THE CONNECTIVE TISSUES. 



15 



canals) bearing nutrient vessels, etc. Arranged in layers con- 

 centric to these canals or parallel to the surface of the bone are 

 the cells, each occupying 

 a space (lacuna) in the yi^ 



dense matrix. These 

 cells are connected by 

 fine, branching, proto- 

 plasmic processes, which 

 run in minute tubules 

 (canaliculi) through the 

 layers (lamellae) of the 

 matrix. Both cartilage 

 and bone are enveloped 

 in a layer of fibrous con- 

 nective tissue, called re- 

 spectively perichondrium 

 and periosteum. 



Many bones, as has 

 just been said, pass 

 through a cartilage stage 

 in their history, the gen- 

 eral outlines being built up in that more yielding substance. 

 Later the matrix is dissolved little by little, and is replaced by 



the lime salts, the 

 cells (osteoblasts) 

 becoming enclosed 

 in the hardened sub- 

 stance. Such bones 

 are called cartilage 

 bones. Other bones, 

 however, have no 

 cartilage stage, but 

 arise from the calci- 

 fication of the inter- 

 cellular substance of 

 membranes, and 

 these are called membrane bones. In either case the process 

 of ossification proceeds from fixed spots (centres of ossification) 



f 7^ uXrr^/^ 



Fig. 17. Bone. A, piece of a long bone 

 showing the appearance under low power ia 

 longitudinal and cross sections ; B, a transverse- 

 section of three lamellae surrounding an Haver- 

 sian canal, from a slice of dried bone; c, bone- 

 corpuscles ; en, canaliculi ; h, Haversian canal %, 

 I, lamellae. 



Fig. 18. Development of membrane bone 

 (mandible of pig). Around the (black) bone are 

 numerous osteoblasts, some of which are included in 

 the bony substance. 



