358 EBEN J. CAREY 



a reciprocating tensional interaction of the muscle layers. This 

 reciprocal elongation is not as strikingly manifested in the heart as 

 in the small intestine because the former organ is composed of 

 muscle layers forming a more compact and complex spirality than 

 the latter one (Carey, '21). That the rhythmicality is lacking in 

 irritable strips composed of fibers only running in the same direc- 

 tion is convincing proof of the reciprocal elongation of the muscle 

 layers. It is the muscular arrangement and their functional ten- 

 sional interaction in the isolated heart that accounts for cardiac 

 rhythmicality when the tensional stimulus of the blood stream is 

 released and not some mysterious seriesof hypothetical explosion. 



The extrinsic tensional stimulus for the genesis of rhythmic beats 



The tensional interaction of the contained fluids, namely, the 

 blood and the urine on the cardiac and vesicular mesenchyne, re- 

 spectively, will be reserved for detailed presentation in a future 

 communication. It will suffice to say at this point as regards the 

 development of the chick heart that the two main cardiac muscle 

 layers, an inner close spiral, and an outer open spiral layer are pro- 

 duced by the heliloidal blood stream flowing through the heart 

 beginning at the sinus venosus at the confluence of the two ompha- 

 lomesenteric veins. The inner cardiac layer is formed when the 

 heart is growing relatively more rapidly in width than in length; the 

 outer open spiral layer when the heart is growing more rapidly in 

 length than in width. The blood stream is the extrinsic efficient 

 agent producing tension in the cardiac mesenchyme comparable 

 to the epithelial tube in the intestinal mesenchyme. The force of 

 the blood stream is many times greater, however, than that of the 

 energy of growth of the epithelial tube. Hence the different .types 

 of muscular products. 



The effect of the tensional action of the blood stream is red cross- 

 striated muscle; of the intestinal epithelial tube, pale smooth 

 muscle. Once the two cardiac groups of muscles have been wound 

 up for action by the blood stream they present a reciprocal action 

 in relation to each other. It is this reciprocal muscular action 

 that keeps the heart beating for seven or eight days (provided the 



