The resistance in the centre is profoundly affected by the condition of the blood. As the blood becomes more venous, impulses pass across the nerve connections with ever-increasing force. Kept in the first instance to definite channels, they spread as the centre becomes more excitable farther and farther afield, reaching one group of muscles after another, and pressing them into the service of respiration. When, in dyspnœa, every muscle which can in any way help the movements of the chest is doing its best, others which are useless for this purpose receive the reflected impulses and join in, producing general convulsions. The increased activity of the respiratory centre which is produced by slight venosity of the blood is shown in the rapid and deep inspirations which are caused by violent exercise. Perhaps it is justifiable to go a step farther, and to assert that there is something in blood which has been rendered venous by muscular activity which is specially exciting to the respiratory centre. If the blood from a limb be prevented from returning to the general circulation, by compressing or tying its great veins, and if the muscles of the limb be strongly stimulated by an electric current, their activity, so long as the passage through the veins is blocked, has no influence upon respiration. But, on relaxation of the pressure on the veins, respiration may become twice as deep and twice as frequent as it was before the muscles were stimulated, although the limb is now in a condition of perfect rest.