THE HEART. 



267 



Fig. 78. 



tinct from each other. (Fig. 78.) All the blood returning from 

 the body by the veins must pass through the lungs before it is 

 again distributed through the 

 arterial system. We have 

 therefore a double circula- 

 tion, and also a double heart; 

 the two sides of which, 

 though united externally, 

 are separate internally. The 

 mammalian heart consists of 

 a right auricle and ventricle 

 (a, b), receiving the blood 

 from the vena cava (i), and 

 driving it to the lungs ; and 

 a left auricle and ventricle 

 C/ 9} receiving the blood 

 from the lungs and driving 

 it outward through the arte- 

 rial system. 



In the complete or double 

 mammalian heart, the differ- 

 ent parts of the organ present 

 certain peculiarities and bear 



certain relations to each other, which it is necessary to understand 

 before we can properly appreciate its action and movements. The 

 entire organ has a more or less conical form, its base being situated 

 on the median line, directed upward and backward ; the whole being 

 suspended in the chest, and loosely fixed to the spinal column, by 

 the great vessels which enter and leave it at this point. The apex, 

 on the contrary, is directed downward, forward, and to the left, sur- 

 rounded by the pericardium and the pericardial fluid, but capable 

 of a very free lateral and rotatory motion. The auricles, which 

 have a smaller capacity and thinner walls than the ventricles, are 

 situated at the upper and posterior part of the organ (Figs. 79 and 

 80) ; while the ventricles occupy its anterior and lower portions. 

 The two ventricles, moreover, are not situated on the same plane, 

 but the right ventricle occupies a position somewhat in front and 

 above that of the left ; so that in an anterior view of the heart the 

 greater portion of the left ventricle is concealed by the right (Fig. 

 79), and in a posterior view the greater portion of the right ven- 

 tricle is concealed by the left (Fig. 80) ; while in both positions the 



CIRCCI. ATir>5 IN MAMMA MANS. n. Right 

 auricle. 6. Right ventricle, c. Pulmonary artery. 

 d. Lungs, e. Pulmonary vein. /. Left auricle, y. 

 Left ventricle, h. Aorta, i. Vena cava. 



