DYNAMICS OF HISTOGENESIS 357 



The reciprocal elongation of muscles is strikingly evident in the 

 intestme. Rhythmic contraction is due to a reciprocal mechanism; 

 each wave is composed of a contraction and an elongation of the 

 imier spiral coat alternating with a contraction and an elongation of 

 the outer elongated spiral or longitudinal muscle coat. At the 

 start the contraction waves of both coats begin together, but, due to 

 the rotary course of the inner wave and the translatoiy course 

 of the outer wave, the former and stronger one will inevitably trail 

 the latter and weaker one. The outer and inner muscles are recip- 

 rocal elongators as peristalsis extends through the intestine. 



"V^Tien the outer and inner muscles are in normal tonic equihbrium, 

 no distortion is evident. As soon as a contraction wave starts the 

 balance is upset. The stronger cephaUc constriction causes an 

 elongation of the outer muscle coat. The wave of the latter 

 follows in the path of the distal region of elongation. The contrac- 

 tion of the outer coat causes an elongation of the inner coat in the 

 region of the caudal dilatation. Subsequently, the contraction wave 

 of the inner coat is seen to occupy the former zone of stretching in 

 the region of the caudal dilatation. There is, therefore, a definite 

 syncopation in the activity of the outer and inner muscle coats as 

 the peristaltic wave travels through the intestine. The muscle 

 coats act as reciprocal elongators; consequently, peristalsis pro- 

 gresses for a variable distance through the gut instead of coming 

 to a dead center. 



In the heart the muscle layers are wound in complex spirals. 

 Some of these spiral layers are elongated hke the outer coat, others 

 closely wound like the inner and intermediate layers (McCallum, 

 '98; Mall, '96). It has been previously observed by Erlanger 

 ('10- '12), Erlanger and Blackmann ('07), Garrey ('U), Moorhouse 

 ('12), and others that various parts of the heart, especially the 

 veno-auricular portion, when isolated in strips show automaticity. 

 The ventricle possesses this property also when strips composed of 

 at least two spiral layers with the long axis of the musculature 

 running in different directions are isolated. If, however, a thin 

 strip from the peripheiy of the heart is isolated it will not show 

 rhy thmicality if the outer layer of muscle composed of fibers running 

 in the same direction has been stripped off. In the heart, there is 



