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. The great instability of neurine, in other words, its high degree of tension, renders it especially apt to disturb the tension of other tissues. It is very variable; and this variability will have to be taken into account in explaining the restriction of discharges to particular centres. A good example of exaggerated tension is furnished by strychnine poisoning. The centres are then so readily excitable that a touch, or a puff of cold air on the skin, will determine convulsions. And it is worthy of remark that for some hours after this convulsive discharge the centres return to something like their normal state; and the animal may then be stroked, pinched, or blown upon without abnormal reactions. But during this interval the centres are slowly accumulating excess of tension from the poisoned blood; and at the close, convulsions will again follow the slightest stimulus. This alternation of exhaustion and recrudescence is noticed by Schröder van der Kolk in the periodicity of the phenomena exhibited in spinal disease.[185] THE PROPAGATION OF EXCITATION.