STRUCTURE OF JOINTS. 



109 



independent of the influence of motility or of muscular action, the articular ends of 

 the bones assume definite shapes such as will allow the motion peculiar to that joint, 

 and (barring the frequent want of perfect coaptation) no other. The common char- 

 acteristics of true joints are articular surfaces covered by hyaline cartilage, so 

 shaped as to determine the nature of the movement, enclosed by a capsule lined with 

 a syyiovial membrane. The articular surfaces are not necessarily formed wholly of 

 bone, since very often increased concavity is secured by the addition of a lip of tibro- 

 cartilage to the margin of the bone ; in other cases ligaments coated with carti- 

 lage complete a socket ; or again, disks of fibro-cartilage loosely attached to the 

 periphery may project into a joint and partially subdivide it, following one bone in 

 certain movements and the other in others. 



Co^npound joints result from the persistence and differentiation of a portion of 

 the tissue uniting the ends of the embryonal bones into a partition which, in the 

 complete compound joint, separates the two synovial cavities developed, one on 

 either side of the septum. The tissue between the bones becomes a fibro-cartilagi- 

 nous disk,' which partially or completely subdivides the cavity. In such a joint, 

 when typical, there are two ends of bone covered with articular cartilage, separated 



Fig. 135. 



B 



Diagrams illustrating formation of joints. A, bones are united by young connective tissue; B, appearance ot 

 joint-cavity; C, differentiation of joint-cavity and capsule; D, development of two joint-cavities separated by 

 fibrous septum, resulting in a compound joint. 



by a fibro-cartilaginous disk or meniscus, and two distinct synovial membranes. The 

 movements are, however, still determined to a considerable extent by the shape of 

 the bones, so that these articulations may be classed as true joints. The fibro- 

 cartilaginous meniscus may be replaced by a row of bones as in the wrist. 



Structure of True Joints. — The opposed ends of the bones, and sometimes 

 other tissues, are coated with hyaline articjilar cartilage, which gives a greater 

 smoothness to the articulating surfaces than is found on the macerated bones. 

 Though following in the main the bony contours, the cartilage does not do so accu- 

 rately ; details are found on the cartilage that are obscure on the bones. It dimin- 

 ishes the force of shocks. Although, as already stated, the shape of the articular ends 

 determines the nature of the motion, it is important to recognize that, as in the case 

 of saddle-joints, the opposed surfaces are not so accurately in apposition that irreg- 

 ular movements cannot and do not occur. Failure to appreciate this fact has given 

 rise to much difficulty in accounting for motions that undoubtedly take place, but 

 which, according to the mathematical conception of the joint, are impossible. 

 Further, the range of individual variation is great ; just as a man may have a long 

 or a short head, so any of the articular ends of his bones may depart considerably 

 from the average proportions. It is ev^ possible in some of the smaller joints that 



' Discus artKularis. 



