He would now briefly trace a few of the more important steps which had
marked the recent study of the relations between the vital phenomena
and those of the inorganic world. No portion of the science of
chemistry was of greater interest or greater complexity than that
which, bearing on the vital functions both of plants and of animals,
endeavored to unravel the tangled skein of the chemistry of life, and
to explain the principles according to which our bodies live, and
move, and have their being. If, therefore, in the less complicated
problems with which other portions of our science have to deal, we
found ourselves often far from possessing satisfactory solutions, we
could not be surprised to learn that with regard to the chemistry of
the living body--whether vegetable or animal--in health or disease, we
were still farther from a complete knowledge of phenomena, even those
of fundamental importance.
Liebig asked if we could distinguish, on the one hand, between the
kind of food which goes to create warmth and, on the other, that by
the oxidation of which the motions and mechanical energy of the body
are kept up. He thought he was able to do this, and he divided food
into two categories. The starchy or carbo-hydrate food was that, said
he, which by its combustion provided the warmth necessary for the
existence and life of the body. The albuminous or nitrogenous
constituents of our food, the flesh meat, the gluten, the casein out
of which our muscles are built up, were not available for the purpose
of creating warmth, but it was by the waste of those muscles that the
mechanical energy, the activity, the motions of the animal are
supplied.
Soon after the promulgation of these views, J.R. Mayer warmly attacked
them, throwing out the hypothesis that all muscular action is due to
the combustion of food, and not to the destruction of muscle.
What did modern research say to this question? Could it be brought to
the crucial test of experiment? It could; but how? In the first place,
we could ascertain the work done by a man or any other animal; we
could measure this work in terms of our mechanical standard, in
kilogramme-meters or foot-pounds. We could next determine what was the
destruction of nitrogenous tissue at rest and under exercise by the
amount of nitrogenous material thrown off by the body. And here we
must remember that these tissues were never completely burned, so that
free nitrogen was never eliminated. If now we knew the heat value of
the burned muscle, it was easy to convert this into its mechanical
equivalent and thus measure the energy generated. What was the result?
Was the weight of muscle destroyed by ascending the Faulhorn or by
working on the treadmill sufficient to produce on combustion heat
enough when transformed into mechanical exercise to lift the body up
to the summit of the Faulhorn or to do the work on the treadmill?
Careful experiment had shown that this was so far from being the case
that the actual energy developed was twice as great as that which
could possibly be produced by the oxidation of the nitrogenous
constituents eliminated from the body during twenty-four hours. That
was to say, taking the amount of nitrogenous substance cast off from
the body, not only while the work was being done, but during
twenty-four hours, the mechanical effect capable of being produced by
the muscular tissue from which this cast-off material was derived
would only raise the body half way up the Faulhorn, or enable the
prisoner to work half his time on the treadmill. Hence it was clear
that Liebig's proposition was not true.
The nitrogenous constituents of the food did doubtless go to repair
the waste of muscle, which, like every other portion of the body,
needed renewal, while the function of the non-nitrogenous food was not
only to supply the animal heat, but also to furnish, by its oxidation,
the muscular energy of the body. We thus came to the conclusion that
it was the potential energy of the food which furnished the actual
energy of the body, expressed in terms either of heat or of mechanical
work.
But there was one other factor which came into play in this question
of mechanical energy, and must be taken into account; and this factor
we were as yet unable to estimate in our usual terms. It concerned the
action of the mind on the body, and although incapable of exact
expression, exerted none the less an important influence on the
physics and chemistry of the body, so that a connection undoubtedly
existed between intellectual activity or mental work and bodily
nutrition. What was the expenditure of mechanical energy which
accompanied mental effort was a question which science was probably
far from answering; but that the body experienced exhaustion as the
result of mental activity was a well-recognized fact.