304 THE AKTICULATIONS OK JOINTS. 



THE DEVELOPMENT OF JOINTS. 



Just as the question of structure determines to a large extent the presence or absence 

 of movement in joints, so in tracing their development it will be found that the 

 manner of their appearance forecasts their ultimate destination as immovable or mov- 

 able articulations. 



All joints arise in mesodermic tissue which has undergone more or less differentiation. 



When this differentiation has produced a continuous membranous layer, in which 

 ossific centres representing separate skeletal segments make their appearance, we get the 

 primitive form of suture. The plane of the articulation merely indicates the limit of 

 the ossific process extending from different directions. If, again, the differentiation of 

 the mesoderm has resulted in the formation of a continuous cartilaginous layer, in which 

 ossification commences at separate centres, the plane of the articulation is marked 

 out by the unossified cartilage in other words, the articulation is a synchondrosis. Ulti- 

 mately this disappears through the extension of the process of ossification. 



To some extent sutures also disappear, although their complete obliteration is not 

 usual even in aged people. Developmentally, therefore, synarthroses or immovable 

 joints do not present any special structural element, and, speaking generally, they have 

 only a temporary existence. 



The development of all movable joints is in marked contrast to that of synar- 

 throses. Not only are they permanent arrangements so far as concerns normal conditions, 

 but they never arise merely as planes which indicate the temporary phase of an ossific 

 process. From the outset they present distinct skeletal units, from which the special 

 structures of the joint are derived. 



The primitive movable joint is first recognised as a mass of undifferentiated meso- 

 dermic cells situated between two masses, which have differentiated into primitive cartilage. 



The cell-mass which constitutes the joint- unit presents the appearance of a thick 

 cellular disc, the proximal and distal surfaces of which are in accurate apposition with the 

 primitive cartilages, while its circumference is defined from the surrounding mesoderm by 

 a somewhat closer aggregation of the cells of which the disc is composed. From this 

 cellular disc or joint-unit all the structures characteristic of amphiarthrodial and diar- 

 throdial joints are ultimately developed. 



Thus, by the transformation of the circumferential cells into fibrous tissue the invest- 

 ing ligaments are produced. Within the substance of the disc itself a transverse cleft, 

 more or less well-defined and complete, makes its appearance. In this manner the disc is 

 divided into proximal and distal segments, separated from each other by an interval 

 which is the primitive articular cavity. This cleft, however, never extends so far as to 

 interrupt the continuity of the circumferential part of the disc which develops into the 

 fibrous tissue of the investing ligaments. From the proximal and distal segments 

 of the articular disc the various structures, distinctive of movable joints, are developed. 



Thus, in amphiarthrodial joints the cellular articular disc or primitive joint-unit gives 

 origin to the following structures : From its circumference, investing ligaments ; from 

 its interior, the fibro cartilaginous plate in which an imperfect articular cavity with 

 corresponding imperfect synovial stratum may be found. 



In the case of a diarthrodial joint the changes take place on a more extended scale. 

 The articular cavity becomes a prominent feature, in relation to which the surrounding 

 fibrous structures form an investing capsule, lined with a synovial stratum. 



When a single cleft arises, but does not extend completely across the longitudinal axi 

 of the articular disc, the undivided portion develops into fibrous interarticular ligaments 

 On the other hand, when two transverse clefts are formed, that portion of the cellula 

 disc which remains between them becomes transformed into a fibro-cartilaginous dis 

 (or in the case of the knee-joint, menisci), which in its turn may either be complete o 

 incomplete, and thus we may obtain two distinct synovial joint cavities belonging to on 

 articulation. 1 



In considering the development of the synovial layer, and the surfaces on which it i 

 found in the interior of a joint, it is necessary to keep clearly in mind that a synovia 

 layer is a special structure, whose function it is to produce a lubricating fluid or synovia 

 and that, therefore, its position is determined by the essential necessity of proximit; 

 to a direct blood-supply. In other words, this condition is provided by all parts c 



1 From a series of observations upon the development of diarthrodial joints, the writer considers th; 

 there is evidence to show that the " cellular articular disc " is directly responsible for the production 

 the epiphyses which adjoin the completed articular cavity, and that, among such amphiarthroses as exi 

 between the bodies of vertebrae, not only the intervertebral fibro-cartilage, but the proximal and dist 

 epiphyses which ultimately unite with the vertebral bodies have a common origin in the joint-unit. 



