I mines] CIRCULATING EXCITATIONS IN HEART MUSCLES 45 



rate of propagation of this change, but the contraction is in no way 

 essential to the propagation. However, at present we have no suffi- 

 cient reason to assume any wave preceding the excited state, and we 

 may return to our discussion of the behaviour of the strip. Suppose 

 it to be in a condition where a wave of excitation is shorter than the 

 strip, and now suppose that the ends of the strip are united so as to 

 form a ring. Under such circumstances, the wave, having made one 

 circuit of the ring would continue to propagate itself, finding the place 

 where it started excitable when it reached it again. 



We do not know at present how to bring the ends of a strip of 

 muscle into physiological continuity, but we can easily obtain closed 

 conducting rings by cutting them from a large piece of heart muscle 



§ 2. Circulating excitations. 



It was shown in 1908, by A. G. Mayer 1 that in a ring of excitable 

 tissue cut from the bell of the large Medusa Cassiopeia it is possible 

 to establish a local block and by stimulating on one side of it to set 

 going a wave which travels in one direction only. Removing the 

 block before the circuit was completed by the wave, the wave continues 

 to circulate round the ring indefinitely. 



In a former paper 2 I have explained the theoretical and experi- 

 mental considerations which led me to seek the production of a similar 

 phenomenon in heart muscle. I there gave a short account of ex- 

 periments in which the circulating waves, or as I prefer to call them, 

 circulating excitations, were set going. 



(a) in rings including portions of auricle from the tortoise. 



(b) in rings cut from the auricles of elasmobranch fishes. 



Last autumn, through the kindness of Professor Yves Delage, I 

 had the opportunity of making further experiments at the Station 

 Biologique in Roscoff. 



I will describe one typical experiment which illustrates several 

 points. 



Large Dog-fish (Acanthias). Killed by decapitation. Spinal 

 cord pithed. Heart excited and placed in a dish with blood. Beats 

 continue regularly, starting in sinus. Scratching the bulbus aortae 

 produces the effect described by Gaskell with great ease. Beats start 

 in bulbus at a faster rhythm and are transmitted backwards over 

 ventricle and auricle for five or six beats, then the normal sequence is 

 resumed. 



After half-an-hour the heart is beating well. Cut away sinus: 

 the auricle and ven tricle stop. Cut off auricle, slit it up to form a ring, 



X A. G. Mayer. Popular Science Monthly. Dec. 1908. p. 481. 

 2 G. R. Mines. Journ. of Physiol. 46. p. 349, 1913. 



