DYNAMICS OF HISTOGENESIS 359 



cardiac musculature has not lost its irritability) after the circulat- 

 ing blood, which is the efficient tensional stimulus that stimulates 

 and maintains the normal cardiac beat, has been removed from its 

 sphere of action. 



In considering the origins of the heart beat it is well to keep in 

 mind two fundamental points : first, the stimulus, and secondly, the 

 reacting body. The structure of the reacting as well as that of the 

 stimulating body, however, contributes to the quality of the effect. 

 As regards the initial heart beat in the chick embryo, the reacting 

 body is the cardiac mesenchyme. This responds, due to its irrita- 

 bility, to the stretching stimulus of the accumulating and subse- 

 quently circulating blood. We may have a modified cross-striated 

 muscular bladder like the subject of this experiment, but if the 

 stimulus is lackmg it has nothing to respond to, there is no rhythm. 

 After the stimulus has been applied to the cardiac mesenchyme for 

 a sufficient period, in order that the two spiral muscle layers may be 

 differentiated, it will continue beating, due to the reciprocating 

 interaction of the musculature, until its energy is expended. This is 

 the factor of safety in cardiac muscle in case of a diminished volume 

 of blood due to hemorrhage or other cause. The heart, however, 

 may be completely inhibited in its rhythm if a sufficient volume of 

 blood is withdrawn in acute hemorrhage. The automaticity may 

 be reestablished by subsequent transfusion (McGrath, '14, fig. 8,). 

 That the heart continues beating for a certain time, provided the 

 irritability of the musculature is maintained, after the circulation 

 has been released completely from the sphere of cardiac action, 

 speaks no more for complete automaticity of the heart muscle 

 than that a cuckoo-clock is completely automatic because it will 

 run for a definite period after the key that wound up its springs has 

 been removed from the sphere of rhythmicaUty. 



We may analyze the heart all we wish, but we are dealing with 

 the irritable reacting body. The extrinsic stimulus that gives rise 

 to the ' inner impulse ' that causes the beat has not been touched in 

 the problem. The neurogenic as well as the myogenic theories of 

 the heart beat have left untouched the extrinsic stimulus — the circu- 

 lating blood. It is the circulation that initiates and maintains the 

 heart beat normally in the chick embryo as well as differentiates 



