chap, vii.] BONE. 57 



dering the primary marrow cavities. The osteoblasts 

 form bone substance, and as this proceeds, the calcified 

 cartilage trabeculce become gradually ensheathed and 

 covered with a layer of osseous substance, the osseous 

 matrix and branched bone corpuscles. Thus the 

 original cartilage gradually assumes the appear- 

 ance of a spongy substance, in which the cavities 

 (primary marrow cavities) are filled with the primary 

 marrow, and are of considerable size, while the 

 trabeculse bordering them are calcified cartilage 

 covered with layers of new bone. The marrow cells, 

 or osteoblasts, continue to deposit bone substance on 

 the free surface of the trabeculse, while the calcified 

 cartilage in the centre of the trabeculse gradually 

 becomes absorbed. 



70. The nearer the centre of ossification, the 

 more advanced is the process, i.e., the more bone the 

 less calcified cartilage is found in the trabeculse, 

 and the thicker the latter. At the " centre of ossifi- 

 cation," i.e., whence it started, the process is farther 

 advanced ; away from it, it is in an earlier stage. 

 At this period of embryo life, between the centre of 

 ossification and a point nearer to the extremity of 

 the shaft of a long bone, all stages described above 

 may be met with, viz., between the solid unaltered 

 hyaline cartilage at the end of the shaft, and the 

 spongy bone with the un absorbed remains of calcified 

 cartilage in the middle of the shaft, all intermediate 

 stages occur (Fig. 34). 



71. After birth, and as long as bone grows, we 

 find in the end of the shaft, and also in the epi- 

 physis, a continuation of the process of endochondral 

 formation described above. In fact, all bones pre- 

 formed in the embryo as cartilage grow in length 

 before and after birth by eiidochondral formation 

 of new bone. The hyaline cartilage at their ex- 

 tremities (intermediate or ossifying cartilage) is the 



