The transfer of afferent impulses into efferent channels occurs in the medulla oblongata. Long ago it was found that if the brain above this level be removed, part by part, respiration is not interfered with until the medulla oblongata is injured. When a cut is made into the floor of the fourth ventricle not far to one side of the middle line, the respiratory movements on that side of the body cease. If the injury be bilateral, even though very limited in extent, respiration stops. This spot was therefore spoken of as the “respiratory centre.” Flourens, who first discovered it, believed that it was a mere spot. He gave to it the fanciful name of _nœud vital_. It is the place at which the afferent nerves which call for respiration are brought into connection with all the various motor nerves which bring about the respiratory movements of nostrils, larynx, chest, and diaphragm. Possibly the knife in Flourens’ incision divides the tract of fibres which distributes afferent impulses, but whether the junction be a defined tract or no, injury to this region of the medulla throws the nervous mechanism of respiration out of gear. At this particular spot lies the “centre” for respiration—the one part of the nervous system which must be intact if the movements of respiration are to be carried out. There is no reason for thinking that respiratory impulses are generated at this spot. It is a centre in the same sense in which Crewe is a centre for distributing the goods of Lancashire and other parts of England to North Wales. The use of the term “nerve-centre” has been very much abused. Centres were supposed to be collections of cells, each group of which had some prerogative of initiation. Reasoning from the analogy of human institutions, it was thought necessary that the nervous system should be organized into departments severally responsible for the administration of the activities of certain sets of muscles: one centre controlled respiration, another the beat of the heart, another deglutition. The centres were dependent one on another; each regulated lower centres, and was governed by those above it, in this bureaucratic scheme. We know nothing of any function of nerve-cells other than that of transmitting impulses. All that we know about nerve-cells is that they place afferent and efferent routes in communication, and interpose resistance into nerve-circuits. Every nerve-cell of the grey matter of the brain and spinal cord gives off processes which ramify. The ultimate twigs into which a branch divides are in connection with other sets of twigs derived from the end-branchings of nerve-fibres or processes of other nerve-cells. A nerve-fibre is but the axis-cylinder process of a nerve-cell. Impulses encounter resistance in passing along the neuro-fibrillæ (_cf._ Fig. 22) contained in the twig-connections of the ramifying processes of nerve-cells. There is no reason for supposing that anything like