DEVELOPMENT OF THE ENDOSKELETON 695 



/. Formation of Joints 



1) Definitions. The word arthrosis is derived from a Greek word meaning 

 a joint. In vertebrate anatomy, it refers to the point of contact or union of 

 two bones. When the contact between two bones results in a condition where 

 the bones actually fuse together to form one complete bone, the condition 

 is called ankylosis or synosteosis. If, however, the point of contact is such 

 that the bones form an immovable union, it is called a synarthrosis; if slightly 

 movable, it forms an amphiarthrosis; and where the contact permits free mo- 

 bility, it is known as a diarthrosis. Various degrees of rapprochement between 

 bones, therefore, are possible. 



2) Ankylosis (Synosteosis) and Synarthrosis. In the development of the 

 bones of the vertebrate skull, two types of bone contact are effected: 



( 1 ) ankylosis and 



(2) synarthrosis. 



In the human frontal bone, for example, two bilaterally placed centers of 

 ossification arise in the connective-tissue membrane, lying below the skin in 

 the future forehead area. These two centers increase in size and spread 

 peripherally until two frontal bony areas are produced, which are separated 

 in the median plane at birth. Later on in the first year following birth, the 

 two bones become sutured (i.e., form a synarthrosis) in the midsagittal plane. 

 Beginning in the second year and extending on into the eighth year, the 

 suture becomes displaced by actual fusion of bone, and ankylosis occurs. 

 In the cat, however, the two frontal bones remain in the sutured condition 

 (synarthrosis). The temporal bone in the human and other mammals is a 

 complex bone, arising by the ultimate fusion (ankylosis) of several bones. 

 In the human at birth, three separate bones are evident in the temporal bone: 



(1) a squamous portion, 



(2) a petrous portion, and 



(3) a tympanic part. 



The squamous and the tympanic bones are of membranous origin, whereas 

 the petrous portion arises through the ossification of the cartilaginous otic 

 capsule. The fusion of these three bones occurs during the first year follow- 

 ing birth. The occipital bone is another bone of complex origin. Five centers 

 of ossification are involved, viz., a basioccipital, two exoccipitals, a squamous 

 inferior, and a squamous superior. The last arises as a membrane bone; 

 the others are endochondral. Ultimate fusion of these entities occurs during 

 the early years of childhood and is completed generally by the fourth to 

 sixth years. In the cat, the squamous superior remains distinct as the inter- 

 parietal bone. Finally, the sphenoid bone in the human represents a con- 

 dition derived from many centers of ossification. According to Bardeen, 

 '10, fourteen centers of ossification arise in the sphenoidal area, ten of them 



