present we must confine our observations to _those_ alterations which are known to be responses to an excitating stimulus; these are the chemical alterations in the metabolism of rest in the living substance. [57] _Nernst_: “Zur Theorie der electrischen Reizung.” Nachrichten der Königl. Gesellsch. d. Wissensch. zu Göttingen. Math. physik. Klasse 1899. If it is asked, which members of the entire metabolic chain are increased primarily by the stimulating excitation of a vital system, we should not be able to answer this question generally for _all_ living systems. To begin with, it appears highly probable that the various forms of vital substances in this respect act quite differently. It is to be regretted that, up to the present, this question has not been treated from a comparative standpoint. This inquiry should be extended to the greatest possible number of organisms. Still there is enough material at hand, obtained from the muscles, glands, ganglion cells, nerve fibers and plants, to show that the complexity is by no means so great as one might at first assume. In considering the two stages of metabolism, assimilation and dissimilation, in their entirety, it appears as a very remarkable fact, that nearly all stimuli produce primarily a _dissimilative_ excitation. We are only acquainted with a primary _assimilative_ excitation, that is, an augmentation of the building up processes, in short, the _formation_ of living substance, occurring as a primary result of stimulation, following increased introduction of _foodstuffs_ extending over a prolonged length of time. With this exception it cannot be proved that _any_ other stimuli, either especially those operative in the activity of the animal organism or any of the physiological nerve impulses which regulate the actions of the different organs and tissues, bring about primarily an assimilative excitation, which leads to an increase of new formation of living substance. The much-discussed teaching of the existence of the trophic nerves has not given us a single case in which there was positive proof that a nerve impulse brought about a primarily assimilative excitation. I have endeavored for nearly fifteen years to discover such a case. My efforts have been, however, without avail. In the most recent critical review by _Jensen_[58] on the subject of the trophic nerves, the same conclusion is reached although certain facts, as, for instance, the excitation of assimilative processes in the green plant cell, produced by light, seems at the first glance to clearly demonstrate a primary excitation of the building up processes resulting from a stimulation. Nevertheless closer observation invariably shows that these conditions are much more complicated and that primarily assimilative excitating reaction of the stimulus cannot be conclusively shown. There remains, therefore, as a primary assimilative excitating stimulus only the increased introduction of nutrition in a living organism.