108 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



by the intervertebral fibrocartilages, and by the connection of the two pubic bones by the inter- 

 pubic fibrocartilage. 



We occasionally find a combination of a synarthrosis (particularly the syndesmosis) with 

 a joint, as in the connection of the sacrum with the innominate bone, or articular spaces may 

 appear within the symphyses, and such arrangements are termed half- joints (amphiarthroses). 



DIARTHROSES. 



The diarthroses or true joints are characterized by congruent cartilaginous surfaces which 

 are separated from each other" by a capillary space, and are provided with a number of struc- 

 tures, the majority of which are absent in the synarthroses. These are the articular capsules, 

 the accessory ligaments, and the articular cartilages. They may also be provided with special 

 structures, such as the interarticular cartilages or menisci, diverticula of the synovial membrane 

 forming synovial bursa, glenoidal lips, joint cushions (designated by different names in the 

 different joints), and bony locking mechanisms. 



The articular capsule usually surrounds the cartilaginous articular surfaces of the con- 

 tiguous bones so^as to form a completely closed articular cavity. It consists of an external firm 

 fibrous layer, the fib rous lay er, and of an internal softer vascular layer, the synovial layer or 

 membrane. The latter gives off microscopic thread-like vascular processes of irregular shape, the 

 synovial villi, and occasionally it forms large structures visible to the naked eye, the synovial folds. 



The articular cavity, usually a capillary space, is filled with a viscid fluid, the synovia. 



The articular capsule varies in thickness; it is sometimes strengthened by accessory liga- 

 ments and sometimes interrupted, so that diverticula of the synovial membrane protrude through 

 its openings and form synovial bursa (see also page 143), whose cavities are consequently 

 directly continuous with that of the articulation (communicating synovial bursa?). 



Accessory ligaments form important components of the articulations, and according to. 

 function the ligaments of the body can be subdivided into the following classes: (1) Ligaments 

 0} fixation; i. e., those ligaments which firmly unite two bones, as in the syndesmoses. (2) Re- 

 inforcing ligaments of the joint capsule; these are more or less adherent to the capsule itself, 

 but sometimes appear as independent structures. (3) Check ligaments, i. e., ligaments which 

 are capable of limiting certain movements of the joint. (4) Ligaments of conduction, i. e., liga- 

 ments whose function is to conduct vessels and nerves to a part of a bone. (5) Ligaments which 

 take the place of bone. These last are found in those situations where neither fixation, nor 

 inhibition, nor any other of the usual functions of a ligament is required; they extend between 

 portions of the same bone or convert a notch into a foramen. 



The interarticular cartilages or articular discs (termed menisci when of a purely fibrous 

 character) serve to adapt mutually articular surfaces which are not completely congruent. They 

 are attached to the inner surface of the joint capsule and extend for a varying distance between 

 the cartilaginous ends of the bones. In the most extreme cases they divide the articular cavity 

 into two portions, so that the articular extremity of one bone is in relation to one side of the 

 articular disc and that of the other bone to the other side. In such a joint the articular 

 extremities of trie two bones are not in immediate contact with each other, and it is consequently 

 possible to distinguish unilocular and bilocular joints. 



