A STUDY OF THE CIRCULATION OF BLOOD 131 



skeleton and muscles of the chest wall and by the lungs, 

 but it is also inclosed in a tough bag of connective tissue 

 called the per-i-car'di-um (Greek peri = around + cardia = 

 heart). This sac is attached to the heart only at its upper 

 or larger end. In size it is considerably larger than the organ 

 it surrounds, and hence the heart has plenty of room in 

 which to expand and contract unhindered. The inner layer 

 of the pericardium and a thin outer layer of the heart (which 

 is continuous with the pericardium) are both formed of 

 smooth, glistening se'rous membrane, which secretes just 

 enough liquid to allow the heart to move freely within its 

 case. 



The Heart a Double Organ. When one makes a dissection 

 of the heart, one finds it to be composed of two halves which 

 are completely separated from each other by a muscular par- 

 tition. It will help a great deal toward understanding the 

 circulation of the blood if this fact is kept constantly in 

 mind, and hereafter the two halves of the heart will be 

 referred to respectively as the right heart and the left heart. 



The Cavities of the Right and Left Hearts. Each heart is 

 divided by a movable partition into an upper and smaller 

 chamber, called the au'ri-de, and a lower chamber called 

 the ven'tri-de (see Figs. 47, 48, 49). An examination of the 

 outside of the heart shows the significance of these terms ; 

 for, at the upper or larger end are the two small ear-shaped 

 auricles (Latin auris = ear -|- cula = little), while at the apex 

 are the ventricles which, taken together, doubtless reminded 

 the early anatomists of a small stomach (Latin venter = 

 stomach + culus little). 



A comparison of these four chambers shows important 

 differences. In the first place, the auricles have relatively 

 thin walls as compared with the ventricles, and the reason 

 for this is evident when we see that their function is simply 

 to receive the blood from the veins and to push it downward 

 into the ventricles. When one compares the walls of the 

 left ventricle with those of the right, one is struck with the 



