288 TEXT-BOOK OF PHYSIOLOGY. 



At the present time it is generally believed as the result of many forms 

 of experimentation that the extra-mural coronary arteries are filled during 

 the time of the systole. For it has been shown that the semilunar valves do 

 not close the openings of the coronary arteries by reason of the presence of 

 blood behind them under a high pressure; that a division of one of the 

 branches of these arteries is followed by a spurt of blood synchronous with 

 the systole. Moreover, if a kymographic trace of the pressure within the 

 coronary artery be compared with the trace of the pressure within the 

 carotid artery, it will be found that there is a complete agreement between 

 them as the pressure in the two vessels rise and fall simultaneously and 

 as a corollary are filled during the systole. Because of the pressure which 

 the heart-muscle must exert upon the smaller arteries and veins within its 

 own substance during systole, it is probable that there is a temporary retarda- 

 tion of the flow of the blood during the systole in the coronary (the extra- 

 mural) vessels, followed by a return of the velocity during the period of 

 diastolic repose. 



During the diastole the blood flows freely from the extra-mural vessels 

 into the intra-mural arteries and capillaries. It is at this time too that the 

 heart-muscle receives from the capillary blood-vessels its nutritive material 

 and returns to the blood the products of its metabolism. During the systole 

 the intra-mural capillaries and veins are compressed and the blood driven 

 into the extra-mural veins. The greater the force and frequency of the beat, 

 the larger the volume of blood passing through the coronary system. 



Vaso-motor Fibers for the Coronary Arteries. The presence in the 

 vagus and sympathetic nerves, of vaso-motor fibers for the coronary arteries 

 has been a subject of much investigation and discussion. By reason of the 

 fact that stimulation of these nerves modifies the rate and the force of the 

 heart-beat, and these in turn modify the flow of blood through the vessels, it 

 is difficult to state whether the observed effects are the result of changes in 

 the caliber of the arteries or to a change in the character of the heart-beat. 

 Moreover owing to the anatomic relation which the arteries bear to the heart 

 muscle, the rapidity of the flow through them must vary with each contrac- 

 tion and relaxation and thereby the difficulty of interpretation is inceased. 

 The results of direct experimental investigations of Porter, however, lead to 

 the conclusion that the existence of vaso-motor (constrictor) fibers for the 

 coronary arteries is quite probable. 



The Effects of Ligation of the Coronary Arteries. As stated in a 

 foregoing paragraph the nutrition of the heart-muscle, its irritability and 

 contractility, depend on the blood-supply derived from the coronary vessels. 

 This is shown by the effects which follow its withdrawal. Ligation of both 

 coronary arteries in the dog is followed by a diminution in the force and 

 frequency of the heart-beat, and in a few minutes by complete cessation. 

 Ligation of even a single branch of a coronary artery of the dog heart, pro- 

 vided it supply a sufficiently large territory e.g., the arteria circumflexa 

 is sufficient to cause arrest in at least 80 per cent, of animals (Porter). With 

 the ligation of this vessel there occurs a gradual diminution in the force and 

 frequency of the systole. As the power of coordinate contraction declines 

 the heart-muscle frequently exhibits a series of independent contractions of 

 individual fibers and cells known as fibrillary contraction. All the results 



