CHAPTER XX 



THE FLOW OF BLOOD IN THE BODY 



298. Arteries. The tubes which conduct the blood 

 away from the heart are called arteries. From the left 



ventricle there goes a single 

 tube called the aorta. It 

 gives off branches, which 

 subdivide again and again, 

 until they are of micro- 

 scopic size and penetrate 

 to every part of the body. 

 From the right ventricle 

 there extends another tube, 

 called the pulmonary artery, 

 which conducts blood only 

 to the lungs, where it is 

 purified. 



299. Structure and action 

 of arteries. An artery is a 

 muscular tube covered with 

 a tough layer of connective tissue and lined with a layer 

 of very smooth, platelike cells. Its muscle can diminish 

 the size of the tube. The arteries are elastic, and are 

 always so full of blood that they are somewhat distended. 

 At each systole of the heart, from two to four ounces of 

 additional blood are suddenly forced into the already full 

 aorta. During the heart's diastole, the elasticity of the 



172 



An artery cut across (X 200). 



a smooth inner coat. 



b middle or muscular coat. 



c outer or connective tissue coat. 



d small artery to nourish the large one. 



