She measures out and
sustains vital force according to the kind and fitness of the raw
material provided for her. When we deal liberally with her, she deals
liberally with us. For everything we give to her she makes a just
return. The stomach, the nutrient arteries, the lungs, have no love, no
patriotism, no pity; but they are perfectly honest. The healthy
digestive organs will extract and pay over to the blood-vessels just so
much of the nutritive elements as the food we eat contains in an
extractible form, and no more; and for this purpose they will demand and
take just so much of the nervous energy as may be needed. The nutrient
arteries will convert into living flesh just so much of the nutritive
elements as the digestive organs give them, and no more. The lungs will
send out from the body as many of the atoms of exhausted and dead flesh
as the oxygen we give them will convert into carbonic acid and water,
and this is all they can do. In these matters, the vital organs are as
honest and as faithful as the boiler, that gives forth steam in the
exact ratio of the heat which the burning fuel evolves and the fitness
of the water that is supplied to it; and neither can be persuaded to do
otherwise. The living machine of bone and flesh and the dead machine of
iron prepare their forces according to the means they have, not
according to the ulterior purpose to which those forces are to be
applied.
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