THE INFLUENCE OF THE DEPRESSOR NERVE ON THE PULMONARY CIRCULATION

1. Excitation of the central end of the cut depressor nerve in the rabbit—the vagi having been previously cut—produces dilatation not only of the systemic but also of the pulmonary arterioles, although the effect upon the latter is usually less pronounced than upon the former. But the effect may be more pronounced in the pulmonary than in the aortic system. 2. The depressor nerve is singularly susceptible to stimulation by mechanical means, such as gentle traction upon a thread used to tie its end for convenience of placing the nerve upon electrodes. 3. During the inhibition of systemic arterioles resulting from stimulation of the depressor nerve a larger amount of blood is conveyed to The right side of the heart. The same applies mutatis mutandis to the pulmonary arterioles and the left side of the heart. 4. In animals which, unlike the rabbit, do not possess a separate depressor nerve, effects similar to those brought about by excitation of that nerve in the rabbit are caused by stimulation of the central end of either vagus—both vagi being cut. The results are, however, apt to be complicated by effects of other afferent fibres in the nerve and especially those from the lungs which influence the movements of respiration, and these may be the cause of blood—pressure changes other than those produced by depressor fibres. 5. Conspicuous Traube waves have been found sometimes to follow excitation of the depressor. 6. If weak stimuli are used, excitation of the depressor does not produce cardiac inhibition even with the vagi intact. The expenses of this investigation have been assisted by grants of money from the Carnegie Trust for the Universities of Scotland, and from the Earl of Moray Endowment for the Promotion of Original Research in the University of Edinburgh. I desire to acknowledge the valuable assistance I have received from Dr R.J.S. M'Dowall in carrying out the experiments recorded in this paper.