PROPERTIES OF THE HEART MUSCLE. 509 



much slower. The ventricles may be beating at 27 per minute and 

 the auricles at 90. In partial block the ratio between the ventric- 

 ular and auricular rate is definite, every second or third auricular 

 beat being followed by a ventricular systole (see Fig. 210). The 

 lesion producing this condition has not yet been determined.* 



In the hearts of the cold-blooded animals the same general 

 results are readily obtained when the tissue between the different 

 chambers is compressed or destroyed. In the frog's heart, for 

 instance, if one ties a ligature (first ligature of Stannius) between 

 the sinus venosus and the auricle, the auricle and ventricle cease 

 beating while the sinus continues pulsating with its normal rhythm. 

 Later the auricle and ventricle may commence beating again, but 

 if this happens their rhythm is slower than that of the sinus and 

 independent of it. So in the terrapin's heart, in which the sequence 

 of beat is so beautifully exhibited, if one ties a ligature between 

 auricle and ventricle, or cuts off the ventricle entirely, the sinus 

 venosus and auricle continue beating at their normal rhythm, while 



Fig. 210. Cardiogram from a case of Stokes-Adams disease, showing two auricular beats 

 (1, 2) to each ventricular beat. (Erlanger.) The time-record marks fifths of a second. 



the ventricle remains entirely quiescent so long as normal blood 

 flows through it. It would seem from these facts that in the mam- 

 malian heart the ventricle when disconnected from the auricle is 

 capable of maintaining a fairly rapid rhythm of its own. At the 

 other extreme, the terrapin's ventricle when similarly treated shows 

 no spontaneous beats at all. These and many other facts that 

 might be quoted support well the general view proposed by Gaskell, 

 that the venous end of the heart possesses the greater rhythmical 

 power and starts the heart beat, and that the wave of contraction 

 is propagated from chamber to chamber through the muscular 

 substance. 



There remains a deeper question as to what occasions this greater rhyth- 

 micity at the venous end, a question that is, of course, bound up with the 

 problem of the ultimate cause or conditions of automatic rhythmicity. In 

 connection with this latter problem the absolute necessity of the presence of 

 certain inorganic salts in certain proportions has been emphasized. In this 

 same general line the author has called attention to the fact that in the ter- 

 rapin the amount of potassium salts present in the blood explains in itself 

 why the sinus sets the heart rate. In blood, or in Ringer's solution con- 



* See Erlanger, " Centralblatt f. Physiologie," 19, 1, 1905. 



