CHAPTER V. 

 THE CIRCULATION OF THE BLOOD 



THE blood is contained in a system of closed vessels through which it is 

 kept in circulation during the life of the individual. The energy to keep up 

 this motion is supplied by the heart, which is a large muscular organ con- 

 sisting of four great divisions, the right and left auricles and right and left 

 ventricles. The right ventricle discharges its blood into the pulmonary 



FIG. 133. Diagram of the Circulation in an Animal with a Completely Separated 

 Right and Left Ventricle and a Double Circulation. Ad, Right auricle receiving the 

 superior and inferior venae cavae, Vcs and Vci; Dth, thoracic duct, the main trunk of the 

 lymphatic system; Ad, right auricle; Vd, right ventricle; Ap, pulmonary artery; P, lung; 

 Vp, pulmonary vein; As, left auricle; Vs, left ventricle; Ao, aorta; D, intestine; L, liver; 

 Vp, portal vein;Z,-y, hepatic vein. (After Huxley.) 



artery, through which it passes to the lungs, returning through the pulmonary 

 veins to the left auricle, and into the ventricle. From the left ventricle 

 the blood is pumped into the great aorta, and through its branches distrib- 

 uted to the entire body. The terminal arteries are continuous with the 

 IT 166 



