THE CIRCULATION OF THE BLOOD AND LYMPH 75 



the capillary electrometer indicates that, in a heart beating 

 normally, the negative change associated with contraction 

 begins at the base and then reaches the apex (p. 644). 



It is not definitely known how in the mammal the beat of the 

 ventricle is co-ordinated with that of the auricle. The alleged 

 absence of muscular connection has led to a very general belief that 

 the link is a nervous one; and certainly there is no dearth of 

 nerves running between the auricles and the ventricles that might 

 serve as such a bridge. But recent work makes it probable that the 

 contraction wave spreads as in the frog's heart, along muscular 

 fibres which, in the form of slender strands, interpenetrate the ring 

 of fibrous tissue between the auricles and ventricles (Kent). For it 

 has been shown that the ventricles (in the dog and cat) continue to 

 beat in unison, even after any nerves connecting them have been 

 almost certainly severed by extensive zig-zag incisions, so long as 

 they are united by a narrow bridge of muscular tissue (Porter). The 

 impulses, by means of which the co-ordination of the ventricles is 

 achieved, are accordingly transmitted along the muscular fibres from 

 one ventricle to the other. And if an excitation can travel in this 

 way from ventricle to ventricle, it is natural to suppose that it can 

 also do so from auricle to ventricle. 



The most conspicuous events in the beat of the heart, in 

 their normal sequence, are : (i) the auricular contraction or 

 systole ; (2) the ventricular contraction or systole ; (3) the 

 pause or diastole. The auricles, into which, and beyond 

 which into the ventricles, blood has been flowing during the 

 pause from the great thoracic veins, contract sharply. The 

 contraction begins in the muscular rings that surround the 

 orifices of the veins, so that these, destitute of valves as they 

 are, are sealed up for an instant, and regurgitation of blood 

 into them is prevented. The filling of the ventricles is 

 thus completed ; their contraction begins either simul- 

 taneously with the relaxation of the auricles or a little 

 before it. The mitral and tricuspid valves, whose strong but 

 delicate curtains have during the diastole been hanging 

 down into the ventricles and swinging freely in the entering 

 current of blood, are floated up as the intraventricular 

 pressure begins to rise, so that, in the first moment of the 

 sudden and powerful ventricular systole, the free edges of 

 their segments come together, and the auriculo-ventricular 

 orifices are completely closed (Fig. 68, p. 181). In the 

 measure in which the pressure in the contracting ventricle 

 increases, the contact of the valvular segments becomes 



