222 THE ORIGIN OF THE NERVOUS SYSTEM 



according to species and conditions. The heart is 

 tubular but evidently is not a fixed physiological gradient. 

 I have already suggested elsewhere (191 yc) that the 

 reversal of the direction of beat is determined by fatigue 

 of the temporarily dominant region with consequent 

 decrease of its dominance. This decrease in dominance 

 permits the other end of the tube, which is recovering 

 or has recovered from fatigue, to initiate a beat and 

 become dominant. The situation is practically identical 

 with the spontaneous reversal which occurs in Pleuro- 

 brachia in the early stages of inhibition (p. 219) in 

 consequence of the decrease in dominance of the more 

 susceptible apical region and the physiological isolation 

 of the oral end of the row. 



Alvarez and his co-workers (p. 48) have demonstrated 

 that the stomach, the small intestine, and the colon of 

 mammals and of at least some other vertebrates represent 

 physiological or metabolic gradients. Here, as in the 

 other cases described, the relations of dominance and 

 subordination exist and determine the normal direction 

 of peristalsis, and physiological isolation and independent 

 function of parts as well as reversal in direction of per- 

 istalsis may be brought about in essentially the same 

 ways as in the plate row. Alvarez' evidence for the 

 existence of the gradient rests on many different lines of 

 experimental investigation and is now very complete. 

 Moreover, he has pointed out that recognition of the 

 presence of the gradient in the alimentary tract of the 

 higher vertebrates and man is not only of theoretical 

 interest but of great practical significance in medicine. 



There is considerable evidence from its functional 

 behavior that the ureter is also a physiological gradient. 



