If the spinal cord of an animal be cut just below the origin of the nerves of respiration, an immediate fall of temperature occurs, and, if the animal be left in a cool room, persists until death ensues. If, however, the victim be put in a warm place, after a time the temperature begins to rise, and finally a most intense fever is developed. Parallel phenomena follow division of the spinal cord in man. Indeed, Sir Benjamin Brodie was first led to experiment upon animals by observing in 1837 an excessive fever follow in a patient a wound of the spinal cord.
I have already explained, in a former number of this Magazine,[2] the nature of the so-called vaso-motor nerves, which preside over the little circular muscles that run round and round in the coats of the blood-vessels. When they are excited, these muscles contract and the size of the arteries is diminished: when they are paralyzed, the arterial inner muscles relax and the vessels dilate. The vaso-motor nerves have their governing centre in that upper portion of the spinal cord which is within the skull, the so-called medulla oblongata. When the spinal cord is divided, the vessels are cut off from the influence of this vaso-motor centre, and at once dilate, profoundly affecting the blood-current by doing so.
The first fall of temperature which follows division of the cord is believed by most physiologists to be due to this dilatation of the vessels. Very probably the blood-stream, flowing sluggishly, does not give the normal amount of stimulus to the tissues, so that at first their chemical actions are lessened, and consequently less caloric than usual is generated in the body. Further, the blood moving slowly through the dilated vessels of the lungs and of the surface of the body, is cooled more completely than it should be; hence, unless the body is protected by being surrounded with warm air, no excessive accumulation of heat in it can occur, and therefore no fever can appear.
Assuming that this explanation of the primary lowering of the temperature after division of the cord be correct—and no better one has as yet been offered—what is the cause of the fever which afterward develops itself? As it occurs only when the animal is exposed to a somewhat elevated temperature, it has been thought by some to be due to the absorption of this external heat. This, however, is certainly not true, as is shown, to omit less decisive proofs, by the experiments of Naunyn and Quincke, who exposed animals for two days to a temperature of 90°, and at the end of that time, their bodily temperature not having risen, cut their spinal cords, after which intense fever was developed in a few hours without any change of atmosphere.
Section of the cord must therefore give rise to an increased chemical movement and heat-production in the body. As already stated, this section affects very greatly the circulation, but the fever is independent of such action. The upper end of the medulla oblongata is continuous with a nervous mass which joins the two brain hemispheres together, and hence is known as the pons or bridge. If, instead of cutting the spinal cord, we separate the medulla oblongata from the pons, an immediate rise of temperature occurs, and continues until death, whether the operation be performed in a cold or heated room.[3]
Cutting the medulla at its junction with the pons causes, then, an immediate and direct elevation of temperature, without disturbance of the circulation. What can this mean? Evidently, only one thing—namely, that by the division of the medulla there has been separated from the general tissues of the body a repressive force—a something which normally controls their chemical activity and the production in them of animal heat.
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.