The texture of the medullary membrane is very little known, because its
extreme tenuity conceals it from our researches; for it is only in the
bones of ricketty subjects, that its morbid increase in thickness has
permitted me to trace it accurately. It has the appearance of cellular
texture; yet its properties and its nature are very different from
this texture; it cannot be referred to any of the three classes of
membranes, the serous, the fibrous or the mucous. Some have pretended
that it was an expansion of the periosteum, which had passed through
the numerous foramina by which the bone is perforated, and entered
into the medullary cavity; but the least parallel made between these
membranes is sufficient to make us see that they are essentially
different in their functions, vital forces, &c. and cannot have the
same texture. A principal vessel penetrates the medullary membrane; it
is the artery, which enters by the only, but very large foramen, which
is seen on the body of the long bones; the two branches of this artery
and those of the corresponding vein, ramify in an opposite direction in
the medullary cylinder, and by their innumerable branches give to it a
very evident reddish colour, that disappears with age. The extremities
borrow their vessels from those of the neighbouring texture of the
cells. We cannot trace any nerve there. Such is sometimes the abundance
of the fluids which penetrate this membrane, and its extreme tenuity,
that it might be said not to exist. To be convinced of its existence,
expose the cylinder that it forms to the intense action of heat; it
contracts, has the horny hardening immediately like all the solids, and
thus becomes more apparent.