away the wasted products of decay.
Also, through the former, the necessary materials for repair and
renovation are brought. [Sidenote: Physiological condition of nerve
action is nerve waste.] There is a definite waste of nervous substance
in the production of a definite mechanical or intellectual result--a
material connexion and condition that must never be overlooked. Hence it
is plain that unless the repair and the waste are synchronously equal to
one another, periodicities in the action of the nervous system will
arise, this being the fundamental condition connected with the physical
theories of sleep and fatigue.

The statements here made rest upon two distinct forms of evidence. In
part they are derived from an interpretation of anatomical structure,
and in part from direct experiment, chiefly by the aid of feeble
electrical currents. The registering or preserving action displayed by a
ganglion may be considered as an effect, resembling that of the
construction known as Ritter's secondary piles.

It will not suit my purpose to offer more than the simplest illustration
of the application of the foregoing facts. When an impression, either by
pressure or in any other way, is made on the exterior termination of a
centripetal fibre, the influence is conveyed with a velocity such as has
been mentioned into the vesicle to which that fibre is attached, and
thence, going forth along the centrifugal fibre, may give rise to motion
through contraction of the muscle to which that fibre is distributed.
[Sidenote: Reflex action of the nervous system.] An impression has thus
produced a motion, and to the operation the designation of reflexion is
commonly given. This reflexion takes place without consciousness. The
three parts--the centripetal fibre, the vesicle, and the centrifugal
fibre--conjointly constitute a simple nervous arc.

[Sidenote: Gradual complexity of the nervous system.] A repetition of
these arcs, each precisely like all the others, constitutes the first
step toward a complex nervous system. Their manner of arrangement is
necessarily subordinated to the general plan of construction of the
animals in which they occur. Thus, in the Radiates it is circular; in
the Articulates, linear, or upon an axis. But, as the conditions of life
require consentaneousness of motion in the different parts, these nerve
arcs are not left isolated or without connexion with each other. As it
is anatomically termed, they are commissured