the distribution of potential energy among the tissues; and
we shall be entirely convinced of it when we reflect upon the conditions
in which the energy is expended and restored. For suppose the
sensori-motor system is a system like the others, of the same rank as
the others. Borne by the whole of the organism, it will wait until an
excess of chemical potential is supplied to it before it performs any
work. In other words, it is the production of glycogen which will
regulate the consumption by the nerves and muscles. On the contrary, if
the sensori-motor system is the actual master, the duration and extent
of its action will be independent, to a certain extent at least, of the
reserve of glycogen that it holds, and even of that contained in the
whole of the organism. It will perform work, and the other tissues will
have to arrange as they can to supply it with potential energy. Now,
this is precisely what does take place, as is shown in particular by the
experiments of Morat and Dufourt.[56] While the glycogenic function of
the liver depends on the action of the excitory nerves which control it,
the action of these nerves is subordinated to the action of those which
stimulate the locomotor muscles--in this sense, that the muscles begin
by expending without calculation, thus consuming glycogen, impoverishing
the blood of its glucose, and finally causing the liver, which has had
to pour into the impoverished blood some of its reserve of glycogen, to
manufacture a fresh supply. From the sensori-motor system, then,
everything starts; on that system everything converges; and we may say,
without metaphor, that the rest of the organism is at its service.

Consider again what happens in a prolonged fast. It is a remarkable fact
that in animals that have died of hunger the brain is found to be almost
unimpaired, while the other organs have lost more or less of their
weight and their cells have undergone profound changes.[57] It seems as
though the rest of the body had sustained the nervous system to the last
extremity, treating itself simply as the means of which the nervous
system is the end.

To sum up: if we agree, in short, to understand by "the sensori-motor
system" the cerebro-spinal nervous system together with the sensorial
apparatus in which it is prolonged and the locomotor muscles it
controls, we may say that a higher organism is essentially a
sensori-motor system installed on systems of digestion, respiration,
circu