22 MAMMALIAN AXATOMY 



cartilaginous layer is active in the formation of new tissue, while 

 in other synchondroses, so far as known, its function is entirely 

 passive. 



2. In a Syndesmosis 1 the uniting substance is fibrous tissue. 

 Genuine syndesmoses are relatively infrequent ; examples may be 

 seen, however, in the way in which the costal cartilages of the small 

 caudal ribs are united, and in the union of borders of adjacent bones 

 by sheets of interosseous membrane. 



3. In the Sutures whereby the thin bones of the skull are joined 

 together, the uniting tissue, fibrous or cartilaginous, is reduced to 

 such an extent that the edges of the bones appear to be in actual 

 contact and immovable. If the edges of the bones are notched so 

 as to interlock, the union is a dentate suture ; if the edges are bevelled 

 and overlap, the union is a squamous suture (Fig. 7, b). Many of the 

 sutures disappear in adult life or old age ; they therefore resemble 

 the epiphyseal synchondroses in permitting, for a time, growth of 

 the bones along their edges. 



4. In a Synsarco'sis'" 1 the bones are joined by muscular tissue. 

 An example of this rare condition is found, in the cat, in the union 

 of the rudimentary clavicle to the sternum and the scapula. 



Synchondroses and syndesmoses may be converted into absolutely 

 immovable unions, or Synosto'ses, 3 by ossification of the intervening 

 tissue. By the same process the sutures of the skull become oblit- 

 erated in old age. 



Diarthroses, or complete joints, are of different forms and the 

 seats of different kinds of motion : some are simple and permit 

 gliding motion ; some are hinge-joints with angular motion ; others 

 permit rotation ; or as ball-and-socket joints have a free motion, known 

 as circumduction. 



Whatever be the form or function of a diarthrosis, it possesses 

 certain general characters, namely : 



(a) The apposing surfaces of the united bones are covered with 

 a layer of articular cartilage (Fig. 8). This layer of cartilage is 

 rarely of uniform thickness : it may deepen an articular cavity by being- 

 thicker round its edge, or it may render an articular elevation more 



1 From (GJr.) syndesmos, a ligament; syn, together, and dein, to bind. 



2 From (Grr.) syn, together, and sarkoun, to make flesh. 



3 From (Gr.) syn, together, and osteon, bone. 



