The Nervous Control of movements of breathing go on without the ne cessity of any interference on our part, without our consciousness of them being necessary. Dur ing sleep their regularity is even greater than when we are awake, and when awake we may alter the movements to some extent, but cannot altogether arrest them. The breath may be held for a time, but speedily the need for breath gets the mastery, and a long deep breath is drawn. Now it has been shown that there is a particular part of the nervous system that presides over the respiratory function. It is situated in the medulla oblongata (p. 150). Destruction of this part is followed by immediate stoppage of breathing and death. On this account this region of the medulla has been called the vital knot (naual vital). Here there is, that is to say, a nervous centre for breathing. It is injury to this centre that proves fatal in cases of broken neck. From it regular discharges of nervous activity pass outwards to the nerves that supply the diaphragm and other muscles of respiration, and stimulate them to the regular or periodical contraction which, as we have seen, is the cause of the movements of the chest, the expansion of the lungs, and the entrance of the air. But the centre is capable of being influenced in various ways. Chiefly it is affected by the condition of the blood. When the blood becomes more venous the action of the centre is excited, and more vigorous respiratory movements occur. On the other hand, if a series of very deep breaths is taken, so that the blood becomes more richly supplied with oxygen than usual, a considerable time will elapse before the need of another breath is felt, and after the interval the breath ing will be resumed, the first inspiration being feeble, and those succeeding gradually increas ing up to the average. The meaning of this is
that the presence af—the unusual quantity of oxygen in the blood has calmed the respiratory centre, so that for a time it has ceased itTroular periodical discharge, till, as the blood begins to become venous, the usual stimulus is restored. This explains how persons may train them selves to remain under water for some time with no arrangements for permitting breathing to go on. They take a series of deep inspirations, take into the blood, as it were, a stock of oxygen sufficient to last for a little time, and before they dive under the surface., The res piratory centre is, then, stimulated bjb the pre sence of excess of carbonic acid in soothed by the presence of excess of ox eft: It is the great excess of carbonic acid present in the blood, in cases of continued obstruction to the breathing, that produces the excessive stimula tion of the centre, indicated by laboured breath ing and the convulsions of suffocation. The centre may, however, be stimulated in other ways. Nerves of sensation supplying the general surface of the body are capable of conveying in fluences to it leading to its excitement, and end ing in vigorous respiratory movements.