It has been shown in this paper that stimulation of the inhibitory nerves of the neurogenic heart of Limulus, which correspond to the vagus nerves of the vertebrate heart, results in a marked diminution of CO2 production in the heart ganglion, while stimulation of the ganglion, leading to increased activity of the heart, leads also to increased CO2 production by the ganglion. This shows that inhibition of the automaticity of this ganglion by the action of its inhibitory nerves consists, not in a process of blocking, but in a diminution of those chemical reactions in the ganglion cells which give rise to the production of CO2.

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