alteration, or detoxification. From the liver the blood flows through the hepatic vein to the inferior vena cava.  The systemic system is divided into the deep and superficial veins. The superficial veins lie immediately under the skin, draining the skin and superficial structures. The deep veins, usually located in the muscle or deeper layers, drain the large muscle masses and various other organs. Deep veins commonly lie close to the large arteries that supply the various organs of the body and typically have the same name as the artery they accompany. V E I N S O F T H E H E A D , N E C K , A N D BRAIN.—The superficial veins of the head unite to form the external jugular veins. The external jugular veins drain blood from the scalp, face, and neck, and finally empty into the subclavian veins. The veins draining the brain and internal facial structures are the internal jugular veins. These combine with the subclavian veins to form the innominate veins, which empty into the superior vena cava (fig. 1-36). V E I N S O F T H E U P P E R E X T R E M - ITIES.—The veins of the upper extremities begin at the hand and extend upward. A vein of great interest to you is the median cubital, which crosses the anterior surface of the elbow. It is the vein most commonly used for venipuncture. Also found in this area are the basilic and cephalic veins, which extend from the midarm to the shoulder. The deep veins of the upper arm unite to form the axillary vein, which unites with the superficial veins to form the subclavian vein. This vein later unites with other veins to form the innominate and eventually, after union with still more veins, the superior vena cava (fig. 1-36). VEINS OF THE ABDOMEN AND THO- RACIC REGION.—The veins from the abdominal organs, with the exception of those of the portal system, empty directly or indirectly into the inferior vena cava, while those of the thoracic region eventually empty into the superior vena cava (fig. 1-36). V E I N S O F T H E L O W E R E X T R E M - TIES.—In the lower extremities (fig. 1-36), a similar system drains the superficial areas. The great saphenous vein originates on the inner aspect of the foot and extends up the inside of the leg and thigh to join the femoral vein in the upper thigh. The great saphenous vein is used for intravenous injections at the ankle. The veins from the lower extremities unite to form the femoral vein in the thigh, which becomes the external iliac vein in the groin. Higher in this region, external iliac unites the internal iliac (hypogastric) vein from the lower pelvic region to form the common iliac veins. The right and left common iliac veins unite to form the inferior vena cava. THE LYMPHATIC SYSTEM LEARNING OBJECTIVE: Identify the parts of the lymphatic system and their function. All tissue cells of the body are continuously bathed in interstitial fluid. This fluid is formed by leakage of blood plasma through minute pores of the capillaries. There is a continual interchange of fluids of the blood and tissue spaces with a free interchange of nutrients and other dissolved substances. Most of the tissue fluid returns to the circulatory system by means of capillaries, which feed into larger veins. Large protein molecules that have escaped from the arterial capillaries cannot reenter the circulation through the small pores of the capillaries. However, these large molecules, as well as white blood cells, dead cells, bacterial debris, infected substances, and larger particulate matter, can pass through the larger pores of the lymphatic capillaries and, thus, enter the lymphatic circulatory system with the remainder of the tissue fluid. The lymphatic system also helps defend the tissues against infections by supporting the activities of the lymphocytes, which give immunity, or resistance, to the effects of specific disease-causing agents. PATHWAYS OF THE LYMPHATIC SYSTEM The lymphatic pathway begins with lymphatic capillaries. These small tubes merge to form lymphatic vessels, and the lymphatic vessels in turn lead to larger vessels that join with the veins in the thorax. Lymphatic Capillaries Lymphatic capillaries are closed-ended tubes of microscopic size (fig. 1-37). They extend into interstitial spaces, forming complex networks that parallel blood capillary networks. The lymphatic capillary wall consists of a single layer of squamous epithelial cells. This thin wall makes it possible for interstitial fluid to enter the lymphatic capillary. Once 1-31