CHAPTER XII 



THE CIRCULATORY SYSTEM 



211. The apparatus distributing the blood throughout 

 the body and keeping it constantly in motion is composed 

 of the heart, the great central 



pump; arteries, tubes to carry 

 blood from the heart ; veins, which 

 are tubes carrying blood to the 

 heart ; and capillaries, a network 

 of small tubes connecting arteries 

 and veins. These constitute the 

 vascular system (Fig. 85, p. 146). 

 In addition to them and forming 

 a part of the complete circulatory 

 system are the lymphatics and lac- 

 teals, sometimes called the lymph 

 vascular system. They are tubes 

 having walls thinner than those of 

 the blood vessels, running from 

 periphery to center, conve}dng 

 lymph and chyle. 



212. The Heart is a hollow, cone- rp . 



The heart is partly covered 



shaped muscle, inclosed in a mem- by the lungs, hut its true out- 

 branous sac called the pericardium, line is shown by a dotted liue ' 

 lying in the thorax between the right and left lungs (Fig. 

 86). Its base is directed backward and upward, while its 



153 



Fig. 86. Front view of the 

 viscera in their natural 

 relations. 



