2. Cartilage.—Pure cartilage enters into the composition of almost all joints, but more particularly of those which are very moveable, and indeed the chief purpose for which it is employed in the economy of adult animals is as an important and valuable element in these moveable joints. Articular cartilage, there fore, constitutes a primary subdivision of this texture by systematic writers. Its hardness, its elasticity, and the limited degree of or ganization which it possesses, peculiarly adapt it for the purposes to which it is applied in the mechanism of the articulations.
Although cartilage is chiefly employed in those joints which possess considerable mo bility, it nevertheless also exists in joints which are limited in their motions, and as it possesses peculiar characters according as it belongs to one or other of these classes of articulations, we may very conveniently subdivide it into— a, cartilage of moveable articulations, or ar ticular cartilage properly so called, or diar throdial cartilage ; b, cartilage of articula tions very limited in their motions, or cartilage of sutures, or synarthrodial cartilage. Under these heads we propose to treat of articular cartilage, a. Diarthroclial cartilage. — The general characters of this class of articular cartilage may be best examined on the articulating ex tremities of the long bones. Here we observe it moulded exactly to the forms of those sur faces, insomuch that, after a little maceration, the cartilage may, by careful dissection, be removed from the bone, to which it adheres with great firmness, and will be found to ex hibit an exact mould of the articular ex tremity; hence these cartilages have been called cartilages of incrustation." This cartilage is perfectly distinct at the early periods of life from the temporary cartilage which forms the nidus of the future bone, and cannot be re garded as a portion of that cartilage left un ossified ; this may easily be seen by examining a vertical section of a femur or tibia at this period ; and the peculiar arrangement of the fibres of the articular cartilage, hereafter to be noticed, constitutes an additional proof that it is completely distinct from that which is after wards transformed into bone.
The physical properties and general charac ters of this form of cartilage do not differ from those of the others ; it possesses the same pearly whiteness—the same apparent homoge neousness of structure—the same elasticity—the same absence of vessels carrying red blood. It is not covered by a perichondrium ; the surface towards the joint is peculiarly smooth and glis tening, and is generally supposed to owe these properties to its being lined by a layer of the synovial sac of the joint ; this point, however, has been controverted, as we shall notice in a subsequent part of the article. The first and
the most complete investigation of the true anatomical construction of articular cartilage was that announced by Dr. William Hunter so long ago as 1743.* His paper still deserves the most attentive perusal, not only for the actual information it affords on its professed subject, but as a specimen of the careful and original method of observation pursued by its distinguished author. To examine the structure of articular cartilages, it is necessary to subject them to boiling or along-continued maceration.t " When an articulating cartilage is well pre pared," says Dr. Hunter, " it feels soft, yields to the touch, but restores itself to its former equality of surface when the pressure is taken off. This surface, when viewed through a glass, appears like a piece of velvet. If we endeavour to peel the cartilage off in lamellae, we find it impracticable, but if we use a certain degree of force, it separates from the bone in small parcels, and we never find the edge of the remaining part oblique, but always perpen dicular to the subjacent surface of the bone. If we view this edge through a glass, it appears like the edge of velvet, a mass of short and nearly parallel fibres rising from the bone, and terminating at the external surface of the carti lage : and the bone itself is planned out into small circular dimples where the little bundles of the cartilaginous fibres were fixed. Thus we may compare the texture of a cartilage to the pile of velvet, its fibres rising up from the bone, as the silky threads of that rise from the woven cloth or basis. In both substances the short threads sink, and bend in waves upon being compressed, but by the power of elasti city recover their perpendicular bearing as soon as they are no longer subjected to a compressing force. If another comparison was necessary, we might instance the flower of any corymbiferous plant, where the flosculi and stamina represent the little bundles of cartilaginous fibres, and the calyx, upon which they are planted, bears analogy to the bone." The total absence of vessels capable of car rying red blood in articular cartilage is proved by the failure of even the minutest injections to pass into the cartilage, and a further confirma tion of this opinion is derived from the fact that madder taken into the system of a young animal does not stain them. The attempts of anatomists to trace lymphatics and nerves into this structure have been equally unavailing.