resists fracture; the central portion contributes
nothing to the amount of that resistance.
The force which tends to break any
straight bar, compresses the particles on one
side and distends those on the opposite side;
hence, a very slight notch in the under surface
of a transverse beam supporting a heavy
weight in the middle, may be sufficient to
cause its destruction. If the beam be arched
with its convexity downwards, the danger is
greatly increased; if, on the other hand, the
convexity be upwards, the danger vanishes,
for then the incumbent weight tends to compress
both surfaces alike. A plain cylinder is
manifestly weaker than one of similar dimensions
with superadded ribs or flutings; and
lastly, the power of resistance may be increased
by widening the surface liable to compression
and augmenting its density, lengthening
the transverse diameter in the direction
of the probable line of fracture, and providing
for the greater cohesiveness of the side liable
to disruption. All these contrivances are
exemplified in the long bones. They are not
quite cylindrical (though so called), but have
a more or less flattened surface on one side,
and opposite to this a longitudinal projecting
ridge or spine. They appear as if slightly
twisted round their own axis, the effect of
which is, that the broad surfaces and the
longitudinal spines present themselves in the
most advantageous positions to resist the
strain of the adjacent muscles. To the same
end, they are sometimes considerably arched
from one extremity to the other, as in the
instance of the human thigh bone. The
density and hardness of their surface vary in
different parts, and are always greatest where
those qualities are most needed.

So much we find set forth in detail by
Arnott and Bell; but no writer has assigned
any satisfactory reason for the fact, that the
long bones of man and of quadrupeds are
filled with marrow. What may be the use of
this substance is a question which remains for
us to solve. It is one which, as Mr. Ephraim
Jenkinson observes of the cosmogony and the
creation of the world, has puzzled philosophers
in all ages. Anatomical writers have long
given it up in despair, or have contented
themselves with the unmeaning conclusion,
that the marrow is there for no very particular
use, but simply as a light material to
fill up vacant space. Assuredly, nature's
workmanship is never disgraced by any such
superfluous cobbling. Our solution of the
question is, that the marrow serves to increase
the rigidity of the bone, by acting as a medium
through which the strength of every part of
its containing walls is simultaneously exerted
to resist an excessive strain at any one point.

It is a well-known principle of hydrostatics,
that a pressure exerted on any part of a mass
of fluid is immediately propagated through
every other portion. If a tightly fitting tube,
furnished with a piston, the surface of which
measures, say one square inch, be inserted
into the head of a full cask, and if a weight of
ten pounds be laid on the piston rod, that
pressure will not be transmitted solely to
the inch of surface at the bottom which
corresponds to the column of fluid directly under
the piston, but every inch of the interior surface
of the cask, top and sides as well as
bottom, will have to bear an additional pressure
of ten pounds. If any one of them is
unable to withstand that additional pressure,
the cask will burst; if they are all able
to do so, the top, sides, and bottom of the
cask will react against the pressure, so that
the equilibrium will be maintained, and the
piston cannot descend. Now, marrow consists
of a delicate network of cellular tissue, and of
a fine oil which occupies its interstices, and
we may consider it practically as a fluid
filling a narrow, elongated, little cask. The
cavity in which it is confined is nearly cylindrical,
whatever be the irregularities of form
on the outer surface of the bone. The
shape of such a cavity cannot be altered
without diminishing its capacity, and
consequentlycompressing its contents. But such
alteration and such compression must necessarily
take place before the bone can break or
even bend considerably; and, in consequence
of the hydrostatic law above stated, the tendency
to this compression at any one or more
points will be resisted by the cohesive force of
every other point in the bony case.

The hollows in the bones of birds of flight
are filled not with marrow but with air,
whereby the specific gravity of the whole
body is diminished. These air cavities are
also in a measure supplementary to the lungs,
and help to furnish the muscles with that
large supply of aërated blood which their
rapid and continuous action demands. Besides
these two functions, the air in the bones fulfils
a third also, analogous to that which we have
ascribed to marrow. Air, being an elastic
fluid, is less capable than oil of resisting pressure;
but that confined air can yet impart
considerable rigidity to the walls of its
chamber, any one may easily convince himself
by handling a blown bladder or gut, such as is
used by sausage-makers. The inferior stiffness
of a bird's bones is no doubt compensated for
by the low specific gravity which imposes so
much the less strain on the skeleton.

May we not hope to see the principle of
these beautiful natural contrivances applied
to a variety of useful purposes in art? We
think we may. Nevertheless there are certain
difficulties which we must not overlook. To
shut up air in cases, whether flat, tubular, or
of any other form, is a simple matter enough;
but it is not quite so easy a thing as it may
seem to fill a tube with a liquid so that it
shall contain no air, and then to seal the tube
hermetically in such a manner that the sealed
end shall be as strong as any other part to
resist a disruptive force from within. We must
also bear in mind how dissimilarly solids
and liquids expand and contract under the