THE CIRCULATORY SYSTEM. 55 



material as may be needed for building or repairing. 

 This is the circulatory system. The blood a bright 

 red fluid with which we are all familiar floats the 

 building material through the channels and reser- 

 voirs of the circulatory system. As the building of 

 a house requires many kinds of material, such as 

 wood, stone, iron, glass, lime, sand and putty, so the 

 structure of the body calls for materials suitable for 

 bone, muscle, nerve, hair, nails, and so on. All this 

 variety of material is carried by the blood in its 

 ceaseless rounds through the body. 



of the organs and vessels of this system 



circulatory are the heart, the arteries, the veins, 

 and the capillaries. The plate before 

 us shows the heart in its position, and gives us a very 

 good idea of the manner in which the arteries and 

 veins run to and from every part of the body. The 

 figures of the right arm and right leg are so drawn 

 by the artist as to show us chiefly the veins of these 

 limbs, while the figures of the left arm and left leg 

 show chiefly the arteries. 



The heart lies near the center of the 



lit* Heart. 



chest, a little to the left of the middle 

 line. A man's heart is about as large as his fist. It 

 is a very strong muscular pump or engine and does an 

 enormous amount of very important work, as we shall 

 soon see. It has four chambers two on each side. 

 The upper chamber of the right side of the heart is 

 called the right auricle (U) ; below this (W) is the 



