The normal, rhythmical, coordinated movements of respiration are not only brought about, but are also regulated by an involuntary nervous mechanism. Since we are unconscious of its action, it certainly is not dependent on the voluntary centres. The upper parts of the brain cannot be needed for regular breathing, since (i) animals born with deficiently developed brains breathe rhythmically; and (2) removal of the brain of birds, etc., or loss of voluntary movement in man (hemiplegia), causes no interruption of the respiratory movements. Injury to the upper part of the spinal cord causes death by stopping respiration. The regulating centre must then be lower than the cerebral centres, and higher than the cervical part of the spinal marrow. The direct evidence of the seat of this centre was found by Flourens, who showed that a localized spot exists in the medulla oblongata, injury of which causes instant cessation of the respiratory movement.

This vital point, nceud vital, is situated in the floor of the fourth ventricle, near the point of the calamus scriptorius, and is now commonly spoken of as the respiratory centre. It is convenient to suppose that there are two groups of cells, one presiding over the inspiratory and the other over the expiratory muscles.

From this centre the impulses which give rise to the all-important respiratory movements rhythmically pass down the spinal cord and nerves. So long as the nervous communication between the centre and the muscles is intact the movements go on with undisturbed regularity; if it be cut off, or the centre be destroyed, respiration instantly stops.