior vena cava, but pass to form the azygos vein, which begins in the cavity of the abdomen, then enters the cavity of the chest, and, as it courses upwards, gradually increases in size by receiving the various intercostal veins, until it finally terminates by joining the superior vena cava. The system of the Inferior Vena Cava consists of both superficial and deep veins, and is arranged as follows : The superficial veins of the foot are separated from the deep veins by the strong membrane or fascia which binds down the muscles. They commence by very fine branches arising from the capillaries of the skin. On the back of the foot the digital veins proceeding from the skin of the toes form an arch, from the inner side of which a vein, called the long saphena, arises. This passes upwards along the inner side of the leg and thigh, increasing consider ably in size in its course, owing to the number of veins joining it from the extensive surface of the skin of the limb. It terminates, at the upper part of the thigh, by passing through a hole in the fascia, and joins the femoral vein. From the outer side of the same arch arises the external saphenous vein, which runs up the back of the leg to the ham, and pierces the fascia to join the popliteal vein. The deep veins begin both on the back of the foot and in the sole. Those which arise on the back of the foot form the anterior tibial veins, and accompany the anterior tibial artery ; they receive a considerable number of branches in their upward course, which proceed from the great mass of muscles lying on the outer side of the leg. The veins which begin in the sole of the foot accom pany the plantar arteries, and then pass upwards, along the inner side, of the ankle-joint, to reach the back of the leg, along which they ascend as the posterior tibial veins, closely accompanying the posterior tibial artery, and receiv ing in their course numerous small veins that proceed from the muscles of the calf of the leg. At the upper part of the leg the anterior tibial veins pass to the back of the leg, and join the posterior tibial veins. The large popliteal vein ; formed by their junction, ascends behind the knee- joint, lying in the ham, along with the popliteal artery. It leaves the upper part of this space, and, passing to the inner side of the thigh, ascends as the femoral vein along with the femoral artery as far as Poupart s ligament, when it enters the cavity of the abdomen. At the upper part of the thigh it receives the profunda vein, correspond ing to the deep artery of the thigh, which conveys bad the blood that has been carried by that vessel to the numerous large and important muscles of the thigh. The femoral vein is also joined at this spot by the long saphena vein. When the femoral vein enters the cavity of the abdomen it becomes the external iliac vein. The external iliac vein receives the smaller veins which ramify in the lower part of the walls of the abdomen, as well as the large internal iliac vein, which corresponds to the internal iliac artery, and by their junction the common iliac vein is formed. The two common iliac veins gradually converge, and, about the level of the last vertebra of the loins, join to form a single large vein, the inferior vena cava. The inferior vena cava ascends at the back of the abdominal cavity lying on the right side of the aorta. Several veins open into it ; some corresponding with the parietal branches of the abdominal aorta, others with the capsular, renal, and spermatic arteries. The greater number of the veins pro ceeding from the organs contained in the cavity of the abdomen do not open directly into the vena cava, but form a large vein called portal. The vena cava passes through the diaphragm, enters the cavity of the chest, and terminates by opening into the right auricle of the heart. The Portal system of veins is formed by the veins which proceed from the large and small intestines, from the stomach, pancreas, and spleen ; they form the inferior mesenteric, superior mesenteric, splenic, and gastric veins, which join together in the neighbourhood of the pancreas to form the portal venous trunk. The portal vein then ascends to the under surface of the liver, which it enters at the portal transverse fissure. In the substance of the liver it subdivides into branches just like an artery, and the finest branches terminate in the lobules of the liver in a plexus of capillaries.- From this plexus the rootlets of the hepatic veins arise, which joining together form the large hepatic vein, which opens into the inferior vena cava before it pierces the diaphragm. Retzius has pointed out that an extra-peritoneal venous plexus exists in the abdo minal cavity, which connects the rootlets of the portal vein with those of the veins of the parietes of the abdomen. The wall of a vein possesses the same number of coats as that of an artery, but the coats are thinner. Veins are also extensively provided with valves, which are absent from the arteries except at the mouths of the aorta and pulmonary artery. LYMPH- VASCULAR SYSTEM. This subdivision of the Lymph; vascular system consists partly of small tubes or vessels, tics - the lymph vessels, and partly of collections of lymphoid or adenoid tissue (p. 849), the lymph glands. The lymph vessels or lymphatics are tubes with delicate transparent walls, which convey the fluid called lymph and chyle. They arise in the tissues and terminate by joining the venous system, so that their contained fluid flows towards the heart. They resemble veins in having a course from periphery to centre; in possessing valves, which are generally two in number and semilunar in shape ; in being divided into a superficial and a deep set the superficial lymphatics being situated, like the superficial veins, in the subcutaneous tissue ; the deep lymphatics accompanying the arteries and deep veins. Lymphatics differ, however, from veins in possessing in their course glandular enlargements, in having thinner coats, in being almost uniform in size, and not uniting into larger vessels as they pass onwards in their course. As a rule they are like fine threads, and their main trunk, the thoracic duct, is not bigger than a crow- quill. The lymph-vessels are divided into lacteal or chyle vessels and lymphatics proper. The lacteal or chyle vessels, named from the milk-like chyle which they contain, arise in the minute processes
called intestinal villi f which project from the free surface
