150 THE ARTICULATIONS IN GENERAL. 



angles to the shafts of the two bones forming the articulation, and longitudinal in the pivot- 

 joint. The condyloid and saddle-joints are biaxial, movement being permitted around two 

 primary horizontal axes at right angles to one another, and necessarily also around any inter- 

 mediate axis ; while in the ball and socket joint a third vertical axis is added, so that 

 movement may take place in any direction about a point in the centre of the spherical head 

 in which the three orthogonal axes intersect each other. 



There are, moreover, certain special cases of articulations which cannot strictly be in- 

 cluded in any of the foregoing groups, for example, the humero-radial articulation and 

 the knee-joint, in both of which movements proper to a hinge- joint and a pivot- joint are 

 permitted. 



In most diarthrodial joints the opposed articular surfaces correspond in shape 

 and curvature, whence they are said to be congruent ; in some cases, however, the 

 cartilage-covered surfaces are incongruent, differing markedly in form. But even 

 among the .articulations with so-called congruent surfaces there are frequently slight 

 differences of curvature, the convex surface for example being a curve of shorter 

 radius than the concave surface, so that the opposed surfaces are never in contact 

 over their whole extent. In such cases the area of contact is slightly increased 

 under the influence of pressure owing to the yielding of the articular cartilage, and 

 the small intervals left are occupied by synovial fluid and by folds of the synovial 

 membrane which project from the periphery of the articulation. In the more 

 marked degrees of incongruence larger folds containing fat fill the angular spaces 

 left by the separation of the articular surfaces in different positions of the joint ; 

 and similar adipose folds or pads are often found within the capsule, correspond- 

 ing to hollows formed by the bones in the neighbourhood of the articular surfaces. 

 Lastly, incongruence of articular surfaces is also compensated in some instances 

 by the interposition of an inter articular plate or meniscus of nbro-cartilage (fibrc- 

 plate Cleland), which may be imperfect, having a crescentic or annular form, or 

 may constitute a disc entirely separating the articular surfaces, thus giving rise to 

 a compound joint, with two distinct synovial cavities (fig. 173, B). 



Development of articulations. Synarthrosis is the primary form of articulation, the 

 adjacent skeletal elements at their first appearance being everywhere connected by indifferent 

 embryonic tissue, and diarthrosis arises by the formation of a cleft in this intermediate 

 substance. Synchondrosis results from the appearance and growth of distinct ossifications in 

 a continuous cartilaginous mass, and suture in a similar way from the approximation of bones 

 developed in a membranous blastema. Synostosis takes place when the ossification extends 

 through the thin intervening layer of the original matrix. In symphysis and syndesmosis 

 the connecting tissue undergoes conversion into the fibro-cartilaginous disc or interosseou-s 

 ligament respectively. In the compound form of diarthrosis the interposed tissue becomes 

 converted into the meniscus, on each side of which a cleft appears. Transitional forms of 

 articulation between synarthrosis and diarthrosis also occur, an imperfect joint-cavity being 

 frequently developed by absorption of the central portion of the disc, in the symphysis pubis 

 for example, and according to Luschka regularly in the intervertebral discs. (For details as 

 to the histological changes in the development of the joints, as well as of the microscopic 

 structure of their constituent parts, reference must be made to the sections on Embryology 

 and Histology in Vol. I.) 



Morphology of ligaments. The capsular ligaments investing diarthrodial articulations 

 may be looked upon as representing a prolongation of the periosteum from one bone to 

 another. The articular ligaments to which special names are given are for the most part 

 portions of the capsule characterized by variations in thickness and the development of 

 definite tracts of fibres. But in addition to these there are bands of different origin occurring 

 either in the neighbourhood of joints, or uniting more distant portions of bone. Such bands 

 may be derived from muscles, in the form of expansions from, or more or less separated 

 portions of the tendons, or as parts of the muscular sheets which have undergone fibrous 

 change in consequence of alteration or loss of function ; or they may represent skeletal parts 

 which, although indicated in the fretus, do not attain full development, as seen in the stylo- 

 hyoid ligament and the internal lateral ligament of the lower jaw ; or they may be specialized 

 portions of fascia, as the stylo-maxillary ligament, and the supraspinous and interspinous 

 ligaments of the vertebras. The annular ligaments of the wrist and ankle, as well as various 



