3d. The order of articulations with implanted surfaces borrows none of its solidity from the mechanism of the part; it owes it entirely to the relation of the surfaces, which are so united and embraced by each other, that displacement is impossible. There is but one example of this articular order, it is the teeth with the jaws. Age does not here efface the articulation, and thus confound the two bones as in the preceding orders, because the medium of union is the palatine membrane, which belongs to the mucous system, and which by its organization has no tendency to ossification; whereas in the preceding cases, the intermediate cartilage has a natural disposition to become encrusted with the phosphate of lime. IV. _Of the Means of Union between the Articular Surfaces._ The articular surfaces would soon separate, if different organs did not retain them in place. These organs are the cartilages and the membranes for the immoveable articulations, the ligaments and the muscles for the moveable. _Union of the Immoveable Articulations._ The two first orders of immoveable articulations, those with inserted surfaces and those with surfaces in juxta-position, have cartilages between the osseous surfaces, the breadth and thickness of which are found so much the greater in proportion as they are examined in subjects nearest infancy. Almost all the bones of the head are held together in this manner, which allows them to yield a little, and consequently prevents their fracture. In the articulations of the pelvis, there are besides the cartilages, ligaments; but as these articulations perform in certain cases small sliding motions, we can consider them as intermediate between the moveable and immoveable articulations; it is on this account that they have the two kinds of organs especially destined to strengthen the articular surfaces of each of the classes, viz. the cartilages and the ligaments. The immoveable articulations with implanted surfaces, an order which comprehends only the teeth, have nothing between the surfaces as a means of union, but a mucous membrane, the palatine. Hence why in the swellings of this membrane, in scorbutic affections, after the use of mercury, &c. the teeth become loose. _Union of the Moveable Articulations._ The moveable articulations with contiguous surfaces have the ligaments especially as a means of union, which are found in the five genera, but under different forms which will be examined hereafter. This kind of organ unites much suppleness with great resistance, a double attribute which it derives from its peculiar texture, and which renders it very proper for this function. Let us observe however that these two properties are in an inverse ratio in the two extreme ages of life, that suppleness is the companion of infancy, that stiffness and resistance are the character of the ligaments in old age. Hence in part the multiplicity of motions in one age, and their slowness and difficulty in the other.