is known as
nervous substance. Cords of similar matter connect his brain of the
lobster, directly or indirectly, with the muscles. Now, if these
communicating cords are cut, the brain remaining entire, the power of
exerting what we call voluntary motion in the parts below the section is
destroyed; and, on the other hand, if, the cords remaining entire, the
brain mass be destroyed, the same voluntary mobility is equally lost.
Whence the inevitable conclusion is, that the power of originating these
motions resides in the brain and is propagated along the nervous cords.

In the higher animals the phenomena which attend this transmission have
been investigated, and the exertion of the peculiar energy which resides
in the nerves has been found to be accompanied by a disturbance of the
electrical state of their molecules.

If we could exactly estimate the signification of this disturbance; if we
could obtain the value of a given exertion of nerve force by determining
the quantity of electricity, or of heat, of which it is the equivalent;
if we could ascertain upon what arrangement, or other condition of the
molecules of matter, the manifestation of the nervous and muscular
energies depends (and doubtless science will some day or other ascertain
these points), physiologists would have attained their ultimate goal in
this direction; they would have determined the relation of the motive
force of animals to the other forms of force found in nature; and if the
same process had been successfully performed for all the operations which
are carried on in, and by, the animal frame, physiology would be perfect,
and the facts of morphology and distribution would be deducible from the
laws which physiologists had established, combined with those determining
the condition of the surrounding universe.

There is not a fragment of the organism of this humble animal whose study
would not lead us into regions of thought as large as those which I have
briefly opened up to you; but what I have been saying, I trust, has not
only enabled you to form a conception of the scope and purport of
zoology, but has given you an imperfect example of the manner in which,
in my opinion, that science, or indeed any physical science, may be best
taught. The great matter is, to make teaching real and practical, by
fixing the attention of the student on particular facts; but at the same
time it should be rendered broad and comprehensive, by constant reference
t