696 THE SKELETAL SYSTEM 



arising in the orbitotemporal region of the primitive chondrocranium. At 

 birth, two major portions of the sphenoid bone are present, the presphenoid 

 and the basisphenoid, being separated by a wedge of cartilage. Ultimate 

 fusion of these two sphenoid bones occurs late in childhood (Bardeen, '10). 

 In the adult cat, they remain distinct. The maxillary bone in the human 

 arises as a premaxillary and a maxillary portion; later these bones fuse to 

 form the adult maxilla. In the cat, on the other hand, these two bones re- 

 main distinct. (Consult also Table 3.) 



The history of the human skull, therefore, is one of gradual fusion (anky- 

 losis) of bones. In many parts, however, fusion does not occur, and definite 

 sutures (synarthroses) are established between the bones, as in the case of 

 the two parietals, the parietal and the occipital, the frontal and the parietals, etc. 



The formation of the association between the parietal bones and neigh- 

 boring bones establishes an interesting developmental phenomenon, known 

 as the fontanels. The fontanels are wide, membranous areas between the de- 

 veloping parietal and surrounding bones which, at birth, are not ossified. 

 These membranous areas are the anterior fontanel, in the midline between 

 the two parietals and two frontal bones, and the posterior fontanel, between 

 the parietals and the occipital bones. The lateral fontanels are located along 

 the latero-ventral edges of the parietal and neighboring bones (fig. 319E). 



3) Diarthrosis. A diarthrosis or movable joint is established at the distal 

 ends of the elongated, cylindrically shaped bones of the body. Diarthroses 

 are present typically in relation to the bones of the appendages. As the bones 

 of the appendages form, there is a condensation of the mesenchyme in the 

 immediate area of the bone to be formed. At the ends of the bone, the 

 mesenchyme is less dense than in the area where the rudimentary bone is 

 in the process of formation (fig. 324A-E). As a result, the area between 

 bones is composed of mesenchyme less compact and less dense than in the 

 areas where bone formation is initiated (fig. 324F, G). This mesenchyme at 

 the ends of the bones thus forms a delicate membrane, tying the bony rudi- 

 ments together, and, as such, forms a rudimentary synarthrosis. As develop- 

 ment proceeds, the miniature bone itself becomes more dense, and, eventually, 

 cartilage is formed. The latter later is displaced gradually by bone (fig. 324E), 

 The areas between the ends of the respective developing bones become, on 

 the contrary, less dense, and a space within the mesenchyme is developed 

 between the ends of the forming bones (fig. 324H). As this occurs, con- 

 nective tissue, continuous with the periosteum, forms around the outer edges 

 of the ends of the bones, tying the ends of the bones together (fig. 324H, I). 

 A cavity, the joint cavity, thus is formed at the ends of the bones, bounded 

 by the cartilage at the ends of the bones and peripherally by connective tissues 

 or ligaments which tie the ends of the bones together along their margins. 

 The membrane which lines the joint cavity is known as the synovial mem- 



