The existence of nerves whose function is to repress action is no new discovery in physiology. Readers of _Lippincott's Magazine_ may remember my description of the pneumogastrics or brake-nerves of the heart, whose duty it is to control the action of that viscus. Nerves which repress or inhibit action are spoken of in modern physiology as inhibitory. The experiments which have been adduced prove that there are nerves whose function it is to control the general vital chemical actions, and that the governing centre of these nerves is situated above the medulla oblongata. To this centre, whose exact location is unknown, the name of the _inhibitory heat-centre_ has been given. The way in which galvanization of a nerve, violent injuries and excessive pain depress the temperature, independently of any action upon the circulation, is now evident. An impulse simply passes up the irritated or wounded nerve, and excites this inhibitory heat-centre to increased action, and the temperature falls because the chemical movements of the body are repressed. The method in which fever is produced also becomes very evident when once the existence of an inhibitory heat-centre has been established. Any poison having the power to depress, and finally paralyze, this centre must, if it find entrance to the blood, produce fever. If the poison, from its inherent properties, or from its being in very small quantity, only diminishes the activity of the inhibitory heat-centre, the controlling influence is not entirely removed from the chemical movements of the body, and only slight fever results; but if the poison actually paralyzes the inhibitory nerves, a very great rise of temperature must rapidly follow the complete removal of the brake-power. As an illustration we may consider the intense rheumatic fever, or the so-called "cerebral rheumatism," such as affected the young Irishman whose case has been narrated in the present article. Without any apparent reason the poison of rheumatism habitually attacks one joint on one day, and another joint on another day, and with as little apparent reason it occasionally falls of a sudden upon the inhibitory heat-centre, and actually paralyzes it. In a few minutes intense fever is developed, and the bodily temperature rapidly approaches nearer and nearer that line on the other side of which is death. In many cases of fever, however, there is no poison in the blood; thus, the local irritation of a boil or other inflammation may cause what is well termed "irritative fever." The way in which this is produced is by an indirect, and not a direct, action upon the inhibitory heat-centre.