Although the heart beats periodically when cut off from the nervous centres, its normal rhythm is under the control of a group of nerve cells in the medulla, from which some fibres of the vagus carry special regulating impulses to the heart. The action of this centre is habitually that of a restraining agent lessening the rate of the heart's contractions, and is, hence, called a tonic inhibitory centre. The activity of the centre is influenced by the condition of many distant parts, such as the cortex of the brain, the abdominal viscera, etc., which exert a kind of reflex action on the heart through the centre. The degree of inhibitory power, as well as the share taken in the action of the centre by automatism and reflexion, differs in different animals. A centre {accelerator) antagonistic to the latter also exists in the medulla. It is weaker in action than the inhibitory centre, and is not tonic.

In the medulla there also exist many other centres connected with the organic functions. Among these, the centres for swallowing and vomiting may be mentioned, For further details on this subject, the reader may consult the chapter on Digestion.