ARTICULATION, the term by which anatomists express the union of the different bones of the skeleton. The junction of any two bones, however firmly or loosely connected, or in whatever mode the union may be effected, is designated by the name of Articulation. Commonly two substances are employed as the media by which the connection is established, namely, a firm and strong membranous tissue termed ligament [LinAmEsT], which may be considered as the band by which the bones aro tied together, and a peculiar substance termed cartilage or gristle [CARTILAGE], which is often interposed between the surfaces of the bones to be united, and which besides serving as the bond of union, accomplishes other purposes.
Of all the parts of the animal fabric, there is none in which mechanism ie more clearly or beautifully shown than in the connections of the bones with each other, and more especially in the structure of joints.
The objects to be obtained in the economy by the union of the several bones of the body are various and even opposite, requiring almost every conceivable variety in the mode of their connection. And such variety actually exists ; but still these varieties admit of classification, and they may all be arranged under three heads, namely, those which form Immoveable, Moveable, and Mixed Articulations.
I. One object to be accomplished by the union of bones is, to form a secure situation for tender and delicate structures. Accordingly the bones are often so dispceed as to inclose cavities in which the organs that need protection are placed ; such, for example, is the cavity of the head which incloses the delicate substance of the brain ; the cavity of the spinal column, which iucloees the no less delicate substance called the spinal marrow ; and the cavities of the chest and abdomen, which inclose soft and tender organs, on the security of which life depends. Bones forming cavities of this class are generally so firmly united that they admit either of no motion whatever, or only of a very slight degree of it, the union being effected sometimes by the .apposition of the surfaces of strong and that lamest ; at other times by the formation of numerous prominences and depressions which mutually receive each other : examples of both these modes of union are found in the articulation of the bones of the head and face. The firmness of the union is sometimes increased by alternate indentations and projections, like the teeth of a saw, formed on the surfaces of bones, the surface of the ono bone being precisely adapted to that of the other; by this mechanism the bones become firmly impacted, and deficiency in extent of contact is compensated by what may be truly called (and it is an admirable example) dove tailing. Suture is the term given to this mode of union, and the bones
of the cranium are nicely adjusted and firmly united to each other in this manner. At other times a ridge is formed in one bone which is received into a groove fissured in another. The bony part of the septum which divides the nostrils affords a specimen of this mode of union, while the teeth are secured in their sockets (that is, a conical surface is firmly impacted in a cavity) very much as a nail is fixed in a board.
2. The Moveable Articulations are those in which the bones are in contact, but not continuous with each other ; such, for example, is the union of the arm with the shoulder, the fore arm with the arm, the wrist with the hand, the lower jaw with the head, the head with the trunk, and so on. In these cases the articulating surfaces are mutually adapted to each other, in general one being convex and the other concave, and the bones are maintained in their situation by the firm and strong membranes termed ligaments. Sometimes the union is assisted by the muscles which surround the joint, as is strikingly exemplified in the shoulder-joint, in which the head of the humerus is kept in contact with the cavity which receives it, partly without doubt by ligamentous substance, but partly also by the surrounding muscles. This is proved by the effect of disease; for if by paralysis, or any other cause, the neighbouring muscles become very much weakened, dislocation of the joint readily takes place. Both the strength of the joint and the range of its motion depend mainly on the extent of its articulating surface, and on the arrangement of the ligamentous substance by which the bones are held in their situations. The extent of contact, and the strength and adjustment of the uniting band, are different in every different joint, the diversity being regulated in every case by the kind and degree of motion which it is intended that the joint should exercise.
3. The Mixed form of Articulation resembles the Immoveable, in having the bones connected by an intermediate substance (cartilage), and the Moveable in admitting some degree of motion between .the surfaces. The articulations between the several bones that form the spinal column afford examples of this mode of union. There are numerous modifications of these several kinds of articulation, which are described with great minuteness in anatomical books, and most of which are distinguished by specific names.