272 BONE. [BOOK i. 



SECT. 4. OSSEOUS TISSUE OR BONE. 



structural T Qe nar d tissue which forms the scaffolding and. 



Elements of support of the soft parts of our bodies, although on 

 Bone. superficial examination, appearing so different from the 



other members of the group of connective tissues, possesses the closest 

 affinity to them, as is evident not merely from developmental 

 considerations but from a study of its chemical composition. 



All the bones of the skeleton are invested by a fibro-vascular 

 membrane, the periosteum, which conveys to them the great majority of 

 the blood-vessels which supply them, and which contains on its inner 

 layer certain cells osteoblasts which are the active agents in the 

 growth of bone, and in virtue of which the periosteum possesses the 

 power of forming new bone. 



Those bones which articulate with others have no periosteal 

 covering over their articular ends, which are tipped with cartilage. 



The external part of all bones has a very dense structure ; the 

 interior is either hollowed out into a cavity termed the medullary 

 cavity, or is occupied by a trellice-work of bony plates which 

 constitute the cancellated tissue. Those bones which possess a 

 medullary cavity present near their articular ends much cancellated 

 tissue. 



The medullary cavity lodges the medulla or yellow marrow, which 

 is composed of fat cells supported by a frame work of connective 

 tissue, and abundantly supplied with blood-vessels; the cancelli 

 or spaces of the cancellated tissue afford support to the so-called red 

 marrow, which is a tissue in which a large number of cells identical 

 with the colourless cells of the blood are found, besides certain cells 

 which resemble the nucleated coloured corpuscles of embryonic blood. 



It also contains large giant cells with many nuclei to which 

 the name of myeloplaxes is applied, and which are identical in 

 appearance and probably in functions with those cells which under 

 the name of osteoclasts are supposed to be the active agents in 

 the formation of the medullary cavities of growing bone. 



Though the chief blood-supply of bone is drawn from the perios- 

 teum, both arteries and veins of considerable size enter by so-called 

 ' nutritious foramina ' and are distributed to the marrow and so-called 

 endosteum, as the connective tissue lining the medullary cavity is 

 called ; some of the branches of the nutrient vessels anastomose with 

 the blood-vessels of the hard tissue. 



The blood-vessels contained in the hard substance of bone lie in 

 canals the Haversian canals. Around these canals the bony sub- 

 stance is arranged in concentric lamellae. ' In these lamellae are 

 cavities, also arranged concentrically, called lacunae, and from these 

 proceed minute canals, the canaliculi, which establish a communica- 

 tion between adjacent lacunae, and between the lacunae which are in 



