The long bones belong in general to the apparatus of locomotion, in
which they form a kind of levers that the muscles move in different
directions. All are placed in the extremities, in which their whole
forms a kind of central column, moveable in different directions. We
see them successively diminishing in length and increasing in number,
when examined from the superior to the inferior part, from the thigh
or the humerus to the phalanges of the toes or the fingers. It follows
from this double opposite arrangement, that the top of the limbs is
characterized by the extent of its motions, and the bottom by the
multiplicity, variety and narrow limits of these motions.
These bones have all an analogous conformation; thick and large at
their extremities, they are more slender and usually rounded in the
middle or body, as anatomists call it.
The size of the osseous extremities exhibits the double advantage, 1st,
of presenting to the articulations large surfaces and consequently more
causes of resistance to different displacements; 2d, of contributing
to the regularity of the forms of the limb to which they belong.
Observe in fact that the muscles and the bones are placed in an inverse
direction in the extremities. The middle of the first, which is their
largest part, corresponds to the middle of the second, which forms
their small portion, whilst the extremities of these compensate by
their size for the smallness of the tendons which terminate the others,
and which are placed at the side of them. The increase of size of the
extremities of the long bones is not sudden; it commences imperceptibly
upon the body. We observe upon these different extremities eminences of
articulation and of insertion.
The middle or the body has no eminence; prominent lines are seen
there, always destined for aponeurotic insertions, and which, when
they are very considerable take from the bone its cylindrical form,
which it however preserves in the interior; thus the tibia is evidently
triangular externally, though within its canal has the form of that of
the femur. In general these lines of insertion, always separated by
plain surfaces, are three in number upon each long bone, as we see on
the humerus, the radius, the ulna, the tibia, fibula, &c. I know not
the reason of this law of conformation. Another general observation
is, that the body of almost all the long bones is twisted, so that the
direction of its superior part is not the same as that of the inferior;
by tracing from above downwards one of these lines of which I have just
spoken, this may be seen; it is however more evident in the adult than
in the fœtus. This change of direction has no uniformity in the course
it pursues.
The internal forms of the long bones are very well seen by sawing them
longitudinally. The texture of the cells fills them to the extremities;
it is, as we shall see, more fine and less abundant in the middle,
where the medullary canal exists.
This canal does not exist in the first month of the fœtus, nor as long
as the bone is cartilaginous; the osseous state is the period of its
formation. All the gelatine of the middle of the bone is then absorbed,
exhalation brings no more there, except in the very delicate texture of
the cells that this canal contains; this function, which is nothing in
the centre, becomes more active on the circumference of the bone. This
increase of activity of the external exhalants favours the formation
of the compact texture, the development of which takes place precisely
at the same time as that of the canal whose parietes it forms; so that
at this period of ossification, exhalation and absorption appear to be
in an inverse state in the two parts of the bone; one is very active on
the exterior in bringing phosphate of lime, with which it encrusts the
already existing parenchyma; the other is very active in the interior
in removing the gelatine whose absence forms the space from which the
medullary canal arises.
There is no well marked medullary cavity except in the humerus, the
radius, the ulna, the femur, the tibia, the fibula and clavicle. The
ribs and the phalanges, which in their forms resemble them, have much
of the ordinary texture of the cells in their centre, and hardly
ever any of that more delicate texture of the cells which occupies
the centre of the bones above named, and which is only found in the
medullary cavity.
This cavity does not extend beyond the body of the bone; where the
compact texture grows thinner, it disappears, and is replaced by a
great quantity of the texture of the cells, which fills the extremity
of the bone. Its form is cylindrical and its direction straight. It
does not vary in its form, on account of its asperities or the external
prominent lines of the body of the bone, which is only thicker in
these places. Its parietes are much smoother in the middle, than at
the extremities, where there are already many considerable cellular
filaments thrown off. There are in many subjects, delicate, horizontal
bony partitions, which interrupt almost entirely its continuity in this
place, and appear to divide it into two or three very distinct parts.
The medullary canal serves not only to lodge and defend the medullary
organ, but also to give more resistance to the bone; for we know, that
of two cylinders formed of an equal quantity of matter, one of which
is hollow, and consequently has a greater diameter than the other
which is full, the first will resist more than the second, because we
can bend and break it with less facility. Full cylinders, equal in
diameter to the long bones, would have prevented by their weight, the
motions of the limbs; whilst other cylinders of the same weight as the
present, but without any cavity, would give too small a surface for
the insertion of the muscles. To unite small weight with a sufficient
space in the middle of the long bones, is then a great advantage of the
medullary canal.
This canal disappears in the first periods of the formation of callus
in fractures, because the whole medullary organ is occupied at this
place by gelatine, and becomes cartilaginous; then this gelatine
gradually re-absorbed, without being replaced, favours the development
of a new cavity, and the communication is re-established between the
superior and inferior parts of the canal.
I have observed that, in the first age, and while the extremities
of the bones are cartilaginous, the medullary canal is shorter in
proportion than in the adult; it hardly forms at birth more than the
middle third of the bone, the superior and inferior thirds being formed
at first by the cartilaginous portion of each extremity, then a texture
of cells intermediate between this portion and the canal; so that as we
advance in age, its length becomes in proportion greater.