2l6 THE CIRCULATION OF THE BLOOD 



closure of a large coronary branch, in the dog, may be occasional and 

 transient irregularity or arrest of the ventricular contractions preceded 

 by irregularities in the force of the contractions and a diminution in the 

 amount of work performed. The force, rather than the rate, of the ven- 

 tricular contractions is closely dependent upon the blood supply to the 

 coronary arteries. Porter and others have shown that the pressure in the 

 coronary vessels follows closely the pressure in the aorta and that there is 

 not, as formerly claimed, a closure of these vessels by the pressure of the 

 systole of the ventricle. 



The vessels of Thebesius, which have been demonstrated to open both 

 into the auricular and ventricular cavities, must now be looked upon, ac- 

 cording to the investigations of Pratt, as an important source of cardiac 

 nutrition. Blood may pass through them by way of connecting branches 

 to the coronary arteries and veins. Pratt succeeded in maintaining cardiac 

 contractions for several hours when the only source of nutrition was from 

 these vessels. This source of nutrition may account for the survival of 

 hearts for years where pronounced arterio- sclerosis of the coronary arteries 

 exists. 



Alteration of Temperature. The effect of cold is to slow the rate of the 

 heart-beat. If the heart of a frog be cooled down to o C. it will stop 

 beating, but when the temperature of the surrounding lymph or blood is 

 again raised, it will renew its spontaneous beats. The effect of heat is to 

 quicken and shorten the heart-beats, but at a moderate temperature, 20 

 C., the contractions are increased in force, figure 177. 



The isolated mammalian heart is influenced by temperature variations 

 in much the same way as that of the frog. It will contract slowly in a low 

 temperature and rapidly in a temperature higher than that normal to the 

 body. The very rapid heart in some high fevers is in part due to the increase 

 in temperatures which affects the heart directly. 



Mechanical Tension. The mechanical factors produced by the heart- 

 beat are so prominent that it would be surprising indeed if there were no 

 reaction of these mechanical conditions on the heart itself. The isolated 

 cardiac muscle responds very quickly to variations in tension. Beginning 

 with a low tension the activity of heart muscle is increased up to a certain 

 optimum tension, after which further increase is unfavorable to the develop- 

 ment of automatic rhythm. A quite strong stretching will paralyze the 

 muscle. 



Tension on the whole heart influences its activity, not only through the 

 effects on the muscle, but indirectly through the nervous mechanism. High 

 tension, such as contracting against a high aortic pressure, stimulates sensory 

 nerves of the heart which, acting through the depressor nerve on the inhibi- 

 tory center, produce reflex slowing of the heart. It also produces reflex 

 vaso-dilatation. Both reflexes relieve the high tension on the heart. This 



