CHAPTER XVII 



P.A. 



THE COURSE OF THE CIRCULATION IN MAMMALS 



THE heart is to be regarded as a double organ, each half consisting 

 of an auricle and a ventricle. The right half contains dark venous 

 blood which has been returned from the body, and is sent to the lungs ; 

 the left heart contains the bright oxygenated blood which has been 

 returned from the lungs, and is distributed to the body. There are 

 thus two circulations the one, the pulmonary, from the right side 



of the heart by the pulmonary artery 

 to the capillaries of the lungs, and 

 back to the left heart by the pulmo- 

 nary veins; the other, the systemic, 

 from the left side of the heart by the 

 aorta to the arteries and capillaries 

 of the body tissues and organs, 

 whence the blood returns by the veins 

 to the right side of the heart. 



A schematic representation is 

 given of the circulatory system in 

 the accompanying diagram (Fig. 51). 



The venous blood flows into the 

 right auricle (R.A.) from the superior 

 and inferior venae cavae, and from the 

 right auricle into the right ventricle 

 through the right auriculo-ventricular 

 orifice. The right ventricle (R.V.) 

 drives through the pulmonary artery 

 the blood received from the right 

 auricle. The right auriculo-ventricular 

 valve, or tricuspid, and the pulmo- 

 nary semilunar valve, are represented 

 directing flow of blood in this direction. 

 From the pulmonary capillaries 



the blood returns by the pulmonary veins (P.V.) into the left 

 auricle (L.A.), and so through the left auriculo-ventricular orifice, 

 guarded by the mitral valve, into the left ventricle (L.V.). By the 

 left ventricle the blood is driven through the aortic orifice, guarded 

 by the semilunar valves, and is distributed to the systemic arteries, 

 and so to the capillaries of the various organs and back to the veins. 

 The muscular wall of the auricles, and that of the right ventricle, 



142 



I.V.C. 



FIG. 51. DIAGRAM OF HEART TO 

 SHOW THE COURSE OF THE BLOOD. 

 (M. S. Pembrey.) 



S.V.C., Superior vena cava; I.V.C., 

 inferior vena cava; R.A., right 

 auricle; T., tricuspid valves; 

 R.V., right ventricle; P., pulmo- 

 nary valves; P. A., pulmonary 

 artery; P. V ., pulmonary vein; 

 L.A., left auricle; M., mitral 

 valves; L.V., left ventricle; a, 

 aortic valves; A., aorta. 



