THE PERIOSTEUM. 



39 



The fibrous layer is composed of bundles of fibrous tissue and sup- 

 ports the larger blood-vessels which, within the deeper parts of the perios- 

 teum, break up into twigs that enter the surface of the bone through the 

 Volkmann canals. The fibro -elastic layer includes a feltwork of elastic 

 fibres and delicate strands of fibrous tissue. The inner surface of the perios- 

 teum is attached to the underlying bone by processes of connective tissue 

 which accompany the blood-vessels into the superficial canals. This relation 



Last formed lamella 

 of bone 



Dense fibrous layer 



Periostea! blood-ves- 

 sel passing into the 

 bone 



Bone-cell within 

 lacuna 



//? if MI ir'.i ; 



w 



: 



--.'<;; \\ raiiV tin 



Remains of osteoge- 

 netic layer 





FIG. 54. Section of young periosteum and subjacent bone. X 275. 



persists from the continuity of the formative tissue of the young periosteum 

 with the early marrow- tissue. 



The osteogenetic layer, during development and growth of the bone, 

 consists of delicate bundles of fibrous tissue and large numbers of round or 

 fusiform connective tissue cells of an embryonal type. Those next the 

 growing bone are of irregular cuboid form and disposed in a single row upon 

 the surface of the developing osseous tissue. Since these cells are directly 

 concerned in producing the new bone, they are termed osteoblasts. Later 

 some of them become imprisoned within the bone-matrix and transformed 

 into bone-cells. After completion of its active role, the osteogenetic layer 

 becomes greatly reduced and inconspicuous, in the adult periosteum being 

 represented by flattened cells, which no longer form a continuous stratum but 

 occur as scattered groups. 



In addition to its important bone-producing function, the periosteum 

 serves as the immediate means by which muscles, tendons, ligaments and 

 fasciae gain attachment to the skeleton. In every case the union is effected 

 by the fusion and blending of the connective tissue of the attached structure 

 with the outer layer of the periosteum. 



Bone-Marrow. The spaces in the interior of bones, whether the large 

 medullary cavities surrounded by the compact substance forming the tubular 



