Each of the organs from the whole of which it results, exhibits it
under the form of a delicate membrane, lining the whole cavity, folded
a great number of times, giving origin to many elongations, of which
some cover the fine threads of the texture of the cells which are met
with in this cavity, others pass, without adhering to any osseous
portion, from one side of the membrane to the other, and of which all
form numerous cells in which the marrow is contained.
We can then form of this organ an idea analogous to that which the
cellular organ gives us; viz. that of a spongy body with communicating
cells. The place that it occupies, gives to it as a whole, a
cylindrical form.
It appears that at the two extremities of the canal, the cells or
membranes do not open into those of the texture of the cells, and that
the medullary fluid of the preceding system does not communicate with
the marrow of this. In fact, the line of demarcation which separates
them is evident; they do not mix in a gradual manner. Air injected from
one side of the medullary cylinder, only penetrates with difficulty and
by tearing the membrane, into the texture of the cells of the opposite
extremity of the bone; yet, notwithstanding these considerations, I
confess that the question is not fully settled. The transudations
in dead bodies have no influence in deciding it, on account of the
permeability that our parts acquire after death.