180 



CIRCULATION OF THE BLOOD 



Before the electrical variations corresponding to the ventricular systole 

 a small diaphasic action current is to be observed (Fig. 64, PQ), which is 

 probably caused by the auricular contraction. This lasts only about 0.13 

 second (Einthoven). 



B. NUTRITION OF THE HEART 



The heart gets its blood supply through its coronary arteries, for distribu- 

 tion of which the text-books of anatomy should be consulted. Here it should 

 be recalled only that they do not anastomose with each other, and are therefore 

 terminal arteries. Besides, the heart wall obtains blood from the heart cavities 

 through the veins of Thebesius, which are in connection with the coronary ves- 

 sels (arteries and veins) by means of capillaries, and with the veins by means 

 of somewhat larger vessels. The capillary network of the heart is very close, 

 and besides this, the smallest vessels proceed directly from relatively large stems. 

 In those places where several muscle fibers unite, spiral vessels are found which 



seem adapted to maintain 

 the blood supply when the 

 fibers shift their position 

 and change their form 

 (Heynemann). 



The following is to be 

 observed with regard to the 

 behavior of the blood flow 

 through the coronary vessels 

 in different phases of the 

 heart's activity. In diastole 

 of the ventricle the vessels 

 of the heart wall are open, 

 and offer no hindrance to 

 the blood stream. When the ventricles contract they must sooner or later 

 exert such a pressure on the capillaries of the heart wall that the blood flow 

 in them is interrupted for a time, and is only resumed at the beginning of 

 relaxation. By this compression of the coronary vessels, blood is driven out 

 into the right auricle. It is found in fact that the quantity of blood flowing 

 in the coronary veins increases during systole. The evacuation of the coronary 

 veins thus brought about has the effect of diminishing the resistance to the 

 blood, so that at the next relaxation they are filled more easily. 



This variation in the caliber of its vessels produced by the movements 

 of the heart to which is to be added possibly a dilatation of the arteries 

 taking place at the beginning of systole causes a greater quantity of blood 

 to flow through the coronary vessels of a beating heart than of a quiescent 

 one, the quantity being in direct proportion to the force and frequency of 

 the heart beat (Porter, Langendorff). On the other hand the volume of blood 

 flow in a heart suffering from loss of coordination of its muscle fibers may 

 be even greater than in a normally beating heart, which is probably due to 

 the fact that no compression of the coronary vessels now takes place, while 

 the waving movements of the cardiac muscle fibers facilitates the passage of 

 the blood by a sort of light massage (Langendorff). The quantity of blood 



FIG. 64. Schematic representation of the action current 

 of the human heart, after Einthoven. 



