164 UNCONSCIOUS NERVOUS OPERATIONS 



the foot in a palmar arch. All these arteries give off 

 numerous other branches all along their course. 



227. The Principal Veins (Fig. 85). The blood con- 

 veyed by the arteries to the capillaries of all parts of the 

 body passes there into the small veins, which unite into 

 larger ones, and, in general, run beside the arteries, and 

 often have corresponding names. They are gathered into 

 the superior vena cava, which collects the blood from the 

 head, arms, and portions of the chest ; the inferior vena 

 cava, by which the blood is returned from the remainder 

 of the body (the lungs and heart excepted) to the heart ; 

 and the coronary vein from the walls of the heart. All 

 these veins empty their contents into the right auricle. 

 Many of the veins are provided with valves to prevent 

 the reflow of the blood. 



228. The Pulmonary Circulation (Fig. 92). From the 

 right auricle the blood flows into the right ventricle, and 

 by its contraction is forced into the pulmonary artery, 

 which, dividing into two, one for each lung, carries the 

 blood to the capillaries of the lungs. It is then collected 

 by the pulmonary veins (two from each lung) and returned 

 to the left auricle, and passes thence to the left ventricle, 

 having completed the circuit of the body. 



In the systemic circulation the arteries convey the pure, 

 oxidized, nutrient blood to the capillaries, while the veins 

 return the impure, deoxidized blood to the heart. In the 

 pulmonary circulation the reverse is true. Impure blood 

 flows through the pulmonary arteries to be oxidized in 

 the lungs, and returned pure by the pulmonary veins. 



229. The Portal Circulation is an accessory and peculiar 

 circulation belonging to the liver. That organ not only 

 receives arterial blood through the hepatic artery, but is 

 also supplied by the portal vein with blood which has 



