DEVELOPMENT OF JOINTS 



213 



(B) Heteromorphic. 



(a) Enarthrodial. Ball-and-socket, allowing the most free movement, e. g., hip- 



and shoulder-joints. 

 (6) Condylarthroses. The convex surface is ellipsoidal, and fits into a corresponding 



concavity, e. g., wrist and metacarpo-phalangeal joints, 

 (c) Giriglymi. One surface consists of two conjoined condyles or of a segment of 

 a cone or cylinder, and the opposite surface has a reciprocal contour. In these 

 joints movement is only permitted round one axis, which may be transverse; 

 e. g., elbow, ankle; or it may be vertical, in which case the joint is trochoid; 

 . e. g., odontoid process of axis with atlas, radius with ulna. 

 Such a classification should be considered as being purely academic and the student must 

 always remember that it is not enough to discuss a joint by assigning it to a particular class in 

 any scheme; for he must be familiar with the actual conditions present in everj^ joint. No 

 classification, however perfect, must be taken as final, and each joint should be studied as a 

 separate thing altogether apart from any general systematic arrangement. 



DEVELOPMENT AND MORPHOLOGY OF JOINTS 



The arrangement of the various parts which constitute an articulation is best appreciated 

 by a study of the development of the various types of joints. In this way it is easy to recog- 

 nise a primitive condition typical of each class; but it must be remembered that various modi- 

 fications take place during growth, that these modifications vary in the individual joints, and 

 produce adult departures from the primitive arrangement which are peculiar to each joint and 

 which must be studied separately. 



In the case of bones ossifying in membrane the articulation will be a suture, the ossifications 

 from neighbouring centres extending until they practically come into contact. 



Fig. 255. — Development of Joints 



A. Stage in which primarj^ embryonic tissue separates the developing cartilages. 



B. Primarjr embryonic tissue transformed into cartilage (synchondrosis), or fibrous connec- 

 tive tissue (sjmdesmosis). 



Cv^egeneration of embryonic tissue with production of a joint cavity (diarthrosis). 



i 



im- 





m 



'-^ 



iT^ 1th cartilage bones the articulation may be either a syndesmosis, a synchondrosis, or a 

 oiartnrosis. The embryonic tissue in which the cartilage is to develop is at first continuous; 

 centres ot chondrification, corresponding in number to the bony elements which are destined to 

 De lormed, appearing in it. As the chondrifications approach each other a small portion of the 

 ty^^7 embryomc tissue persists between them (fig. 255), and it is the subsequent fate of 

 tms intermediate tissue that determines the nature of the articulation. 



(1) When the ossification of the cartilage occurs to form the articulating bones, the inter- 

 meoiate tissue may undergo transformation into cartilage (fig. 255), a synchondrosis being 

 inus produced. (2) Or the intermediate tissue may be converted into fibrous connective-tissue 



