The bronchi contain muscular fibres in their walls, which appear to maintain a state of tonic contraction similar to that of the arteries. The motor fibres which supply these muscles are contained in the vagi. When one vagus is cut the bronchi of the corresponding lung expand, and when the peripheral end of the cut vagus is stimulated, the bronchi contract so much as sometimes almost to close completely; but the vagi appear to contain bronchial-dilating fibres, as well as bronchial-constricting, so that irritation of the peripheral end of a cut vagus may sometimes cause marked dilatation instead of contraction, and sometimes primary contraction followed by dilatation. The vagi also contain afferent fibres, passing from the bronchi to the nerve-centres, and these afferent fibres have also a twofold action, so that when the central end of one cut vagus is irritated, the irritation may cause either reflex contraction or reflex dilatation of the bronchi in the other lung. It is probable that there are two cerebro-spinal centres: one producing dilatation and the other contraction. Atropine completely paralyses either the constricting fibres of the vagus or their terminations in the bronchi, so that after a very small dose stimulation of the peripheral end of the cut vagus no longer causes contraction. Ether probably paralyses the cerebro-spinal centre for contraction, so that irritation of the central ends of a divided vagus causes expansion instead of contraction in the bronchi of the other lung. Small doses of nicotine have a powerful effect in expanding the bronchi, but the mode of action of the drug has not been determined.1