The laws of biological energetics are three in number. First of all, there is the fundamental principle which we have just developed, and which is, so to speak, laid down _a priori_; and there are two other principles, those established by experiment and summing-up, as it were, the multitude of known physiological effects. Of these two experimental laws, one refers to the _origin_ and the other to the _termination of the energies developed in living beings_. § 3. SECOND LAW OF BIOLOGICAL ENERGETICS. _The Origin of Vital Energy._—Vital energies have their origin in one of the _external or common energies_—not in any one we choose, as might be supposed, but in one only: chemical energy. The third principle will show us that they terminate in another energy or a few others, also completely fixed. It follows that the phenomena of life must appear to us to be a circulation of energy which, starting from one fixed point in the physical world, returns to that world by a few points, also fixed, after a transient passage through the animal organism. Or more precisely, it is a transposition from the realm of matter into the world of energy, of the idea of the _vital vortex_ of Cuvier and the biologists. They defined life by its most constant property—nutrition. Nutrition was exactly this current of matter which the organism obtains from without by alimentation, and which it throws out again by excretion; and the even momentary interruption of which, if complete, would be the signal of death. The cycle of energy is the exact counterpart of this cycle of matter. The second truth taught us by general physiology, a truth which physiology learned from experiment, is enunciated as follows:—_The maintenance of life consumes none of its energy. It borrows from the external world all the energy which it expends, and borrows it in the form of potential chemical energy._ This is a translation into the language of energetics of the results acquired in animal physiology during the last fifty years. No comment is needed to exhibit the importance of such a truth. It reveals the origin of animal activity. It reveals the source from which proceeds that energy which at some moment of its transformations in the animal organism will be a _vital energy_. The _primum movens_ of vital activity is, therefore, according to this law, the chemical energy stored up in the immediate principles of the organism. Let us try to follow, for a moment, this energy through the organism and to specify the circumstances of its transformations.