44 LIBBIE H. HYMAN. 



auricular region. Meantime the auricular evaginations are dis- 

 integrating. The sinus region at this stage cannot be seen very 

 well as it is buried in the body. Stages beyond the third day 

 have not been studied. 



It thus appears that there is in the heart before it has begun 

 to beat a gradient in activity from the sinus towards the arterial 

 end and that this gradient is the cause of the direction and 

 sequence of the heart beat. This gradient appears to be es- 

 tablished simply by the mode of formation of the heart by the 

 backward growth of the anterior intestinal portal, just as a 

 similar postero-anterior gradient is established at the rear end 

 of the chick embryo by the backward growth of the growing 

 point of the primitive streak, as set forth in the preceding paper. 

 In the latter case, the gradient does not persist and hence this 

 embryonic postero-anterior gradient in the axis leaves only 

 morphological signs behind it, namely, the segmentation of the 

 vertebrate body. There may be some functional paths of the 

 postero-anterior type also in the nervous system as permanent 

 records of the embryonic physiological history. But in the 

 heart the postero-anterior, i.e., the sin-arterial, gradient is 

 permanently retained. Why? We cannot answer this question 

 very well except to suggest that the isolation of the heart from 

 the dominance of other structures permits it to retain its high 

 embryonic rate of metabolism. 



In the history of physiology, much effort, argument, and 

 paper have been expended on the question of the "cause" of the 

 heart beat, whether neurogenic or myogenic or due on further 

 analysis to the chemical conditions within the heart. This 

 question like many other biological questions has never received 

 any adequate or satisfactory answer; and the reason for this is, 

 as in many other cases, that the question is a false one, it is 

 wrongly put. It seems to me to be really not of the slightest 

 significance whether the heart beat be neurogenic or myogenic; 

 the real question is: what property of any tissue makes that 

 tissue automatic? The crux of the matter would seem to lie 

 in the nature of stimulation. It is evident that some organs 

 function without extrinsic stimulation, while others do not. 



Elsewhere I have made a suggestion as to the essential nature 



