chap vii.] BONE. 55 



of varying length and thickness, and composed of 

 lamellae of bone substance. 



According to the arrangement of the trabeculse, 

 the spongy substance is a uniform honey-combed sub- 

 stance, or appears longitudinally striated, as in the 

 end of the shaft. In the latter case the marrow spaces 

 are elongated and the trabeculse more or less parallel, but 

 anastomosing with one another by transverse branches. 



68. Development of bone. Bone is developed 

 in the embryo, and continues to be formed also after 

 birth as long as bone grows, either in the cartilage, or 

 independently of this directly from the osteogenetic 

 layer of the periosteum. The former mode is called 

 endochondral, the latter periosteal, or intermembranous 

 formation. 



All the bones of the limbs and of the vertebral 

 column, the sternum, and the ribs, and the bones 

 forming the base of the skull, are preformed in the 

 early embryo as solid hyaline cartilage, covered with 

 a membrane identical in structure and function with 

 the periosteum, which at a later period it becomes. 

 The tegmental bones of the skull, the bones of the face 

 with the lower jaw, except the angle, are not pre- 

 formed at all. Only a membrane identical with the 

 future periosteum is present, and underneath and 

 from this the bone is gradually deposited. 



69. Endochondral formation. The stage 

 next to the one (1) in which we have solid hyaline 

 cartilage covered with periosteum is the following 

 (2) : Starting from the " centre, or point, of ossifica- 

 tion," and proceeding in all directions, the cartilage 

 becomes permeated by numbers of channels (cartilage 

 channels) containing prolongations (periostea! processes 

 of Virchow) of the osteogenetic layer of the periosteum, 

 i.e., vessels and osteoblasts, or marrow cells. This is 

 the stage of the vascularisation of the cartilage. In 

 the next stage (3) the cartilage bordering on these 



