What is the special action of the vagus nerve? Its superior laryngeal branch checks inspiration and induces expiration, as already said. The impulses which pass up its main trunk bring about ordered movements. They are not dependent for their generation upon the condition of the blood in the lungs. When the chest is filled with nitrogen, inspiration and expiration alternate in the usual way, although the blood is growing steadily more venous. The failure of inspiration to bring about aeration of the blood does not lead to a prolongation of the inspiratory effort. Inspiration is cut off and expiration established in regular sequence. In performing “artificial respiration” (_cf._ p. 184) for the purpose of saving life, in cases in which respiration has ceased owing to the lungs being filled with water, or for other reasons, the chest is enlarged by raising the arms above the head, and diminished by pressing the elbows against the sides. Enlargement promotes a tendency to expiration, compression a tendency to a natural inspiratory effort. Evidently there is a connection between the movements of the chest and the stimulation of the respiratory centre. If respiration is being carried on artificially, by forcing air from a bellows into the trachea, the nostrils dilate as the chest is distended, and contract as it is emptied, so long as the vagus nerve is intact, just as they do in normal respiration. This shows that, when the chest is emptied, a message is sent through to the nucleus of origin of the nerve which supplies the dilator muscles of the nostril. When the lungs are full, a message calls upon the nostrils to contract. The only factor which is common to pressing in and pulling out the ribs, and filling and exhausting the lungs with a bellows, is the alteration in the form of the lungs which is produced by the two methods. It is impossible to resist the conclusion that the stretching of the tissue of the lungs stimulates the nerve-endings of the vagus. The impulses thus induced automatically stop inspiration, and lead to an expiratory effort. There are many indications that the nervous mechanism of respiration is a double one, certain stimuli inducing expiration, with inhibition of inspiration, others inhibiting expiration and inducing inspiration. There are, however, many difficulties in the way of formulating a satisfactory theory of the relation of these antagonistic actions. We may frequently observe indications of such an antagonism between the two phases of the respiratory mechanism. Cold water dashed on the back of the head (when the head is being shampooed) induces a long inspiration with inhibition of expiration. A blow in the pit of the stomach “knocks all the wind out of a man.” Expiration is prolonged until the lungs are unusually empty, and yet the victim of the blow feels as if he would never again be able to draw breath.