86 THE CIRCULATION OF THE BLOOD AND LYMPH 



sinus and auricles, hesitates for a moment at the auriculo- ventric- 

 ular junction, and then with a certain suddenness invades the 

 ventricle. N 'In the mammalian heart the contraction likewise com- 

 mences, so far as can be ascertained by inspection or the study of 

 tracings, in the region near the mouths of the veins opening into the 

 auricles. ) It will be seen, when the question of the origin of the 

 rhythmical beat is being discussed (p. 141 ) f '' that the actual starting- 

 point is probably the sinus tissue of the right auricle (p. 142) near the 

 opening of the superior vena cava, which is richly provided with 

 muscular fibres akin to those of the heart. But the wave advances 

 so rapidly that it is difficult to trace in its course a regular progress 

 from base to apex, although the ventricular beat undoubtedly 

 follows that of the auricle, and in a heart beating normally the 

 electrical change associated with contraction of the ventricle 

 begins at the base, then reaches the apex (p. 806), and finally passes 

 towards the orifices of the great arteries. 



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, each followed by relaxation, (3) the 

 pause. The auricles, into which, and beyond which into the ven- 

 tricles, blood has been flowing during the pause from the great 

 thoracic veins, contract sharply, the right, perhaps, a little before 

 the left. The contraction begins in the muscular tissue that 

 surrounds the orifices of the veins, so that these, destitute of valves 

 as they are, are functionally, at least, if not anatomically, sealed up 

 for an instant, and regurgitation of blood into them is to a great 

 extent, if not entirely, prevented. Apparently, complete closure 

 of the inferior cava is unnecessary, the pressure of the blood in it 

 being sufficiently high to hinder any important back-flow. The 

 action of the circular fibres of the veins in closing their orifices is 

 reinforced by the contraction of a band of muscle (the Uenia ter- 

 minalis) in the roof of the right auricle. This band compresses 

 especially the mouth of the superior vena cava. The filling of the 

 ventricles is thus completed. The actual amount of extra blood 

 injected into the ventricles by the auricular contraction is not large. 

 The ventricles are already nearly charged, but the auricles, so to 

 speak, ram the charge home. The ventricular contraction follows 

 hard on the relaxation of the auricles. The mitral and tri cuspid 

 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. 98, p. 204). In the measure in which the pressure in the 

 contracting ventricles increases, the contact of the valvular seg- 



