tory on
account of the shock the entire system receives in vivisections, and
accordingly, artificial evidence can, for the most part, be used only in
a corroborative way. But, as Cuvier observed, the hand of Nature has
prepared for us these very experiments without that drawback. The animal
series, as we advance upward from its lowest members, proves to us what
is the effect of the addition of new parts in succession to a nervous
system, as also does any individual thereof in its successive periods of
development. It is one of the most important discoveries of modern
physiology that, as respects their nervous system, we can safely
transfer our reasonings and conclusions from the case of the lowest to
that of the highest animal tribes.

The articulata present structures and a mode of action illustrating in a
striking manner the nervous system of man. Lengthwise upon their ventral
region is laid a double cord, with ganglia, like a string of beads;
sometimes the cords are a little distance apart, but more generally they
are coalesced, each pair of ganglia being fused into one. [Sidenote:
First introduction of governing ganglia.] To every segment of the body a
pair is supplied, each pair controlling its own segment, and acting
toward it automatically, each also acting like any of the others. But in
the region of the head there is a special pair, the cephalic ganglia,
receiving fibres from the eyes and other organs of sense. From them
proceed filaments to the ventral cord, establishing communications with
every segment. So every part has two connexions, one with its own
ventral ganglia, and one with the cephalic.

It is not difficult to determine experimentally the functions of the
ventral ganglia and those of the cephalic. If a centipede be
decapitated, its body is still capable of moving, the motion being
evidently of a reflex kind, originating in the pressure of the legs
against the surface on which they rest. [Sidenote: But thus far actions
are only instinctive.] The ventral cord, with its ganglia, is hence
purely an automatic mechanism. But if, in making the decapitation, we
leave a portion of the body in connexion with the head, we recognize
very plainly that the cephalic ganglia are exercising a governing power.
In the part from which they have been cut off the movement is forward,
regardless of any obstacle; in that to which they are attached there are
modifications in the motions, depending on sight or other specia