THE BLOOD-VESSELS. 391 



the pulmonary artery, by which it is carried to the lungs, 

 where it is exposed to the air, and its color changed from a dark, 

 bluish red to a bright scarlet, and all impurities removed. It 

 is then carried back to the left auricle by the pulmonary veins, 

 and from the left auricle it is forced through the mitral valves 

 into the left ventricle, which contracts and forces it through the 

 semilunar valves, at the mouth of the aorta, into the aorta, to 

 be sent to all parts of the body, and again returned to the 

 heart b}^ the veins, to traverse the same rounds as before. 



It is easy to see how the arteries, after starting from the 

 heart in one trunk, divide and divide again, like the branches 

 of a tree, and how the veins return, like many little streams 

 flowing into still larger ones, and finally terminating in one 

 large stream ; but it is not so easy to see how the blood gets 

 from the fine branches of the arteries into those of the veins. 

 This is effected by means of 



The Capillaries. — These are a sort of net-work of little ves- 

 sels, too small to be seen by the naked eye, which run through 

 every j^art to which blood goes, and which form the connection 

 between the arteries and veins. 



The branches of the arteries frequently unite with each 

 other in their course, and the veins much more frequently; 

 so that if one branch should get clogged up, the other will sup- 

 ply the parts with blood. 



The Arteries. — I shall only attempt to give a general idea of 

 the most important branches of the arteries. I shall not bur- 

 den the reader with their unfamiliar names, but merely indicate 

 their locality. 



The great aorta, after leaving the left ventricle, gives oif a 

 small branch which supplies the substance of the heart with 

 blood. A little further, and it divides into the anterior, or for- 

 ward, aorta and the posterior, or hinder, aorta. 



The forward aorta divides into two other branches, called 

 the right and left innominata. These give oif branches which 



