78 A MANUAL OF PHYSIOLOGY 



becomes continuous. This is the state of affairs in the vascular 

 system. The intermittent action of the heart is toned down in the 

 elastic vessels to a continuous steady flow. 



The Beat of the Heart. In the frog's heart the contrac- 

 tion can be seen to begin about the mouths of the great veins 

 which open into the sinus venosus. Thence it spreads in suc- 

 cession over the sinus and auricles, hesitates for a moment at 

 the auriculo- ventricular junction, and then with a certain sud- 

 denness invades the ventricle. In the mammalian heart the 

 starting-point of the contraction is likewise the mouths of the 

 veins opening into the auricles (especially the superior vena cava) , 

 which are 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 the capillary electrometer indicates that, in a heart 

 beating normally, the electrical change associated with contrac- 

 tion begins at the base, then reaches the apex (p. 730), 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 relaxa- 

 tion, (3) the pause. 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 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 anatomi- 

 cally, 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 tcenia terminalis) 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 com- 

 pleted ; their contraction begins either simultaneously with the 

 relaxation of the auricles or a little later.* The mitral and 

 tricuspid valves, whose strong but delicate curtains have during 

 the diastole been hanging down into the ventricles and swinging 



* It has often been debated whether any appreciable interval exists 

 between the end of the auricular and the beginning of the ventricular 

 systole of the warm-blooded heart. According to Chauveau, not only is 

 this period (the intersystole) well marked and sharply delimited (in the 

 horse), but it is occupied by a definite series of events, including the con- 

 traction of the papillary muscles. 



