580 CIRCULATION OF BLOOD AND LYMPH. 



respects it differs from the typical heart muscle of the vertebrate, 

 but the difference is perhaps sufficiently explained by the discovery 

 (p. 571) that the crustacean heart, in one form at least, is not an 

 automatically rhythmical tissue. Its rhythmical contractions, like 

 those of the diaphragmatic muscle in the higher vertebrates, depend 

 upon rhythmical impulses received from nerve centers. 



The Compensatory Pause. It has been observed that when an extra 

 systole is produced by stimulating a ventricle it is followed by a pause longer 

 than usual; the pause, in fact, is of such a length as to compensate exactly 

 for the extra beat ; so that the total rate of beat remains the same. The pro- 

 longed pause under these conditions is therefore frequently designated as the 

 compensatory pause. It has been shown,* however, that the exact compen- 

 sation in this case is not referable to a property of heart muscle, but is due to 

 the dependence of the ventricular upon the auricular beat. When the auricle 

 or ventricle is isolated and stimulated the phenomenon of exact compensation 

 is not observed. In an entire heart, on the contrary, the beat originates at 

 the venous end of the auricle and is propagated to the ventricle. If the latter 

 chamber is stimulated so as to give an extra beat out of sequence it will remain 

 in diastole until the next auricular beat stimulates it, and will thus pick up 

 the regular sequence of the heart beat. 



The Normal Sequence of the Heart Beat. The normal 

 rhythm of the heart beat is first a contraction of the auricles, 

 then one of the ventricles. Many efforts have been made to 

 determine the precise spot in which the contraction of the heart 

 normally starts. Formerly it was supposed that the contraction 

 began in the great veins just before they pass into the auricle, 

 and it was implied that this initiation of the beat might occur in 

 the pulmonary veins as well as in the vense cavae. More recent 

 experiments f which have been made largely upon the isolated 

 heart while perfused with a Ringer-Locke solution have shown 

 pretty conclusively that the most rhythmic part of the heart 

 and the part from which the beat, in all probability, normally 

 starts is an area of the wall of the right auricle lying between 

 the openings of the venae cavae, or, according to the most recent 

 views, in that remnant of the sinus tissue known as the sino-auricular 

 node which lies in this region, and which is connected with the 

 auricular muscle and with the auriculoventricular bundle (p. 

 541). When this portion of the heart is warmed or cooled the 

 rate of beat of the whole heart is correspondingly increased or 

 decreased, while, on the contrary, warming or cooling of the ven- 

 tricles themselves, the auricular appendages, the left auricle, etc., has 

 no effect upon the heart-rate. From the point of confluence of the 

 venae cavae the wave of contraction spreads over the auricles and 

 through the auriculoventricular bundle to the ventricles. This 

 sequence from venous to arterial end is beautifully shown in the 



* Cushny and Matthews, "Journal of Physiology," 21, 227, 1897. 



t Consult especially Adam, "Archiv f. d. ges. Physiol.," Ill, 607, 1906; 

 Erlanger and Blackman, "American Journal of Physiology," 19, 125, 1907, 

 and Flack, "Journal of Physiology," 41, 64, 1910. 



