163. Vitality is characterized by incessant molecular movement, both of composition and decomposition, in the building up of structure and the liberation of energy. The life of every organism is a complex of changes, each of which directly or indirectly affects the statical and dynamical relations, each being the resultant of many co-operant forces. In the nourishment of every organite there is an accumulation of molecular _tension_, that is to say, stored-up energy in a latent state, ready to be expended in the activity of that organite; and this expenditure may take place in a steady flow, or in a sudden gush. The molecular movements under one aspect may be called convergent, or _formative_: they build the structure, and tend to the state of equilibrium which we call the statical condition of the organite, i. e. the condition in which it is not active, but ready to act. Perfect equilibrium is of course never attained, owing to the incessant molecular change: indeed Life is inconsistent with complete repose. Under another aspect the molecular movements may be called _discharging_: they constitute the dynamic condition of the organite, in which its functional activity appears. The energy is now diverted, liberated, and the surplus, over and above that which is absorbed in formation, instead of slowly dribbling off, gushes forth in a directed stream. The slow formation of a secretion in a gland-cell, and the discharge of that secretion, will illustrate this; or (if muscular _tone_ be admitted) the incipient contraction of the chronic state, and the complete contraction of the dynamic state, may also be cited.
164. The discharge which follows excitation may thus be viewed as a _directed quantity_ of molecular movement. Because it is always strictly relative to the energy of tension, and is inevitable when that tension attains a certain surplus over what is required in construction, there is a limit, 1°, to the growth and evolution of every organite, and every organism (comp. Problem I. § 118), and, 2°, to its dynamical effect. When there is no surplus, the organite is incapable of discharge: it is then exhausted, i. e. will not respond to stimulus.
165. The speciality of nerve-tissue is its pre-eminence in directive energy. Like all other tissues, it grows, develops, and dies; but above all others it has what we call _excitability_, or readiness in discharging its energy in a directed stream. By its topographical distribution it plays the functional part of exciting the activity of other tissues: it transmits molecular disturbance from periphery to centre, from centre to centre, and from centre to muscles, vessels, and glands. When a muscle is excited it moves, and when a gland is excited it secretes; but these actions end, so to speak, with themselves; the muscle does not directly move any other muscle;[184] the gland does not directly excite any other gland. The nerve, on the contrary, has always a wide-spreading effect; it excites a centre which is continuous with other centres; and in exciting one muscle, usually excites a group. Hence the nervous system is that _which binds the different organs into a dynamic unity_. And Comparative Anatomy teaches that there is a parallelism between the development of this system and the efficient complexity of the organism. As the tissues become more and more specialized, and the organs more and more individualized, they would become more and more unsuited to the general service of the organism, were it not that a corresponding development of the nervous system brought a unifying mechanism.