THE HEART 109 



The heart of the higher animals has a distinct arterial and 

 venous system, upon which its nourishment depends. The 

 arteries in the human heart each supply a given area of the 

 muscle, not invading the area of its neighbors, and no collateral 

 circulation can be established between them. If, therefore, 

 an artery is plugged by embolism or thrombosis, the part 

 of the heart wall that it supplies dies, becoming dull white 

 or faintly yellow in color, granular in cross-section, and is 

 soon replaced by connective tissue. Such an area is known 

 as an infarct. The result of closure of the arteries of the heart 

 depends upon the size of the vessel operated upon. Sometimes 

 no effect is produced, or the ventricles may stop beating and 

 fall into fluttering, twitching movements known as fibrillary 

 contractions. The auricles will, perhaps, continue beating 

 for a short time. As the arrest of the heart draws near the force 

 of the ventricular beat becomes irregular, but the pressure 

 in the heart gradually lessens during systole and becomes 

 greater during diastole. The cause of the arrest is not the 

 mechanical injury done to the heart, but the sudden anemia 

 produced. Anemia brought about by hemorrhage produces 

 a different series of symptoms because the heart works against 

 decreasing resistance in the arteries, which is not the case 

 when a branch of the coronary artery is ligated, for then the 

 peripheral resistance continues to be high. Closure of the 

 coronary veins produces fibrillary contractions in a rabbit in 

 from fifteen to twenty minutes, but is without effect upon 

 the dog, owing to the fact that some of the blood passes into 

 the cavities of the heart through the venoe Thebesii, and is 

 sufficient in amount to maintain the nutrition of the heart. 



The contractions of the heart favor the passage of the blood 

 through the coronary arteries in two ways : 



1. By the pressure produced in the aorta. 



2. By rhythmically compressing the walls of the bloodvessels 

 in the heart muscle. 



It has been found that the volume of the blood passing, 

 through the coronary circulation, unless it varies very much, 

 does not influence the rate of the beat, but does modify the 

 force of the contraction. 



The various constituents of the complex fluid, blood, have 

 different values in maintaining the activity of the heart. This 



