96 PHYSIOLOGY AND HEALTH. 



but it closes again when the water above presses downward, 

 and prevents its return to the well. 



209. The heart is the centre of the circulating system. 

 It sends all the blood to the whole body through the arteries, 

 and receives it back again through the veins. There are two 

 sets of these blood vessels, each consisting of arteries and 

 veins. One set begins at the heart and extends through the 

 whole body. The other set reaches from the heart to the 

 lungs. The arteries open from the ventricles or lower cham- 

 bers of the heart, and carry the blood out. The veins open 

 into the auricles or the upper chambers of the heart, and 

 carry the blood back. 



210. The arteries are round tubes. They are firm in their 

 structure, and retain their cylindrical form when empty. 

 They are composed of three coats. The outer coat is dense 

 and strong, and is the principal means of resistance to pres- 

 sure. The' middle coat is thick and elastic, and expands 

 when the blood flows in, and contracts when it flows out. 

 The inner coat is very delicate, and forms a polished surface, 

 on which the blood flows easily. The arteries have the same 

 structure throughout the body. There is a valve between 

 the ventricle and the aorta, which allows the blood to pass 

 from the heart into the artery, but not to flow backward 

 from the vessel into the heart. 



211. One large artery, called the aorta, leads out from the 

 left ventricle upward, (Fig. VII. e, Fig. IX. c, d, e,) and then 

 turns downward, (Fig. IX. d.) It passes then along the 

 back-bone, through the chest and the abdomen. Great 

 branches pass out from this artery to the various parts of the 

 body. Two branches, called the carotids , (Fig. IX. /*, f,) 

 pass off from the arch and go to the head, one on each side 

 of the neck. Two branches, called the subclavians, (Fig. 

 IX. g, g,) go to the arms and hands. While passing through 

 the chest, the aorta sends branches to the walls of the chest, 

 (Fig. IX. A, h.) In the abdomen, the aorta sends the cceliac 

 artery to the stomach, the hepatic artery to the liver, the 

 renal arteries to the kidneys, &,c., and other branches to the 



