new sliiiiiil.iliiiLi' iiuillcr is hein;:- fonncd aiid .iflcr ;i ccrlain linie 

 (Mjiial lo llic normal itcriod il will liaxc ((hlaiiicd ciioiiuli iiilciisih lo 

 cause aiiollicr conlraclioii. So llie following spoiilaiicoiis svslolc will 

 not fall ill llic iiioniciil it would liaxc done so if an cxlra svslolc 

 had iiol liccii scl lip. hut just so niiu'li i'<trln'r as llic extra contrac- 

 tion rcaclicil tlic N'ciia ca\a hi'/on- llic iiioiiiciil in w liicli tlic follow inn- 

 s|tonlancoiis contraction wonld have occurred. 



In the dia.i^rains 11 and ill an attempt has heen made at making' 

 these ol)ser\ati(tns clear lor a |)articulai' case. 



Ve — 



t 



.t^ 



i ^ i 



■Uf 



J z 



;z 



i 



V 



■^' 



/6 



J.W 



^\ 



/'/ 



^r 



'U. 



F.-^X 



h^o 



Tji fig'. II the aui-icle is ai'tiücially stimulalcd respectively iH, 15 

 and 12 units of time after the previous si)ontaiieons conti-action; 

 auricle and \entricle follow the extra stimulus; in the firsl two cases 

 the exlra contraction moving to the vena cava intercepted the sjxni- 

 laneons conlracliou coming from the vena cava. I]i the third case it 

 arrives in the vena cava just at the same time as the i)hysiological 

 stimulus becomes active. In all these cases the i-hylhm remains nn- 

 dislurhed and the comjiensaforv pause is complete for the auricle as 

 well as for the ventricle: the interval between the systole j)rece(ling 

 the extra systole and the one following it is double the period of 

 the heai"t, iji this case = 40. 



In fig. Ill an earlier stimulation of the heart is shown, 10. i-esp. 

 <S and 5 units of time after the pi-eceding systole the auricle is 

 stimulated. The extra systole formed by the tirsi stimulus arrives in 

 the vena cava 4 units he/on' the following spontaneous conliaction. 



The stimulating matter present at that moment is destroyed and 

 a certain time = 20 has to [)ass before the stimulus has increased 

 to suHicient strength. So the interval of the spontaneous contractions 

 is not =40 but =16 + 20 = 86. 



Accoi'ding to the extra auricular contraction falling earlier, this 



