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