184 THE CIRCULATION OF THE BLOOD 



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 beat- 

 ing. 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 in- 

 crease 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 inhibitory 



