Page:Popular Science Monthly Volume 26.djvu/757

. This division is peculiar, because it does not affect nutrition or the ordinary organic processes. In a limb whose nerves are severed there is a loss of sensation; there is also a loss of movement; the limb continues to live, but for all limb purposes it might as well be dead. Nerve-matter, therefore, preserves the higher bodily unity.

In examining this general nerve-function we discover the distinctive tasks of fibers and cells. The fibers convey, while the cells originate,



motions. Fibers may convey motions from without inward, or from within outward; in the former case they are called afferent, in the latter efferent. The nerve-arc is composed of an afferent and an efferent fiber and cell matter. The arrangement of the arc is such that the outer end of the afferent fiber terminates at the surface of the body, the inner end at the cell-matter, while the outer end of the efferent fiber terminates in a muscle, its inner end being also in the same cell-matter. Nothing more than this arc is necessary to produce nerve-action. If an impression be made at the surface of the body, the motion there occasioned is carried by the afferent fiber to the cell-substance; through this substance the motion is transferred to the efferent fiber, along which it passes to the muscle causing muscular contraction. Since the cell liberates motion, and, being much more unstable than the fiber, liberates motion freely, it often happens that a slight impression at the surface is followed by a very violent contraction of the muscle.

Our nerve-arc is not a nervous system. We need only one additional element, however, to form such a system, and this is an