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]
166. Understanding, then, that the propagation of an excitation depends on the state of tension of the tissue, and always follows the line of least resistance, whichever that may be at the moment, we have to inquire whether the transmission takes place only in one direction, from periphery to centre in sensory nerves, and from centre to periphery in motor nerves? By most physiologists this is answered affirmatively. Indeed a special property has been assigned to each nerve, in virtue of this imaginary limitation of centripetal and centrifugal conduction. The “nerve-current” (accepted as a physical fact, and not simply a metaphor) is supposed to “flow” from the central cells along the motor nerve to the muscles; but by a strange oversight the current is also made to “flow” _towards_ the central cells which are said to produce it! Now although the fact may be, and probably is, that normally the sensory nerve, being stimulated at its peripheral end, propagates the stimulation towards the centre, and the motor nerve propagates its central stimulation towards the periphery, the question whether each nerve is not capable of transmission in both directions is not thus answered. _A priori_ it is irrational to assert that nerves fundamentally alike in composition and structure are unlike in properties; and we might as well suppose that a train of gunpowder could only be fired at one end, as to suppose that a nerve could only be excited at one end. And how does the evidence support this _a priori_ conclusion? Dubois Reymond proved that each nerve conducted electricity in both directions; but as Neurility has not been satisfactorily shown to be identical with the electric current, this may not be considered decisive. Such a doubt does not hang over the following facts. M. Paul Bert, pursuing John Hunter’s curious experiments on animal _grafting_, has grafted the tail of a rat under the skin of the rat’s back, the tip of the tail being inserted under the skin, its base rising into the air, so that there is here an inversion of the normal position. In the course of time Sensibility gradually reappears in this grafted tail; and at the end of about twelve months the rat not only feels when the tail is pinched, but knows _where_ the irritation lies, and turns round to bite the pincers.[186] Here we have a case of a sensory nerve reversed, yet transmitting stimulation from the base to the tip of the tail, instead of from the tip to the base, as in a normal organ. Vulpian and Philippeaux having divided two nerves, united the central end of the sensory nerve with the peripheral end of the motor nerve; when the organic union was complete, and each nerve was formed out of the halves of two different nerves, the effect of pinching one of these was to produce simultaneously pain and movement, showing that the excitation was transmitted upwards to the centre, and downwards to the muscles.[187] It may be compared with a train of gunpowder having a loaded cannon at one end and a bundle of straw at the other, when if a spark be dropped anywhere on this train, the flame runs along in both directions, explodes the cannon, and sets alight the straw.