CHAP. iv.J THE VASCULAR MECHANISM. 301 



beat and is followed by fibrillar contractions of parts of the ven- 

 tricles. 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 

 or from excess of one or more of the normal constituents, may 

 unfavourably or, it may be, favourably affect the heart beat, by 

 directly influencing the cardiac tissues through the coronary 

 arteries. These changes in the blood may of course also work 

 upon the heart through the central nervous system, and this 

 indirect effect may possibly be different from the direct effect. 

 Thus, when the breathing is interfered with, the too highly 

 venous blood, while it acts directly on the cardiac tissues and that 

 unfavourably, also stimulates the cardio-inhibitory centre, whereby 

 the heart is slowed and its expenditure of energy lessened. 



168. As is well known, the beat of the heart may become 

 temporarily or permanently irregular. That many hearts go on 

 beating day after day, year after year, without any such irregu- 

 larity is a striking proof of the complete balance which usually 

 obtains between the several factors of which we are speaking. 

 Sometimes such cardiac irregularities, those of a transient nature 

 and brief duration, are the results of extrinsic nervous influences. 

 Some events taking place in the stomach, for instance, give rise to 

 afferent impulses which ascending from the mucous membrane of 

 the stomach along certain afferent fibres of the vagus to the 

 spinal bulb, so augment the action of the cardio-inhibitory centre 



