ments are effected. So that where a nervous system exists, with its
complementary sense-organs and motor apparatus, everything should happen
as if the rest of the body had, as its essential function, to prepare
for these and pass on to them, at the moment required, that force which
they are to liberate by a sort of explosion.

The part played by food amongst the higher animals is, indeed,
extremely complex. In the first place it serves to repair tissues, then
it provides the animal with the heat necessary to render it as
independent as possible of changes in external temperature. Thus it
preserves, supports, and maintains the organism in which the nervous
system is set and on which the nervous elements have to live. But these
nervous elements would have no reason for existence if the organism did
not pass to them, and especially to the muscles they control, a certain
energy to expend; and it may even be conjectured that there, in the
main, is the essential and ultimate destination of food. This does not
mean that the greater part of the food is used in this work. A state may
have to make enormous expenditure to secure the return of taxes, and the
sum which it will have to dispose of, after deducting the cost of
collection, will perhaps be very small: that sum is, none the less, the
reason for the tax and for all that has been spent to obtain its return.
So it is with the energy which the animal demands of its food.

Many facts seem to indicate that the nervous and muscular elements stand
in this relation towards the rest of the organism. Glance first at the
distribution of alimentary substances among the different elements of
the living body. These substances fall into two classes, one the
quaternary or albuminoid, the other the ternary, including the
carbohydrates and the fats. The albuminoids are properly plastic,
destined to repair the tissues--although, owing to the carbon they
contain, they are capable of providing energy on occasion. But the
function of supplying energy has devolved more particularly on the
second class of substances: these, being deposited in the cell rather
than forming part of its substance, convey to it, in the form of
chemical potential, an expansive energy that may be directly converted
into either movement or heat. In short, the chief function of the
albuminoids is to repair the machine, while the function of the other
class of substances is to supply power. It is natural that the
albumino