6 HUMAN ANATOMY. 



connecting the lacunas with each other and with the Haversian 

 canals. Each canaliculus contains an artery, vein, and lym- 

 phatic (Schaffer). In this manner every part of the osseous 

 substance communicates. 



Development. The development of bone is effected in two 

 ways (1) from cartilage, the intra-cartilaginous ; and (2) 

 from membrane, the intra-membranous ; the former occurring 

 at the base of the skull for the protection of vital centres, or 

 in the extremities to secure rigidity of the parts. 



In the intra-cartilaginous the parts are first formed in 

 cartilage and then converted into bone. The process is as fol- 

 lows: The cartilage cells at the "centre of ossification" become 

 enlarged and arranged in rows. The cartilaginous matrix also 

 increases and separates the cells. Lime salts are deposited be- 

 tween the rows of cells, inclosing them in oblong spaces called 

 primary areolce. 



Blood-vessels from the deep or osteogenic layer of the 

 periosteum, carrying numerous osteob lasts (bone-forming cells),, 

 and osteoclasts (bone-absorbing cells), pass into the area of 

 calcified cartilage (centre of ossification). The osteoblasts re- 

 place the calcified cartilage, which is absorbed, with new bone, 

 which latter is absorbed in part (tunneled) by the osteoclasts to 

 form the medullary spaces or marrow cavity. 



The walls of the spaces are gradually thickened by suc- 

 cessive layers of osteoblasts, forming lamella? of bone, till noth- 

 ing remains but a channel the Haversian canal. 



The perichondrium having been in the meantime converted 

 into periosteum, the osteogenic, or vascular, layer of this mem- 

 brane furnishes a layer of osteoblasts that form enveloping, cir- 

 cumferential layers of bone. 



Thus, from the osteoblasts all the structures of bone are 

 produced; the remains of a group of cells constitute an Haver- 

 sian canal, the layers deposited by them and the adjacent cells, 

 the lamella?, and the isolated, persistent cells (osteoblasts) form 

 the contents of the lacunas, or "true bone-cells," with their nu- 

 trient canaliculi diverging from them. 



In the intra-membranous form the parts are first formed in 

 membrane, and from one or more centres of ossification lime 

 salts are deposited in radiating spiculas, or osteogenic fibres, in- 

 closing the osteoblasts. 



From these fibres the process spreads, and vessels from the 

 neighboring parts pass into it and form Haversian canals. 



The formation of the lamella? and the lacunas is essentially 

 the same as in the intra-cartilaginous, and in most bones both 

 processes go on simultaneously. 



