The cartilages are found only in two kinds of immoveable articulations,
viz. in those with surfaces in juxta-position and those with indented
surfaces. They form in all a very thin layer, continued upon both bones
which articulate together, arising from their osseous portion, like
those described before, being of the same nature as it, and forming a
band so much the more close as we advance in age.
On the head these cartilages are very numerous; those of the cranium
have more thickness on the convex than on the concave side, hence the
more rapid disappearance of sutures in the last than in the first
direction.
Wherever they are found, they contribute to unite the bones which form
the cavities; hence, as we have said, there is less danger for these
from external bodies, since the motion lost then in part in their soft
texture, produces a less effect than if the cavity was only one osseous
piece.
It appears that these cartilages have much more affinity to the
phosphate of lime, than those of the moveable articulations, which
rarely ossify, whilst at an advanced age these always become osseous,
of which the cranium especially furnishes us examples. I would observe
however that the cartilages of the indented surfaces have much more
tendency to ossification than those of the surfaces in juxta-position.
At least it is more common to see the parietal bones united with the
occipital and frontal, than to see the union of the ossa maxillaria,
ossa palati, &c.