I02 HUMAN ANATOMY. 



ter of the shaft as well as in thickening of the osseous wall separating the medullary 

 cavity from the surface. In order, therefore, to maintain the balance between the 

 longitudinal growth of the medullary cavity, effected by the absorption of the endo- 

 chondral bone, and its lateral expansion, the removal of the innermost portions of 

 the subperiosteal bone soon becomes necessary. Absorption of the older internal 

 trabeculae thus accompanies the deposition of new osseous tissue at the periphery ; 

 by this combination of destructive and formative processes the thickness of the 

 cylindrical wall of the compact substance of the diaphysis is kept within the proper 

 limits and the increased diameter of the medullary cavity insured. 



Throughout the period of early growth the increase in length of the bone is 

 due to the addition of new cartilage at the ends ; later, the cartilaginous increments, 

 contributed by the chondrogenetic layer of the perichondrium, are supplemented by 

 interstitial expansion following the multiplication of the existing cartilage-cells. On 

 attaining the maximum growth and the completion of epiphyseal ossification, a por- 

 tion of the cartilage may persist to form the articular surfaces. After the cessation 

 of peripheral growth and the completion of the investing layer of compact substance, 

 the osteogenetic layer of the periosteum becomes more condensed and less rich in 

 cellular elements, retaining, however, an intimate connection with the last-formed 

 subjacent bone by means of the vascular processes of its tissue, which are in con- 

 tinuity with the marrow-tissue within the intraosseous canals. In addition to being 

 the most important structure for the nutrition of the bone, on account of the blood- 

 vessels which it supports, the periosteum responds to demands for the production of 

 new osseous tissue, whether for renewed growth or repair, and again becomes active 

 as a bone-forming tissue, its elements assuming the role of osteoblasts in imitation 

 of their predecessors. 



