Health Bulletin Jor leacher

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Factors of Safety in Body Structure and Function

““wyactor of safety” is a term used by engineers to designate the margin

EF of safety required in building engines, bridges, houses, and other
structures. It is not enough to make a structure strong enough to bear
expected maximum loads. It must be capable of withstanding stresses
several times the size of such loads. In a paper read before the Harvey
Society of New York in December 1906, Dr. S. J. Meltzer first drew
attention to the fact that the human body is provided with factors of
safety on a lavish scale. Since then several other physiologists have
developed this theme.

The remarkable thing about many of our factors of safety is that they
are so seldom called upon. Barcroft cites as an example of a factor of
safety always available, but never used by the great majority of man-
kind, the duplication of mechanism associated with the initiation of the
heart beat. The electrical impulses which set off each contraction of
the heart originate in a little node of nervelike material located in the
wall of the right auricle. The impulses are conducted from the auricle
to the ventricle by a bridge of nerve and muscle fibers called the bundle
of His. However, if this bundle ceases to conduct in its upper portion,
a new rhythm arises lower down and causes the ventricles to beat but
with a slower rhythm than before. Thus “if the normal machinery is
wrecked, the heart does not on that account cease to beat, as the beat is
initiated elsewhere.”” And then Barcroft goes on to ask: “Is it a happy
accident? Are we to suppose that all of us carry an accident so happy
in our hearts, that it will save the lives of a few persons whilst func-
tionally it never enters the economy of more than a negligible proportion
of the race? Are there such accidents?”

Barcroft himself believes, and many other scientists agree with him,
that duplication is so frequent that it can scarcely be accidental. It is
a definite feature in the architecture of the body, “the more impressive
because it is achieved in such different ways...” So many ways indeed
that Barcroft concludes wistfully, “It is always surprising to me that
there is only one heart!’’*

Factors of Safety in the Circulatory System. We have only one heart,
it is true, but that single heart has a superabundance of volume and force.
At any moment it is able to beat twice as fast as it ordinarily does,
and put forth twice the usual amount of blood at each beat against an

 

*JosepH Barcrorr. Features in the Architecture of Physiological Function. Cambridge, at the Uni-
versity Press. 1934. Chapter XIV.

 

Vol. XII, No. 8 : APRIL 1941