, nerve fibres passing from
each to its neighbours, and each is thus brought into sympathy or
connexion with all the others.

[Sidenote: First appearance of special ganglia.] The next advance is a
very important one, for it indicates the general plan on which the
nervous system is to be developed: it is the dedication of special nerve
arcs to special duties. Thus, in the higher articulates and molluscs,
there are such combinations expressly for the purpose of respiration and
deglutition. Their action is altogether of the reflex kind; it takes
place without consciousness. These ganglia are commissured for the sake
of sympathetic action, and frequently several of them are coalesced for
the sake of package.

This principle of dedication to special uses is carried out in the
introduction of ganglia intended to be affected by light, or sounds, or
odours. The impressions of those agencies are carried to the ganglion by
its centripetal fibres. Such ganglia of special action are most commonly
coalesced together, forming nervous masses of conspicuous size; they are
always commissured with those for ordinary motions, the action being
reflex, as in the preceding case, though of a higher order, since it is
attended with consciousness.

[Sidenote: They are automatic mechanisms.] Such being the elementary
construction of a nervous system, it is plain that animal tribes in
which it exists in no higher degree of complexity must be merely
automata. In this remark many insects must be included, for the instinct
they display is altogether of a mechanical kind, and, so far as they are
concerned, without design. Their actions are uniformly alike; what one
does under given circumstances, under the same circumstances another
will certainly do. They are incapable of education, they learn nothing
by experience, and the acts they are engaged in they accomplish as well
at the first trial as ever after.

Of parts like those described, and of others of a higher order, as will
be presently seen, the most complex nervous system, even that of man, is
composed. [Sidenote: Evidence to be used in these investigations.] It
might, perhaps, be expected that for the determination of the duty of
each part of such complex system the physiologist must necessarily
resort to experiment, observing what functions have been injured or
destroyed when given portions have been removed by his knife. At the
best, however, evidence of that kind must be very unsatisfac