110 HUMAN BIOLOGY 



cavity to be removed; one would then see the soft, pink 

 lungs on either side, nearly filling the chest cavity, and 

 between them the heart ' (Fig. 2). 



156. Chambers of the heart. — We have seen (A. B., 99) 

 that a fish's heart has two chambers, an auricle to receive 

 the blood from all parts of the body, and a muscular ventricle 

 to force the blood into the arteries which carry it to the organs 

 of respiration (gills) and thence by another system of arteries 

 to all parts of the fish's body. In the himian circulatory 

 system, the blood, after returning to the heart from the organs 

 of the body, is likewise forced through an auricle, a ventricle, 

 and arteries, and so reaches the breathing organs (lungs). 

 Unlike the circulation in the fish, however, the blood does 

 not pass from the breathing organs to the other parts of the 

 body directly, but returns by veins to the heart, and so an- 

 other auricle and ventricle are provided on the left side of 

 the heart. These receive the blood from the organs of 

 respiration, and force it to all parts of the body. Thus'we 

 see that we have two hearts, the chambers of which are 

 completely separated by a muscular partition; the right 

 heart receiving the blood from all over the body and pump- 

 ing it to the lungs ; the left heart receiving the blood from 

 the lungs and pumping it over all the body. 



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 



' The heart is not only surrounded by the skeleton and muscles of 

 the chest wall, but it is also inclosed in a tough bag of connective 

 tissue called the pericardium (Greek, peri = around + cardia = 

 heart). 



