DEVELOPMENT OF BONE 73 



connective tissue, which distinguish it from a structure that has 

 merely been calcined, as may happen in certain disease processes. 



PERIOSTEUM 



The surface of bone is covered by an envelope called periosteum r 

 which has two layers a dense, outer, fibrous layer and a looser, 

 inner, vascular layer. The inner layer contains the cells, osteoblasts, 

 which form osseous tissue, hence this layer is called the osteogenetic 

 layer of the periosteum. In operating on bone, surgeons guard the 

 periosteum very carefully, on account of its blood-vessels and its- 

 osteogenetic elements. 



MARROW 



The spaces of bones are everywhere filled with marrow. The 

 largest of these spaces are the medullary cavities of long bones. ID 

 the medullary cavities the marrow is yellow, owing to the deposit 

 of fat in the cells. The smaller spaces of bone are filled with red 

 marrow. Red marrow contains marrow -cells, which are similar to 

 connective tissue cells, supported in a very vascular connective 

 tissue framework. The marrow cells are identical with the osteo- 

 blasts of the periosteum and with bone-corpuscles. Immense cells, 

 giant cells, containing many nuclei, also occur in red marrow. On 

 account of their function of absorbing superfluous bone, they are 

 called osteoclasts (Fig. 50). Red marrow also contains nucleated 

 cells, tinged with hemoglobin, that are connected with the forma- 

 tion of red blood -corpuscles, which probably takes place exten- 

 sively in the red marrow. 



DEVELOPMENT OF BONE 



The majority of the bones of the body are first formed in the 

 embryo as hyaline cartilage, which is subsequently replaced by true 

 bone endochondral ossification . 



'The bones of the face and of the vault of the cranium and a 

 portion of the lower jaw are the principal exceptions intramem- 

 branous ossification. In the latter case, the basis of the forming 

 bone is an embryonic fibrous tissue, and this form of ossification 

 differs from the endochondral in not being carried on in a carti- 

 laginous basis, which is, however, a temporary structure. 



