CHAP, iv.] THE VASCULAR MECHANISM. 347 



that the miss be due not to vagus inhibition but to intrinsic 

 events) is unfavourable for a succeeding beat: the mysterious 

 molecular changes, by which the actual occurrence of one beat 

 prepares the way for the next, are missing, the favourable 

 influences of the extra rush of blood through the coronary arteries 

 due to a preceding beat are missing also, and even the distension 

 of the cardiac cavities, at first favourable, speedily passes the 

 limit and becomes unfavourable. And these untoward influences 

 accumulate rapidly as the first miss is followed by a second, arid 

 by a third. In this way a heart, which has been brought into a 

 state of unstable equilibrium by disordered nutrition, (as for 

 instance by imperfect coronary circulation, such as seems to 

 accompany diseases of the aortic valves leading to regurgitation 

 from the aorta into the ventricle, in which cases sudden death is 

 not uncommon), which is able just to accomplish each beat, but 

 no more, which has but a scanty saving store of energy, under 

 some strain or other untoward influence, misses a leap, falls, and is 

 no more able to rise. Doubtless in such cases could adequate 

 artificial aid be promptly applied in time, could the fallen heart 

 be stirred even to a single good beat, the favourable reaction 

 of that beat might bring a successor, and so once more start 

 the series ; but such a period of grace, of potential recovery, 

 is a brief one. Even a coarse skeletal muscle, when cut off 

 from the circulation, soon loses its irritability beyond all recovery, 

 and the heart cut off from its own influence on itself runs down 

 so rapidly, that the period of possible recovery is measured chiefly 

 by seconds. 



192. Turning now to the minute arteries and the peripheral 

 resistance which they regulate, we may call to mind the existence of 

 the two kinds of mechanism, the vaso-constrictor mechanism, which, 

 owing to the maintenance by the central nervous system of a 

 tonic influence, can be worked both in a positive constrictor, and 

 in a negative dilator direction, and the vaso-dilator mechanism, 

 which, as far as we know, exerts its influence in one direction only, 

 viz. to dilate the blood vessels. The latter dilator mechanism seems, 

 as we have seen, to be used in special instances only, as seen in the 

 cases of the chorda tympani and nervi erigentes ; the use of the 

 former constrictor mechanism appears to be more general. Thus 

 the relaxation of the cutaneous arteries of the head and neck, which 

 is the essential feature in blushing, seems due to mere loss of tone, 

 to the removal of constrictor influences previously exerted through 

 the vaso-constrictor fibres of the cervical sympathetic. Though 

 probably dilator fibres pass directly along the roots of the cervical 

 and of certain cranial nerves to the nerves of the head and neck, 

 we have no evidence that these come into play in blushing; 

 as we have seen, blushing may be imitated by mere section of 

 the cervical sympathetic. So also the " glow " and redness of 

 the skin of the whole body, i.e. dilation generally of the cutaneous 



