Skeletal Structures. 17 



deposition of bone lamellae is, as described above, that the transverse 

 diameter of the bone is greatly increased. The enlargement of the 

 marrow cavity, with which this is associated, is produced by the absorp- 

 tion of bone from the interior. 



In young animals both the epiphysial centres and the masses of cartil- 

 age in which they are formed are sharply marked off from the body of the 

 bone (cf. Figs. 12-13). This is largely because the formation of the 

 epiphysial centres tends to lag behind that of the main centres, and thus 

 the cartilage extremities of the bones are evident long after the forma- 

 tion of the shaft is under way. In the epiphysial centres the bone 

 formation is endochondral. The bone masses which they form are distin- 

 guished as epiphyses. During the period of growth they are connected 

 with the body of the bone by plates of epiphysial cartilage, into which the 

 surrounding perichondrium extends as an ossification ridge. In this 

 region bone formation takes place, with the result that the whole 

 structure is greatly increased in .length. 



After the period of growth, the duration of which differs in different 

 bones, the epiphyses become firmly coossified with the body of the bone, 

 although the lines of junction or epiphysial lines may still be visible. 

 Thus in the distal extremities of the radius 

 and ulna, in the proximal extremities of ? °' 



the fibula, or in the bodies of the lumbar 

 vertebrae, the epiphysial lines appear even 



in old animals. In the foregoing figure jM$t '-t >u \ \ - c ^- 

 (9) of the divided femur it will be 

 seen that the position of the epiphysial 



lines is indicated by bands of compact ^ ffl[ f.m. ^ ||.....ex.D.. 

 tissue. Finally, in thoroughly macerated 

 bones of young animals, the epiphyses are 

 usually found to be readily separable from 

 the bones. 



In a comparison of the adult skeleton FlG u The occipital portion rf the 



With the more primitive embryonic skull in a three-day-old rabbit: b.o., 



skeleton, several differences in the arrange- cartfiagfrf 1 chondrocra°ni'um° CCiP c'o a ! 

 ment of the elements are evident. Thus occipital condyle; f.m foramen 



magnum; ex.o., exoccipital; s.o., 



many bones, notwithstanding their pos- supraorbital. 

 session of several centres of ossification, 



are to be looked upon as individual either in the cartilage or in the 

 bone condition. In other cases, as in the basal portion of the skull, 

 separate bone elements are produced in a mass of cartilage primarily 

 continuous. These either remain distinct throughout life, or, as in the 

 occipital region, (Fig. 11), become fused together to form compound or 

 composite bones. In still other cases, as in the vertebrae, the apparently 

 single elements of the adult condition are the products not only of 

 originally distinct bones, but also of primarily separate cartilage masses. 

 The bones of the skeleton are united or articulated with one another 

 by connective tissue in the form of ligaments, by cartilage, or in some 

 cases by both together, i.e., by fibrocartilage. Ligamentous union, 

 distinguished as syndesmosis, is the most general type of articulation. 



