4 2 



THE VITAL PROCESSES 



of the pericardial sac, as seems at first glance to be the 

 case, but its relation to this space is like that of the hand 

 to the inside of an empty sack which is laid around it 

 (Fig. 14). The inner layer of the pericardium is closely 

 attached to the heart muscle, forming for it an outside cov- 

 ering. The outer layer hangs loosely 

 around the heart and is continuous 

 with the inner layer at the top. The 

 outer layer also connects at certain 

 places with the membranes surround- 

 ing the lungs and is attached below 

 to the diaphragm. Between the two 

 layers of the pericardium is secreted 

 a liquid which prevents friction from 

 the movements of the heart. 



Cavities of the Heart. The heart 

 is a hollow, muscular organ which has 

 moved. A. Place occu- j ts interior divided by partitions into 



pied by the heart. B. . -. --^i 



Space inside of perkar- four distinct cavities. The mam par- 

 dial sac. a. inner layer tition extends from top to bottom and 



of pericardium and outer diyides the heart into two similar por- 

 lining of heart, b. Outer 



tions, named from their positions the 

 right side and the left side. On each 

 side are two cavities, the one being di- 

 rectly above the other. The upper cavities are called auri- 

 cles and the lower ones ventricles. To distinguish these 

 cavities further, they are named from their positions the 

 right auricle and the left auricle, and the right ventricle 

 and the left ventricle (Fig. 15). The auricles on each side 

 communicate with the ventricles below ; but after birth there 

 is no communication between the cavities on the opposite 

 sides of the heart. All the cavities of the heart are lined 

 with a smooth, delicate membrane, called the endocardium. 



D 



FIG. 14. Diagram 

 of section of the peri- 

 cardial sac, heart re- 



layer of pericardium. 

 Covering of lung. 

 Diaphragm. 



