THE CIRCULATION. 



275 



DIAGRAM OF THE HEART AND PULMONARY CIR- 

 CULATION IN MAMMALIANS. a. Right auricle. 

 b. Right ventricle, c. Pulmonary artery, d. 

 Lungs, e. Pulmonary vein. /. Left auricle, g. 

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



through the lungs, its left cavities to that through the general system. 

 It is of a somewhat conical form ; 

 its base, situated upon the me- 

 dian line, being directed upward 

 and backward, while its apex, in 

 man, points downward, 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 ven- 

 tricles, are situated at its upper 

 and posterior part, while the 

 ventricles occupy its anterior 

 and lower portions. The two 

 ventricles, moreover, are upon 

 different planes. The right ven- 

 tricle is somewhat in front and 

 above the left ; so that in an 

 anterior view the greater por- 

 tion of the left ventricle is con- 

 cealed by the right, and in a 

 posterior view the greater por- 

 tion of the right ventricle is concealed by the left; while in both 

 positions the apex of the heart is- constituted altogether by the point 

 FlG 58 of the left ventricle. 



The cavities of the heart and 

 of the adjacent blood-vessels on 

 each side, though continuous 

 with each other, are partially 

 separated by certain construc- 

 tions, The orifices by which 

 they communicate are known 

 by the names of the auricular, 

 auriculo-ventricular, and aortic 

 and pulmonary orifices ; the 

 auricular orifices being the pas- 

 sages from the vena3 cavas and 

 pulmonary veins into the right 

 and left auricles ; the auriculo- 

 ventricular orifices leading from 

 the auricles into the ventricles ; 

 and the aortic and pulmonary 

 orifices leading from the ven- 

 tricles into the aorta and pulmonary artery respectively. 



The auriculo-ventricular, aortic, and pulmonary orifices are fur- 



RIGHT CAVITIES OF THE HEART; Auriculo-ventricu- 

 lar Valves open, Arterial Valves closed. 



