fcl.KUU.1. UJKJ1. 



Cartilage 

 / articularis 



whose function it is to secrete a lubricating fluid, the synovia or joint-oil ; (d~) a 

 plate of hyaline cartilage coating each of the opposing surfaces of the bones 

 concerned. All the joints belonging to this group occur in the median plane of 

 the body. It includes the symphysis pubis, the joints between the bodies of the 

 vertebras, and the joint between the manubrium sterni and the body of the sternum. 

 A diarthrosis (Fig. 291) is the most elaborate as well as the most complete form 

 of articulation. It is characterised by (a) capability of movement which is more or 

 less free in its range ; (6) a reduction of the 

 uniting structures to a series of retaining liga- 

 ments ; (c) an articular cavity which is limited 

 only by the surrounding ligaments; (d) 

 the constant presence of synovial membrane ; 

 (e) cartilage articularis (hyaline encrusting 

 cartilage) which clothes the opposed surfaces 

 of the articulating bones. The majority of the 

 joints in the adult belongs to this group. This 

 series of joints has been subdivided into a 

 number of minor sections, in order to 

 emphasise the occurrence of certain well- 

 marked structural features, or because of the 

 particular nature of the movement by which 

 they are characterised. Although in all 

 diarthroses there is a certain amount of 

 gliding movement between the opposed surfaces of the bones which enter into 

 their formation, yet, when this gliding movement becomes their prominent feature, 

 as in most of the joints of the carpus and tarsus, they are termed arthrodia. But 

 bones may be articulated together so as to permit of movement in one, two, or 

 more fixed axes of movement, or in modifications of these axes. Thus in uniaxial 

 joints the axis of movement may lie in the longitudinal axis of the joint, in 

 which case the trochoid rotatory form of joint results, as in the proximal and 

 distal radio-ulnar articulations ; or it may correspond with the transverse axis of 

 the articulation, as in the elbow-joint and knee-joint, when the gmglymus or hinge 

 iety results. If movement takes place about two principal axes situated at 

 rht angles to each other, as in the radio-carpal joint, the terms ellipsoid (biaxial 

 condyloid) are applied. Movements occurring about three principal axes placed 

 right angles to each other, or in modifications of these positions, constitute 

 mltiaxial joints, in which the associated structural peculiarities provide the 

 alternative terms of enarthrosis or ball-and-socket joints. 



Stratum 

 synoviale 



Cartilage/ 

 articularis 



FIG. 290. DIAGRAM OF A DIARTHRODIAL 

 JOINT. 



STRUCTURES WHICH ENTER INTO THE FORMATION OF JOINTS. 



The structures which enter into the formation of joints vary with the nature 

 of the articulation. In every instance there are two or more skeletal elements, 

 whether bones or cartilages, and in addition there are the uniting media, which are 

 either simple or elaborate according to the provision made for rendering the joint 

 more or less rigid, or capable of movement. We have already seen that the uniting 

 medium in synarthrodial joints is a remnant of the common matrix, whether fibro- 

 vascular membrane or hyaline cartilage, in which ossification has extended from 

 separate centres. Among the amphiarthroses there is still extensive union between 

 the opposing surfaces of the articulating bones, but the character of the uniting 

 medium has advanced from the primitive embryonic tissue to fibrous and fibro- 

 cartilaginous material, as well as hyaline cartilage. These, with very few exceptions, 

 are permanent non-ossifying substances, such as may be seen between the opposing 

 osseous surfaces of two vertebral bodies. The joint cavity, more or less rudimentary, 

 is confined to the centre of the fibro-cartilaginous plate, and may result from the 

 softening or imperfect cleavage of the central tissue. It may also present rudiments 

 of a synovial membrane. 



In the diarthrodial group the extensive cavity has produced great interruption 

 in the continuity of the uniting structures which originally existed between the 



