CHAP. iv.J THE VASCULAR MECHANISM. 367 



by altering in different directions these metabolic changes, even 

 though the basis of the metabolism, the supply of blood to the 

 cardiac tissues, may remain the same in quantity and quality, 

 that the inhibitory and augmentor nerves produce their respective 

 effects. In old age the cardiac substance through intrinsic changes, 

 the accumulated result of the events of a lifetime, is unable to 

 avail itself fully of the advantages which the blood, though like the 

 heart somewhat deteriorated, is still able to furnish ; and we may 

 conceive that, in a somewhat analogous manner, apart from 

 changes of the blood supply and from extrinsic nervous influences, 

 the beat of the heart may vary by reason of intrinsic molecular 

 changes, whose origin we cannot at present trace. But the more 

 obvious and direct cause of changes in the nutrition and so in 

 the behaviour of the heart lies in changes in the quantity and 

 quality of the blood supplied to the cardiac tissues. In the 

 mammal this means the quantity and quality of the blood flowing 

 through the coronary arteries. 



If in a mammal the coronary arteries be tied or otherwise 

 occluded the heart in a few seconds comes to a standstill ; this, 

 which always results if both arteries be tied, sometimes if one only 

 be tied, is preceded by an irregularity or by changes in the beat and 

 is followed by fibrillar contractions of parts of the ventricles. 

 This is an extreme case, but it illustrates in a striking manner 

 how closely the rhythmic contraction of the cardiac fibres is 

 dependent on the blood supply. 



The quantity of blood flowing through the coronary arteries is 

 dependent on the pressure in the aorta, or rather on the difference 

 between that pressure and the pressure in the right auricle into 

 which the coronary veins open, and on the resistance offered by 

 the coronary vessels. Hence with a high aortic pressure, more 

 blood passes to the cardiac tissue. This is at least favourable to 

 the development of the beat, and may be the direct cause of a 

 stronger stroke ; indeed observations seem to shew this. Thus a 

 high aortic pressure itself helps the heart to the effort necessary 

 to overcome that high pressure. Conversely a low aortic pressure 

 would similarly tend to spare the heart an unnecessary exertion. 

 As to how the heart may be influenced by changes in the width 

 of the coronary arteries brought about by vaso-motor action, we 

 have at present but little definite knowledge. 



More important still than the quantity is the quality of the 

 blood flowing through the coronary vessels. We shall have 

 occasion in treating of respiration to speak of the manner in 

 which blood deficient in oxygen or overladen with carbonic acid 

 affects the beat of the heart ; and we may here be content to point 

 out that every change in the constitution of the blood, whether 

 arising from the presence of substances such as drugs and poisons, 

 introduced from without, or of substances manufactured in this 

 or that tissue of the body or resulting from the absence or paucity 



