THE SKELETAL SYSTEM 



47 



U'. TTia. 



rrw. 



Fig. 24. Outline sketch of the 

 proximal end of the femur of a 

 young animal: cf., principal epi- 

 physis for the head of the femur. 

 The accessory epiphyses are for 

 the great (tr.ma.), lesser (tr.mi.), 

 and third (tr.t.) trochanters. 



blasts lying directly in the perichondrium, or later the periosteum, 



a process of formation of intramem- 

 branous bone goes on, continuing to 

 the end of the period of growth, and 

 the result of this peripheral deposition 

 of bone lamellae is, that the trans- 

 verse diameter of the bone is greatly 

 increased. The enlargement of the 

 marrow-cavity, with which this is 

 associated, is produced by the ab- 

 sorption of bone from the interior. 



In young animals both the epi- 

 physial centres and the masses of 

 cartilage in which they are formed 

 are sharply marked off from the body of the bone (cf. Fig. 26). 

 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 formation of the shaft is under 

 way. In the epiphysial centres the bone 

 formation is endochondral. The bone 

 masses which they form are distinguished 

 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 ex- 

 tends 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 co-ossified with the 

 body of the bone, although the lines of junction or epiphysial lines 

 may still be visible. Thus in theMistal 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 figure 12, which represents a divided femur, it may 

 be seen that the position of the epiphysial lines is indicated by bands 



Fig. 25. The occipital portion 

 of the skull in a three-day- 

 old rabbit: bo, basi-occipital 

 bone; ch, occipital portion^ of 

 chondrocranium; co, occipital 

 condyle; eo, exoccipital; fm, 

 foramen magnum; so, supra- 

 occipital. 



