BONE 



117 



(<:), radiating from them in all directions. Both lacunae and 

 canaliculi commonly appear black, owing to their being filled 

 either with air or with bone-dust produced in grinding the 

 section. 



In a section of decalcified bone (Fig. 36) the marrow is 

 seen to be surrounded by lamellae of a delicate fibrous 

 substance, arranged in two layers, an outer (<5), having the 

 periosteum (p) closely investing it, and an inner (^'), in 

 contact with the marrow. In the fibrous substance of the 

 lamellae are cell-spaces, corresponding with the lacunae of the 



B 



WmKpM 



FIG. 36. Transverse section of decalcified frog's femur under a low power. B, portion 



of the same under a high power. 



b. outer, and b' . inner layer of bone ; b. c, bone-cells ; ni. marrow ; o. layer of 

 osteoblasts in connection with periosteum ; o' . layer of osteoblasts in connection 

 with marrow ; p. periosteum. (After Howes.) 



dried bone, and each containing a bone-cell (b.c\ which sends 

 off delicate branched processes of its protoplasm into the 

 canaliculi. Thus the bone, like connective-tissue and 

 cartilage, consists of cells with an intercellular substance : 

 the latter is in the form of concentric layers and is impreg- 

 nated with lime-salts. 



The long bones of the frog grow in two directions. 

 Between the periosteum and the bone is a layer of cells, the 



