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 deg., 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.