[93] Rubner, *Die Gesetze des Energieverbrauches bei der Ernährung*, Leipzig u. Wein, 1902. The so-called “regulation of heat” in warm-blooded vertebrates is an instance of a special function which can be said to be regulatory in itself. There exists a normal blood heat for each species, which is maintained no matter whether the temperature of the medium rise or fall. It might seem at first as if in this case there were a little more of an adaptive regulation than only its well-known primary type; no reversion, one might say, of the direction of one and the same process occurs in the regulation of heat production, but one kind of process is called into action if it is necessary to raise the temperature, and another whenever it is necessary to lower it. Even in the dilatation and constriction of capillary vessels there are different nerves serving for each operation respectively, and far more important are the increasing of transpiration for cooling, the increasing of combustion for heating--two radically different processes. But, nevertheless, there is a certain unity in these processes, in so far as a specific locality of the brain has been proved to be the “centre” of them all; it is to this centre of course that the analysis of heat production considered as a kind of regulation or adaptation must be directed. Such an ultimate analysis, it seems to me, would have to classify heat regulation under the primary type of adaptations in physiology without any restriction. The centre acts in one sense or in the other, if stimulated by any temperature beyond a very limited range, and it is in the action of the centre that the “regulation” of heat consists.[94] [94] The phenomenon of fever we leave out of account here; it is regarded by some as regulation, by others as a disturbance of heat regulation. Of course, if the first view should ever prove to be the right one, fever might be classified among the real regulations of the secondary type.