OTHER INFLUENCES WHICH AFFECT THE HEART 205 



conditions influence the heart's action in the normal body, conditions that 

 are not discussed directly under any of the categories treated above. Of 

 these may be mentioned the coronary circulation, temperature, mechanical 

 tension, age. 



The Coronary Circulation. The contractions of the heart cannot long 

 be maintained without a due supply of blood or other nutrient fluid. The 

 nutrient fluid for the heart of man and the mammals is supplied from the 

 coronary arteries and the vessels of Thebesius. The coronary arteries arise 

 from the base of the aorta, where they receive the benefit of the highest arterial 

 pressure. The coronary arteries are terminal arteries; that is, they do not 

 permit the establishment of a collateral circulation when one of their branches 

 is blocked. If the block be complete, that portion of the heart wall supplied 

 by the branch dies. The immediate effect of the 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 ventricular 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, and if the heart of a frog be cooled down to o C. it will stop 

 beating. It is said that the frog's heart may be frozen, and when thawed 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. 



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 



