venous systems in the body: the pulmonary, por- tal,  and  systemic. The  PULMONARY  SYSTEM  comprises  four vessels,  two  from  each  lung,  which  empty  into  the left atrium. These are the only veins in the body that  carry  freshly  oxygenated  blood. The PORTAL SYSTEM consists of the veins that drain venous blood from the abdominal part of the digestive tract (except the lower rectum), spleen, pancreas, and gallbladder and deliver it to  the  liver.  There  it  is  distributed  by  a  set  of venous capillaries. The blood in the portal system conveys absorbed substances from the intestinal tract to the liver for storage, alteration, or detox- ification. 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. They  usually  lie  close  to  the  large  arteries  that supply the various organs of the body (fig. 3-31) and usually have the same name as the artery they accompany. The  superficial  veins  of  the  head  unite  to  form the external jugular veins. They 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 in- nominate  veins,  which  empty  into  the  superior vena  cava  (fig.  3-31). The veins of the upper extremity begin at the hand and extend upward. An extremely valuable vein, the median cubital, crosses the anterior sur- face  of  the  elbow.  It  is  the  vein  most  commonly used  for  intravenous  injections  and  infusions. 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 later unites with other veins to form the innominate and even- tually,  after  union  with  still  more  veins,  the superior  vena  cava. In  the  lower  extremity  (fig.  3-32),  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. This  vein  is  sometimes  used  for  intravenous  in- jections   at   the   ankle.   The   superficial   venous system of the leg often becomes varicose, or ex- cessively  dilated,  particularly  in  persons  whose occupations   require   long   periods   of   standing. When  this  develops,  the  venous  valves  become  in- competent,  allowing  stagnation  of  blood  in  the dependent extremity. Under these circumstances varicose  ulcers  frequently  develop.  Ligation  at several  points  along  the  system  will  force  the venous return into the deep venous system and restore  normal  venous  circulation. 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,  it  unites  with  the  hypogastric  vein  from the lower pelvic region to form the common iliac vein.  The  two  common  iliac  veins  unite  to  form the  inferior  vena  cava. 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. LYMPHATIC   SYSTEM 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 circulation by means of venous capillaries, which feed  into  larger  veins.  Large  protein  molecules that have escaped from the arterial capillaries can- not reenter the circulation through the small pores of  the  venous  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  circulation  with  the  remainder  of the tissue fluid. LYMPH Lymph  usually  is  clear,  but  following  inges- tion  of  a  fatty  meal  the  lymph  contained  in  the lymphatic that drain the small intestine appears milky because of the fat globules that have been absorbed.  This  milky  lymph  is  called  CHYLE. LYMPH VESSELS Lymph vessels and lymph nodes form a net- work  throughout  the  body.  Capillaries,  like  veins, 3-25