THE CIRCULATING SYSTEM. 



407 



Physiology," Lesson II. 8, 9, and 10. Nevertheless, it may 

 be well here to recapitulate certain leading facts. 



The heart is enclosed in a shut sac the pericardium one 

 of those closed membranous bags called " serous " from the 

 serous fluid they contain, the largest of which is the " peri- 

 toneum." Not only the heart itself, but also the roots of the 

 great vessels which spring from it, are invested by the peri- 

 cardium. 



'JW , & 



FIG. 354. THE HEART, GREAT VESSELS, AND LUNGS. FRONT VIEW. 



R V, right ventricle ; LV, left ventricle; RA, right auricle; LA, left auricle; 

 Ao, aorta; PA, pulmonary artery ; PV t pulmonary veins; RL, right lung; 

 LL, left lung ; VS, vena cava superior; SC, subclavian vessels ; C, carotids ; 

 R and LJV, right and left jugular veins ; VI, vena cava inferior ; T, trachea ; 

 B, bronchi. All the great vessels but those of the lungs are cut. 



The heart is placed in the thorax between the lungs and 

 entirely above the diaphragm. Of strong muscular structure, 

 it is divided by a complete partition into two halves, a right 

 and a left, and each half is subdivided, by an incomplete 

 partition, into a smaller cavity or auricle, and a larger cavity 

 or ventricle. The auricles alone give admission to blood 

 into the heart ; the ventricles alone expel blood from it. 



The openings of the auricles into the ventricles are guarded 

 by membranous valves which prevent regurgitation. 



The valve which guards the entrance into the right ven- 

 tricle is called tricuspid, and consists of three flaps attached 



