When we take a general view of the first and second periods in the early life of warm-blooded animals, we find that they are under the influence of two general conditions relative to calorifica tion ; conditions which, acting inversely, tend to compensate each other mutually ; on the one hand, the celerity of the motions ; on the other, the imperfection of the nutrient and ner vous functions. The celerity of the motions of circulation and respiration diminishes, whilst the development of the nutritive and nervous functions increases with age. These two con ditions influencing the production of heat are, therefore, in an inverse ratio to one another. And according to the nature of these relations will the temperature vary. Were the opposite effects equal, there would be exact compensa tion in the whole phases of the evolution, from the moment of birth to that of perfect adole scence, and the temperature of the body would be the same at every period of life. But the progression in the celerity of the movements on the one hand and of corporeal development on the other, is unequal ; and there is but a single epoch in the whole course of childhood when such an equality or balance exists, and at which consequently the temperature of the child is the same as that of the adult. Previous to this epoch, the nutritive and nervous func tions are so imperfectly developed, that their influence, inimical to the production of heat, surpasses the favourable tendency to this end, which we have in the celerity of the motions of circulation and respiration. It follows that the
temperature of the body is inferior at the pre ceding limit or to that of the adult state; with the progress of time, however, the child attains this limit, and then we have a new relation established. The evolution of the nutritive and nervous functions continues, and although it have not yet attained its ultimate term, the de fect of heat which results from this is all but compensated by the celerity of the motions, which is still sufficiently great, to surpass in a marked degree the celerity of the motions in the adult. The temperature at this period will, therefore, be above that of the adult. This pe riod lasts for several years in childhood or youth ; but then comes a gradual retardation in the motions both of respiration and circula tion, and with this a reduction of the tempera ture to the standard of the adult.
There are consequently four states of the temperature from birth up to adolescence inclu sive. In the first period the temperature is at the minimum ; in the second, it attains the adult degree ; this might be entitled the period of the mean temperature ; in the third, the temperature exceeds that of the adult; finally, in the fourth, it sinks to the mean, that is, the temperature of the adult.
There are, therefore, constitutions in the same class of animals which are more or less favour able to the production of heat ; for it is so among individuals that differ in age in the limits between the moment of birth and that at which adolescence is completed ; and this leads us to new considerations.