196 



Messrs. J. A. Gunn and F. JB. Chavasse. [Dec. 18, 



on the superior vena cava is unique, there are apparently only two ways in 

 which its action on the quiescent superior vena cava can be explained. 



On the neurogenic hypothesis of the rhythmicity of the heart, it is possible 

 to hold that the continuous stimulation by adrenin of the sympathetic 

 myo-neural junctions so raises the excitability of the muscle that previously 

 subminimal rhythmic impulses from intrinsic motor ganglia (hypothetically 

 present in the ring preparation) are now adequate to elicit rhythmic 

 contractions. 



On the myogenic hypothesis, on the other hand, stimulation of the myo- 

 neural junctions of the sympathetic nerve causes the muscle to enter into the 

 rhythmic activity which is inherent in it. 



All that can be said at present is that the latter explanation seems some- 

 what more probable. If, however, we should be able to elicit, by adrenin, 

 rhythmic contractions in a ring of the superior vena cava in which subsequent 

 histological investigation can reveal no ganglia, then it would furnish a 

 cogent argument in favour of the myogenic hypothesis of the rhythmicity of 

 the heart. 



Further, this kind of investigation has afforded, and with further experi- 

 ment, it is hoped will afford with greater accuracy, a physiological method of 

 determining how far the rhythmically contractile tissue extends up the great 

 veins, and where it merges into non-rhythmic contractile tissue. The 

 difference in physiological reaction can be controlled by subsequent histological 

 investigation. In the meantime it can be said that the rhythmically ' 

 contractile tissue extends up the superior vena cava of the sheep for at least 



Fig. 4. 



6 — 8 mm. from the veno-auricular junction. Fig. 4 shows that 16 mm. from 

 this junction, a ring of the same superior vena cava which gave the rhythmic 

 contractions shown in fig. 3 responds to adrenin by simple contraction. It 

 must be emphasised that a negative result in inducing rhythmic responses is 

 not in itself conclusive, because, even at 6 mm. distance from the heart,, 

 adrenin does not always induce rhythmic contractions. Under the conditions 

 of the present experiments it is not possible for the veins to reach the 

 laboratory always in the same condition of excitability ; and we have found 



