2704. The nervous system becomes differential either through its two kinds of substances, or through extraneous influence; in the first case it thinks or moves, in the second it feels.
2705. Sensation is in the nervous what motion is in the fibrous system, namely, a polar condition, in which the two ends have a tendency to come together, in order to discharge or unload.
2706. In the sensation the extremity of the nerves strives to approach that of the brain; it is therefore a contractile effort in the nerves, like motion is in the muscular fibre. Juxta-posited granules could not, however, shorten; therefore, the already calculated polarity courses over them.
2707. All external stimuli act upon the nerves and polarize them, but not the muscular fibres.
2708. The fibre becomes directly polarized only through the internal stimuli, or by the blood, galvanic tension, &c.
2709. The most general function of the nervous system consists in its assuming polarity from the world, and imparting this to other systems, apart namely, from the alternate operation of its two substances.
Nervous and Motor System.
2710. The first system is that of motion. The tranquil presence of the nerve in the fibre produces indifference in the latter; so that it has no susceptibility for the polarization that is effected by the artery. Doubtless the arterial blood passes, for the most part, close by the fibre through other capillary vessels.
2711. But if the nerve be polarized it is then basic, negative, and enters into opposition with the arterial blood, which now streams into the capillary vessels of the fibre, and renders the two ends of the latter heterogeneous.
2712. Thus, if the nerve be cut across, it remains always indifferent, and no motion can any longer ensue. But this does ensue so soon as the nerve is galvanized. A proof that the nervous influence is homologous to the galvanic tension.