strongly, towards that insertion of the nerves, according as the action of the nerves is increased or diminished by its centricity; and it is thus that they determine the muscular contraction, and all animal motion. The following figure will explain the contraction and relaxation of the muscle: a b represents the relaxed muscle, and c the nerve in its usual state; d e the contracted muscle, and c2 the nerve in increased action. In the second figure, the contraction of the muscle by means of a diminution of volume is explained by the closer approximation of the points to the centre.
But besides this twofold termination, the nervous system finally branches out also into the vegetative sphere of the body, thus partly determining both sensation and motion, and partly constituting the bond of unity between the different organs of the vegetative system. In the latter point of view the very form of the nervous fibres is remarkable, since they all have an evident tendency to inclose the intestines and vessels in a kind of network. In the same point of view it becomes clear also that, as the inclosing of the lower system in the higher one is characteristic of animal organization, and the rudiments of the nerves always show this peripherous type, the higher animals, and man in particular,—in whom the spinal nerves encompass the common cavity of the trunk, somewhat similarly to the bending of the ribs,—possess a system of nerves appropriated to the vegetative structure, and perfectly analogous to the nervous system of the inferior animals, namely, the ganglionic system, or that of the great sympathetic nerve.
The osseous system is developed uniformly with that of the nerves, and issues out of the vertebral column, as the nervous system does out of the spinal marrow; whilst the vertebral column forms at first but an isolating sheath around the latter, as an earthly substance most powerfully attracted by the nervous system, acting like the sun, and for the most part antagonistically upon the other parts of the body. In a somewhat similar manner, among falling bodies of different specific gravities, the heaviest will always lie undermost and nearest to the attracting centre of the earth. Where the osseous system has acquired its most perfect structure (in the skull) it presents also the original type, the spherical form; on the contrary, like the radii of the nervous system, it branches out more and more in the extremities, a fact which is clearly seen in the increasing number of the bones in each member, from their root in the trunk to the ramification in the fingers and toes.
