THE CAUSATION OF THE HEART-BEAT 1085 



break up into a meshwork of capillaries around all the fibres. The '<, 

 coronary arteries do not anastomose with one another, so that occlu- jj 

 sion of one artery permanently cuts off the supply of blood to the 

 parts within its area of distribution. The blood enters the coronary 

 arteries from the aorta both during systole and diastole, though the 

 systole of the ventricles exercises a direct effect in increasing the 

 resistance to the flow of blood through the heart, and squeezes out the 

 contained blood into the coronary veins. On account of this pumping 

 action of the muscular walls the flow of blood through the coronary 

 system is greater in a beating heart than in a heart which is quiescent. 



In an excised heart which is being perfused through its coronary vessels the 

 rate of beat of the heart has been found to be in direct proportion to the pressure 

 at which the fluid is being perfused. This is the case even when the fluid is non- 

 nutritious, such as liquid paraffin or gum solution, and has therefore been 

 ascribed to a direct excitatory effect transmitted from the coronary vessels to the 

 surrounding heart-muscle. The influence of the rate of beat, on the intra- 

 coronary pressure is shown in the following Table (Guthrie and Pike) : 



Pressure in Rate of beat 



mm. Hg. per minute 



90 .. 90 



140 . . 138 



156 . . 162 



175 . . 204 



104 .. 96 



Variation in pressure = 94'4 per cent. 



Variation in rate = 126*6 per cent. 



This statement can only apply to cases in which the heart is being insuffi- 

 ciently fed. In a heart nourished by blood and doing its normal work, when it is 

 entirely cut off from the central nervous system, the rate of beat is unaltered by 

 changes in the arterial pressure. This can be shown very easily in the heart - 

 lung preparation described on p. 1028. In fact, under such conditions the rate of 

 the heart-beat depends exclusively on the temperature, and is unaltered by 

 changes in arterial pressure, venous filling, or moderate variations in the gaseous 

 contents of the blood. 



Since the coronary arteries do not anastomose, ligature of a branch 

 of one of them deprives the corresponding part of the heart-muscle of its 

 blood- supply, with the result that coagulation necrosis of this part 

 sets in. If the branch ligatured be a large one, the heart very often 

 beats for one or two minutes with unimpaired force, then a beat is 

 dropped occasionally, and very shortly afterwards the heart stops 

 altogether and the blood pressure falls to zero. On inspection of 

 the heart immediately after the blood pressure has fallen, its mus- 

 cular wall is seen to be in a state of fibrillar contractions, or ' delirium 

 cordis.' All the strands of muscle fibres are contracting more cr 

 less rhythmically, but the rhythms of no two parts coincide, so that 

 the heart dilates and is incapable of carrying on the circulation. It 

 is probably in this way that sudden deaths occur in cases where the 



