The design of articular as means to break the violence of shocks, is well illus trated by comparing the different arrangement of the cartilaginous incrustation on convex arti cular surfaces from that on concave. In the former, we observe the layer of cartilage to be very thin at the circumference of the articular surface, its thickest portion being in the centre, while the opposite arrangement obtains on con cave surfaces : there the thinnest portion of the cartilage is in the centre, and the layer increases in thickness as it approaches the circumference.
" The articulating cartilages are most hap pily contrived to all purposes of motion in those parts. By their uniform surface they move upon one another with ease : by their soft, smooth, and slippery surface mutual abra sion is prevented by their flexibility, the con tiguous surfaces are constantly adapted to each other, and the friction diffused equally over the whole : by their elasticity, the violence of any shock, which may happen in running, jumping, &c. is broken and gradually spent ; which must have been extremely pernicious, if the hard surfaces of bones had been immediately contiguous. As the course of the cartilaginous fibres appears calculated chiefly for this last advantage, to illustrate it, we need only reflect on the soft undulatory motion of coaches, which mechanics want to procure by springs, or upon the difference betwixt riding a chamber-horse and a real one."t b. Synarthrodial cartilage.—The cartilages of synarthrodial articulations are destined in some degree -to act as bonds of union, as well as means of separation and for the prevention of the effects of concussion. They are simply cartilaginous laminae interposed between the osseous articular surfaces, very adherent to both, and adherent likewise by their margins to the periosteum or ligamentous expansions which may pass from one bone to another. We find instances of these cartilages in the sacroiliac symphysis, or synchondrosis, as it has been called from the junction of the bones by carti lages ; also in the sutures, where there are very thin cartilaginous laminae interposed be tween the osseous margins. These laminae will be found to be triangular in their section, the thin edge or apex being internal, which, as Meckel observes, may in some degree account for the earlier obliteration of the sutures on the internal than on the external surface of the cranium. These cartilages of sutures are not
strictly permanent ; they disappear with age : and according to lieclard, hold the midway, as to frequency of ossification, between permanent and temporary cartilages.f The cartilages of the ribs perform in some degree the office of articular cartilage ; they are situated between two osseous surfaces ; they form bonds of union, and their elasticity is eminently essential to the full performance of the movements of the thorax.
In fishes most of the moveable articulations are provided with elastic cartilages, which serve the double purpose of forming bonds of union as well as of permitting motion by their elasticity.
3. Fibro-cartilage.—This remarkable struc ture, called by the older anatomists ligamentous cartilage or cartilaginiform ligament, is made much use of in the articulations ; and it is well adapted for a means of union, by reason of its great strength, which it owes to its ligamentous part, and of its elasticity, for which it is indebted to its cartilaginous portion. We find fibro cartilage to be connected with the joints under three forms : a. In the form of laminae, free on both sur faces to a greater or less extent, and lined to the same extent by the synovial membrane re flected upon them/ These are the interarti cular cartilages or menisci of authors. They are found in the temporo-maxillary, sterno clavicular, and tibio-femoral articulations, some times in the acromio-clavicular, between the bodies of the cervical vertebrae in birds, and in general in those joints where there is constant and extensive motion, and consequently where the articular surfaces are exposed to consider able friction. The principal use of these fibro cartilaginous laminae must unquestionably be to guard against any bad consequences likely to arise from this continued friction ; this is particularly obvious in the sterno-clavicular articulation. To increase the depth of an arti cular excavation is another object, as appears from the semilunar cartilages of the knee-joint ; and moreover, in conjunction with the attain ment of these two objects, to ensure in all the motions of the joint a perfect adaptation of the articular surfaces to one another, as will appear obvious to any one who carefully considers the construction of the temporo-maxillary or even of the knee-joint.