210 



TEXT-BOOK OF EMBRYOLOGY. 



it may form a number of distinct and separate pieces, as in the vertebral column, 

 the pieces being united by a certain amount of the undifferentiated embryonic 

 connective tissue. 



Synarthrosis. Syndesmosis. — When ossification begins at one or more 

 centers, either in cartilage or in embryonic connective tissue, the centers grad- 

 ually enlarge and approach each other, and the bone so formed comes in contact 

 with the bone formed in neighboring centers, (a) In a case where more than one 

 center appears for any single adult bone, they may come in contact and fuse so 

 completely that the line of fusion becomes indistinguishable, (b) In the case of 



Talus (astragalus) 



Phalanx 



<K Epiphyses of 



phalanges 



Fig. 189. — Skeleton of right foot of a boy 3 years old, showing ossification centers. Toldt, 



adjacent bones the fusion may not be so complete; that is, the two bones may 

 simply articulate, leaving a visible line of junction or suture. Such joints are 

 immovable and are represented in the sutures of the skull. 



Synchondrosis. — In some cases a small amount of embryonic connective 

 tissue remains between adjacent bones, (a) In time, this embryonic connective 

 tissue gives rise to cartilage which unites the bones quite firmly, thus producing 

 a practically immovable joint, as in the case of the sacro-iliac joint, (b) Or the 

 cells in the center of the cartilage disintegrate or become liquefied so that a small 

 cavity is produced {articular cavity) . This type of joint makes possible a slight 

 degree of mobility and is exemplified by the symphysis of the pubic bones. Such 

 a type is also represented by the joints of the vertebral column. In place of 

 cavities, however, are the pulpy nuclei which are remnants of the notochord. 



