90 



HUMAN ANATOMY. 



osteum contains an additional stratum, the osteogenetic layer, which lies closely asso- 

 ciated with the exterior of the bone. After growth has ceased, the osteogenetic 

 layer becomes reduced to an inconspicuous stratum included as part of the fibro- 

 elastic constituent of the periosteum. 



The fibrous layer is composed of closely placed bundles of fibrous connective 

 tissue, and serves to support larger blood-vessels which break up within the deeper 

 parts of the periosteum into the minute twigs entering the canals opening onto the 

 surface of the bone. 



Fig. 1 20. 





^»**- 



'^^-- 





Sharpey's 

 fibres 



Lacuna 



Sharpey's 

 fibres 



Oblique section of decalcified tibia, showing fibrous character of lamellae and groups of Sharpey's fibres. X 420. 



Fig. 121. 



Ci- 



ec 



V-^v. 



^\X 





The fidro-elash'c layer consists of a rich felt-work of elastic fibres, often arranged 

 as several distinct strata ; the elastic tissue is separated from the surface of the bone 

 by a layer of fibrous tissue comparatively rich in flat, plate-like connective-tissue 

 cells, the remains of the elements of the osteogenetic layer. The inner surface of 

 the periosteum is intimately attached to the osseous tissue by means of delicate 

 processes of connective tissue which accompany the blood-vessels into the nutrient 

 canals ; this relation persists from the continuity of the formative tissue of the young 



periosteum with the early marrow-tissue. 

 Between the fibrous bundles next the bone 

 numerous cleft-like lymph-spaces exist ; 

 these are imperfectly lined by the endothe- 

 lioid connective-tissue cells and communi- 

 cate with the lymph-channels within the 

 bone. 



The osteoge7ietic layer, conspicuous 

 during the development and growth of the 

 osseous tissue, consists of delicate bundles 

 of fibrous tissue and large numbers of 

 connective-tissue cells of an embryonal 

 type. Those next the growing bone as- 

 sume a low, irregular columnar form, and 

 are disposed in rows upon the surface of 

 the developing osseous tissue ; since these 

 cells are concerned in the production of the 

 latter, they are appropriately termed osteoblasts. Later some of them become sur- 

 rounded by the bony matrix, and are thus transformed into bone-cells. The osteo- 

 genetic layer is rich in blood-vessels which, as the bone is formed, are continued into 

 the primary marrow-cavities. 



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

 medullary cavities surrounded by the compact substance forming the shaft of the 

 long bones or the irregular interstices between the trabeculae composing the cancel- 



^V 



Bone-cells lying within the lacunae. X 700. 





