ARTICULATION (in anatomy), synony ,mous with joint. (Gr. apepOY. Lat. articulus, arthrosis, junctura. Fr. articulation. Germ. Articulation, Gelenk. Ital. articolo).
The power of motion, to an extent however limited, seems to be inseparable from our idea of an animal, and in looking through the animal series we find none which do not appear to be endowed with this power whether for the pur pose of progression, or simply of altering the po sition or condition of some part of their bodies with respect to the others. The organic structure which is the immediate agent in this motive power (the muscular fibre), is one and the same throughout the whole chain of animals, va riously modified according to the degree and force of the motions necessary for the particular individual. The mechanism by which this structure acts upon the different parts of the body varies considerably, and increases in complexity as the forms of the animals them selves become more complex. In the lowest grade of animals the structure is so soft and pliant that nothing more is required to produce motion than this contractile tissue, which acts in obedience to certain stimuli. But when hard parts are superacided to the structure of the animal, we then find a peculiar me chanism to allow of the motion of these hard parts on each other without the risk of injury. It is that such motion could not take place were these hard parts united in one piece. hence we find that they are subdivided into segments, and these segments are joined to each other through the medium of some struc ture_more flexible than that of the segments themselves, or by an apparatus of such a con struction as to admit of the motion of one segment upon the other. It is to these join ings of different segments of an animal body that the term articulations or joints has been applied.
An articulation may, therefore, be defined to be the union of any two segments of an animal body through the intervention of a structure or structures different from both.
The most perfect and elaborate forms of articulations are those which are seen in ani mals that possess a fully developed internal bony skeleton, and in none may they he studied with more advantage than in man. We propose to treat of the forms and structure of the ar ticulations in man, and at the same time to in quire what modes of mechanism are employed for analogous purposes in the lower classes. In the human subject and in the vertebrated ani mals generally, we shall, indeed, have particular occasion to admire the articulations, as mira biles commissuras, et ad stabilitatem aptas, et ad artus finiendos accommodates, et ad mo turn et ad omnem corporis actionem.* It will be observed that the definition here given of articulation is of the most compre hensive nature. In most instances, in man, two parts articulated together are joined by their solid portions, which are never in immediate apposition with each other, but have some elastic structure interposed which may or may not form a bond of union ; and it is obvious that the fact of the intervening substance being, or not being also a bond of union will greatly in fluence the extent of motion of which the joint is capable. Before inquiring into the variety
of forms of joints, we shall first examine briefly the various structures which enter into their composition, and which essentially contribute to the perfection of their mechanism.
These parts may be enumerated as follows, and we propose to observe the same order in treating of them : 1. Bone. 2. Cartilage. 3. Fibro-cartilage. 4. Ligament. 5 Synovial membrane.
1. Bone. — The osseous or an analogous structure constitutes the fundamental portion of an articulation in all the vertebrated animals, in the mollusca, and in some of the articulated classes. In the human subject and all ver tebrated animals we find that certain parts of the bones have surfaces marked upon them in correspondence with similar surfaces on others with which they are connected, or that, as in the long bones, the extremities are expanded or enlarged, and present sur faces which are adapted to similar surfaces on contiguous bones. In this way are formed the articular portions of the bones, and we observe that these portions present considerable varieties in their characters according to the nature of the articulation which they con tribute to form. In fact, in examining these articular portions of the bones we cannot fail to notice the diversity of their form, so that some are adapted to each other in such a manner as evidently to favour motion, and others are so framed as to limit and restrict it. The articular sur!lices in dry bones are ge nerally characterised by a peculiar smooth ness, indicative of the existence on them of a cartilaginous incrustation in the recent con dition. The expansion of the extremities of the long bones on which the articular surfaces are formed is to be attributed to the accu mulation there of a considerable quantity of the reticular texture, covered by a thin lamina of compact tissue, whereby a large surface is obtained without the inconvenient increase of weight which would necessarily result did that portion of the -bone contain compact tissue to any extent. In the neigh bourhood of the articular portions of the bones we find certain eminences, depressions, or rough nesses, which indicate the points of attach ment of those bonds of union by which the joints are secured and strengthened. In ge neral it may be observed that the long bones are articulated with each other by joints which possess a considerable extent of motion; the flat bones, again, have articulations very limited in their mobility, and this is likewise the case with the irregular bones.