hus that scientific views of the nature of man can only be
obtained from an examination of his nervous system, and that the right
interpretation of the manner of action of that system depends on the
guiding light of comparative anatomy and physiology, I shall, in the
following exposition, present the progress of discovery on those
principles.

[Sidenote: The rudimentary nervous system is automatic.] In those low
tribes of life which show the first indications of a nervous system, its
operation is purely mechanical. An external impression, as a touch, made
upon animals of that kind, is instantly answered to by a motion which
they execute, and this without any manifestation of will or
consciousness. The phenomenon is exactly of the same kind as in a
machine of which, if a given lever is touched, a motion is instantly
produced.

[Sidenote: Two elementary forms of nerve structure.] In any nervous
system there are two portions anatomically distinct. They are, 1st, the
fibrous; 2d, the vesicular. It may be desirable to describe briefly the
construction and functions of each of these portions. Their conjoint
action will then be intelligible.

[Sidenote: Structure of a nerve fibre.] 1st. A nerve fibre consists
essentially of a delicate thread--the axis filament, as it is
called--enveloped in an oil-like substance, which coagulates or congeals
after death. This, in its turn, is inclosed in a thin investing sheath
or membranous tube. Many such fibres bound together constitute a nerve.

[Sidenote: Function of a nerve fibre is conduction.] The function of
such a nerve fibre is indisputably altogether of a physical kind, being
the conveyance of influences from part to part. The axis filament is the
line along which the translation occurs, the investing material being
for the purpose of confining or insulating it, so as to prevent any
lateral escape. Such a construction is the exact counterpart of many
electrical contrivances, in which a metallic wire is coated over with
sealing-wax or wrapped round with silk, the current being thus compelled
to move in the wire without any lateral escape. Of such fibres, some
convey their influences to the interior, and hence are called
centripetal; some convey them to the exterior, and hence are called
centrifugal. No anatomical difference in the structure of the two has,
however, thus far been discovered. As in a conducting wire the
electrical current moves in a progressive manner with a defin