The constancy of the body temperature with regard to time is on the other hand something which has been attained at a late stage in the development of the animal kingdom. The so called "warm blooded animal," in reality the animal whose temperature remains at an even level, did not appear until long after the vertebrate level had been reached. Yet this device was developed apparently independently both in birds and mammals.
So manifest are the advantages of a constant temperature that its late development seems almost surprising. The body from one point of view consists of innumerable and ceaseless chemical reactions. These reactions take place at given velocities, the velocity of each reaction depending upon the reaction itself and the temperature at which it is taking place. Imagine the whole machine adjusted perfectly to work at 30° Cent., each reaction progressing at a velocity which (a) is most suitable for its own purpose, and (b) fits into the general scheme so nicely that the whole multitude of reactions take place harmoniously. Now alter the temperature from 30° to io°; some of the reactions are affected much by the alteration, some little ; chaos must result and the whole machine be thrown out of gear. Therefore a third condition (c) is necessary, namely, that the effect of temperature on the velocities of the reactions must be such that when the temperature is altered the reactions alter in speed harmoniously with one another. This fresh condition naturally limits the num ber and nature of the chemical processes, which are available. Some chemical operation might be most desirable and could take place at 37° C, but its insertion into the body would be merely vicious if, by refusing to keep pace with its fellows, it brought the mechanism to a standstill at io° C. By maintaining a uni form temperature condition (c) is dispensed with and the body is free to select what chemical reactions are most suitable under the constant condition without reference to the extent to which they may be affected by alteration of the temperature.
The constancy of the body temperature is maintained by a nice adjustment of the heat loss of the body to the heat forma tion; for the mechanism of this adjustment see the article on ANIMAL HEAT ; here it need only be said that the adjustment is controlled entirely by the nervous system.
Every action of the body tends to upset this equilibrium usually in the sense of putting more acid into the blood; muscular exer cise for instance adds carbonic and sometimes lactic acid to the circulating medium. How then is the constancy maintained? First by the action of the respiratory system. Increase of the hydrogen ion concentration of the blood leads to increased activity of the respiratory centre, and greater consequent loss of carbonic arid by the body in the breath. Secondly the kidney comes into play and if the concentration of acid in the blood falls alkali is secreted in correspondingly excessive amounts in the urine. Whether the central nervous system controls the output of alkali by the kidney is not known. The actual secreting cells of the kidney are not regarded as being operated by nerves; if therefore the nervous system plays a part it probably does so by controlling the blood supply.
Two examples have been taken for consideration but the reader can from any book on physiology learn of many more, such as the constancy of sugar, of protein, of the calcium-phosphorus ratio of oxygen, of haemoglobin and so forth. Yet these are not absolutely constant in the blood but their variation is within small limits—any considerable deflection from which is in health soon redressed.