The fibrous membranes have in general a very compact texture, of a
remarkable thickness; they are formed only of a single lamina. The
dura-mater seems to be an exception to this rule, as its folds form
the falciform process and the tentorium cerebelli; but except at the
place of the sinuses, it is difficult and even impossible, to find two
distinct laminæ.
These membranes have more vessels than all the other divisions of the
fibrous system; they are perforated by a great number of foramina for
the passage of these vessels, most of which only pass through them,
and afterwards go to the organs they cover. These foramina, each of
which is larger than the branch it transmits, form also a character
of the fibrous membranes, distinct from the serous, which are always
folded up, and never open to allow the vascular system to penetrate
their respective organs.
The particular description however of the membranes of which we are
treating, will be added to that of the organs they surround. I shall
except the periosteum, whose description belongs to and may be made in
a general way, whether because clothing the whole osseous system, we
cannot consider it separately, or because, as I have said, it is the
centre from which arise and to which go all the organs of the fibrous
system, so that its functions relate still more to this system than to
that of the bones.