Papers from the Marine Biological Laboratory at Tortugas. 



in the opposite direction. When the two waves meet 180 away from their 

 common point of origin they interfere with and annul each other, and a 

 period of quiescence ensues until another contraction-stimulus is sent forth 

 from a marginal sense-organ. 



Under normal conditions the two side waves are of practically equal 

 magnitude, and thus one can not overpower the other and travel constantly 

 around the circuit in one direction. Such an accident is prevented by the 

 interference and consequent suppression of the two waves, one by the 

 other; but the protection is not perfect, for on several occasions I have 

 started such a wave through a severe electrical or mechanical shock, and 

 then the sense-organs, being exhausted by the wave which set them into 

 play one after another, were powerless to control the pulsation, and the 

 single wave rushed constantly around the subumbrella annulus, causing each 

 and every part of the medusa to pulsate successively as it passed. The 

 rate of pulsation under these unusual conditions was fully twice that of 

 the isolated, recurrent contractions initiated by the sense-organs. 



A 



FIG. 5.- A , showing that a pulsation-wave may pass across newly 

 regenerated tissue (dotted area) which contains no muscles. 

 B, showing that a pulsation-wave can not pass through mus- 

 cles (ruled area) from which the nervous network has been 

 peeled away. 



Such a circuit wave can not take possession of the vertebrate heart, 

 for here each wave of contraction normally originates in the region of 

 the sinus, then spreads over the auricles, and finally over the ventricle, 

 whence it can not immediately return over its path. The pulsations of the 

 heart are recurrent, and are rhythmical only in the sense that the separate 

 pulsations follow one another, at sensibly equal intervals of time. 



In the Scyphomedusffi the pulsation-stimulus is conducted by the dif- 

 fuse nervous system of the subumbrella, and this stimulus causes the muscles 

 to contract. The stimulus will pass through tissue which contains no muscles 

 and can not contract, or through tissue wherein the muscles have been 

 rendered incapable of contracting through the effects of distilled water, 

 magnesium, curare, carbon dioxide, alcohol, etc. 



On the other hand, the pulsation-stimulus can not pass through or 

 be conducted bv a muscle from which the nervous connections have been 



