The comprehension of _the metabolism of rest_ demands a closer consideration. On closer observation we must say that this much-used conception is merely an abstraction nowhere realized in a strict sense. In truth, there is nowhere in nature a metabolism of rest. No cell exists which in a mathematical sense remains for even two successive moments under absolutely the same external conditions. If we imagine a single living cell of the simplest kind living in a fluid nutritive medium, and if we suppose its body and surroundings so magnified that the single molecules and atoms were respectively of the size of cannon and rifle balls, the boundary between cell and medium would represent a battlefield, on which a heavy bombardment is constantly taking place. The rain of shot of food and oxygen molecules penetrating into the cell from the medium, would produce an explosion in the existing ammunition depots, now at one point, now at another, creating great breaches through which new masses of shot would reach the interior. The fragments of these exploding molecules would be flung out here and there into the medium and would stem, now at this, now at that point the besieging masses of shot. In this wild confusion on the whole boundary line between cell and medium there can be no question of rest or even equilibrium at any point. The human mind, superior to the material world as we may deem it, is yet always dependent upon the results of experience, and even in its highest flights cannot become wholly emancipated from the concrete objects. For this reason it is of great purport to conceive processes whose dimensions cannot be observed even microscopically, as enlarged and transformed to that method of expression most familiar to the human mind, namely, in the field of optical presentation. This method is of great help in aiding our understanding, and likewise here, even in the resting state, the cell is constantly exposed to local effects of stimulation, now at one point, now at the other. The conception of the metabolism of rest is, therefore, in a strict sense fiction.