46. The organic substances are what analysis discovers in organized substances, but none of them, not even the highest, is living, except as organized. Albumen alone, or Stearin alone, is as incapable of Vitality, as Plumbago, or Soda; but all organic substances are capable of playing a part in vital actions; and this part is the more important in proportion to their greater molecular variety. Organization is a special synthesis of substances belonging to all three classes; and the organized substance, thus formed, alone merits the epithet living. We see how organized substances, being constituted by principles derived from the inorganic world, and principles derived from the organic world, have at once a dependence on the external Medium, and an independence of it, which is peculiar to living beings. An analogous dependence and independence is noticeable with respect to the parts; and this is a character not found in inorganic compounds. The organism, even in its simplest forms, is a structure of different substances, each of which is complex. While one part of a crystal is atomically and morphologically identical with every other, and is the whole crystal “writ small,” one part of an organism is unlike another, and _no part_ is like the whole. Hence the dependence of one organ and one tissue on another, and each on all. Yet, while every part is, so to speak, a condition of existence of every other, and the unity of the organism is but the expression of this solidarity,--wherever organized substance has been differentiated into morphological elements (cells, etc.), each of these has its own course of evolution independently of the others,--is born, nourished, developed, and dies. 47. The interdependence of nerve and muscle is seen in this, that the more the muscle is excited the feebler its contractions become; this decrease in contractility is compensated by an increased excitability in its nerve; so that while the muscle demands a more powerful stimulus, the nerve acquires a more energetic activity. Ranke’s curious and careful experiments seem to prove that this depends on the wearied muscle absorbing more water, owing to the acids developed by its activity, and on the nerve losing this water--a nerve being always more irritable when its quantity of water diminishes.