THE HEART. 



319 



Fig. 99. 



a right auricle and ventricle (a, 6), receiving the blood from the vena 



cava (i), and driving it to the lungs; and a left auricle and ventricle 



(/, g) receiving the blood from 



the lungs and propelling it out- 



ward through the arterial sys- 



tem. 



In the mammalian heart, the 

 different parts of the organ pre- 

 sent certain peculiarities and bear 

 certain relations to each other, 

 which influence its action and 

 movements. The heart itself is 

 suspended somewhat freely in 

 the cavity of the chest, attached 

 to the spinal column mainly by 

 the great bloodvessels passing 

 through the superior and pos- 

 terior mediastinum. It is of a 

 more or less conical form ; its base, 

 situated upon the median line, 



being directed upward and back- 



,,.,., . , , 



Ward, While its apex points down- 



ward, forward, and to the left, 

 , , , ,, . ,. 



surrounded by the pericardium, 



but capable of a certain degree 



of 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. 100 and 101); while the ventri- 



CIRCULATION IN MAMMALIANS. a. 

 Right auricle . b . Right ventricle, c. Pulmon- 

 ary artery, d. Lungs, e. Pulmonary vein. 

 /. Left auricle, cr. Left ventricle, h. Aorta. 



< Vena cava> 



Fig. 100. 



Fig. 101. 



HUMAN HEART, anterior view. 

 a. Right ventricle. 6. Left ventricle. 

 c. Right auricle, d. Left auricle, e. 

 Pulmonary artery, /. Aorta. 



HUMAN HEART, posterior view. 

 a. Right ventricle. 6. Left ventricle. 

 c. Right auricle, d. Left auricle. 



cles occupy its anterior and lower portions. The two ventricles, more- 

 over, are not situated on the same plane. The right ventricle occupies 



