60 ANATOMY OF THE HAT 



ricular appendices) are the ventral extensions of the right 

 and left atria, respectively, Avhich extend ventrally on 

 each side of the pulmonary artery. Notice on the surface 

 of the heart the injected coronary artery, which carries 

 blood to the tissues of the heart, and the coronary veins, 

 which drain the venous blood back into the general 

 circulation. 



The heart is a muscular pump which propels the blood 

 throughout the body. Its complete failure to function 

 means almost instant death. Venous blood returning from 

 the capillaries of the muscles, brain, digestive organs, kid- 

 neys, etc. of the body enters the right atrium through the 

 superior and inferior venae cavae. Contraction of the 

 right atrium forces the blood into the right ventricle. The 

 two atria contract simultaneously, but their contraction 

 alternates with the simultaneous contraction (systole) of 

 the ventricles. When the right ventricle is full it drives 

 the blood through the pulmonary artery to the lungs, then 

 relaxes (diastole) to receive more blood from the atrium. 

 The excess of carbon dioxide carried by the blood is elimi- 

 nated in the lung capillaries into the air contained in the 

 minute air sacs (alveoli) of the lung. At the same time 

 oxygen from the alveolar air penetrates the walls of the 

 air sacs and capillaries and unites chemically with the hemo- 

 glohin, or red coloring matter of the blood. The blood 

 then traverses the pulmonary veins to the left atrium of 

 the heart, thence to the left ventricle, and finally out 

 through the aorta to the tissues of the body. The internal 

 mechanism of the heart may now be elucidated by an exam- 

 ination of the dissected organ. 



Insert one point of the scissors into the severed end of 

 the pulmonary artery, carefully slit it lengthwise, continu- 

 ing the cut along the lateral wall of the right ventricle. 

 Bend back the cut walls and carefully wash out the coagu- 



