Close by the respiratory centre is the centre which controls the circulation. But enough has been said in the section on reflex action, wherein the process of fainting was described, to give an idea of the part it plays in the body; so it need not detain us here. We cannot, however, pass over its neighbour, the centre of temperature, so briefly. Its methods not only afford one of the most striking and interesting examples of harmonious regulations by reflex action, but the subject of temperature itself is so important that we must describe in some detail how that of the body is kept level. As we said when discussing protoplasm generally, life—that is, the change always going on in the protoplasmic substance—is influenced by temperature: the single cell becomes less active at a low temperature, and dies at a high one; so obviously there is a temperature at which its functions are most easily carried on. Inside the body the cells are all kept at the temperature best for them by the circulation of the blood; but the absolute temperature of the whole body depends upon the heat which is generated within it by chemical action, and the heat which it loses to, or receives from, its surroundings. Under normal conditions this temperature in man is 98·4° F., when the production of heat from its own metabolism is balanced by the loss of heat by radiation. If, however, the atmosphere be very hot, less heat is developed in the body, the general metabolism being slower; and more heat is lost, since by reflex action the skin is bathed in sweat and cooled by its evaporation, and the small bloodvessels under the skin are dilated, so that more blood being brought to the surface, its chance of being cooled by radiation is thereby increased. If, on the other hand, the atmosphere is cool, the loss at the surface is minimized by constriction of the cutaneous bloodvessels, and a checking of the perspiration and consequent evaporation; while internally more heat is generated by increased metabolism. The cells which are mainly responsible for the production of heat are those of the muscles; and when much heat is required they increase in activity, not only in their general tone, but even by a visible movement, which we describe as shivering. So, within reasonable limits, whatever the temperature of its surroundings may be, that of the body remains the same, and though we may raise or lower our temperature by lying in a hot or cold bath, reflex adjustment of the sweat glands, bloodvessels and muscles brings it quickly back to normal when we emerge. With a passing mention of the cerebellum, the three-lobed organ shown in Diagram 61, and seen again in a more advanced stage in Diagram 63, we may dismiss the two hinder divisions of the brain.