THE VASCULAR SYSTEM 395 



Below the knee the popliteal artery terminates in the posterior and 

 anterior tibial arteries which are deeply situated beneath the calf 

 muscles. 



The posterior tibial artery extends to the foot. In the leg its branches 

 supply the leg muscles and the skin of the lower leg. One of its branches, 

 the peroneal artery, parallels the posterior tibial artery as far as the ankle. 

 In the foot the latter terminates as the lateral and medial plantar arteries. 

 The anterior tibial artery extends along the front of the leg to the foot 

 where it terminates as the dorsalis pedis artery with its metatarsal and 

 other branches. 



The Systemic Veins. Blood is returned to the heart from the head, 

 the upper part of the body, and from the arms by the precaval vein or 

 vena cava superior. The precava is formed by the union of the right and 

 left innominate veins. From the thorax blood is returned to the precava 

 by the azygos vein. (Fig. 327) 



Each of the innominate or brachiocephalic veins is formed by the 

 confluence of the internal jugular and subclavian veins. The left iimomi- 

 nate crosses the median plane of the body just above the arch of the 

 aorta, and unites with the right innominate vein immediately to the left 

 of the aortic arch. 



Blood from the anterior part of the scalp and face is returned to the 

 facial vein, a branch of the internal jugular. Venous blood from the 

 back of the scalp and neck enters the external jugular veins, and thus is 

 carried to the subclavian vein. The two chief veins tributary to the 

 common facial vein are the anterior and posterior facial veins. 



The external jugular vein is formed by the union of the posterior 

 auricular vein and the communicating branch of the posterior facial 

 vein. Other tributaries of the external jugular are the occipital, posterior 

 external jugular, transverse scapular, and anterior jugular vein. For 

 purposes of description, it is customary to divide veins into two classes, 

 superficial and deep. Most of those of the head and neck mentioned 

 belong to the group of superficial veins. 



The deep veins of the head include the veins of the diploe, which 

 drain the cancellous bones of the skull, the venous sinuses of the brain 

 membranes, and the veins of the brain, nose, eye, ear, pharynx, and larynx. 

 The chief deep veins of the neck are the internal jugular veins, which have 

 as tributaries the lingual from the tongue and the superior th3n:oid from 

 the thyroid gland. The vertebral veins which bring blood from the 

 vertebrae unite with the innominate veins, which also receive blood from 

 the deeper neck muscles through the deep cervical veins. From the 

 thyroid gland come the inferior thyroid veins and the thyroidea ima vein 

 which is usually single and median. Veins from the thymus gland, 

 trachea, and esophagus also open into the left innominate vein. 



