HEART 425 



o-rown out to reach the heart, they play a not unimportant 

 part in initiating contraction. 



(1) There is the negative evidence that, if during Hfe the 

 apex of the frog's ventricle has been separated from the rest 

 by crushing, it remains passive. 



(2) Chloral, apparently by poisoning the nervous struc- 

 tures, may stop the rhythmic action of the heart, but leave 

 the muscle capable of responding to stimulation 



(3) Carlson has shown that the heart of the king crab 

 stops if the nervous structures are dissected off it. But the 

 muscle of this heart is of the type of skeletal muscle, and it 

 is perhaps unsafe to apply these results to ordinary heart 

 muscle. 



(4) The contractions normally start in the sinus region, 

 a part of the heart richly supplied with nerve cells and 

 fibres (p. 388), The importance of this part of the heart in 

 originating the movements of the rest of the organ is shown 

 by experiments on the heart of the frog (Practical Physiology). 



If a ligature be applied between the sinus and auricles, 

 the sinus goes on beating while the rest of the heart stops 

 (Stannius' Experiment). TJiis shows the dominant influence 

 of the sinus. But, if now a ligature be applied between the 

 auricles and ventricles, these latter generally begin to beat 

 with a slower rhythm than the sinus {Practical Physiology). 



The second part of this experiment seems to indicate 

 that each part of the heart has the property of rhythmic 

 contractility. It has also been shown that if the ventricle 

 be made to beat faster than the sinus, the contraction wave 

 may travel in the reverse direction, from ventricle to sinus. 

 It, and not the sinus, becomes the " pace-maker." 



The whole question of the relative parts played by nerve 

 and muscle in starting contraction is still unsettled. The 

 evidence seems to indicate that the rhythmicity is a function 

 of the primitive cardiac tissue which is at first purely 

 muscular, but which later contains nervous elements, and in 

 connection with which, in the mammal, the chief masses of 

 nerve cells occur. 



The maintenance of this rhythmic contraction and relaxa- 



