vn BOXE 117 



cavities, the lacuna (Ic), with delicate, branching tubes, 

 the canaliculi (c), 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 (b), having 

 the periosteum (p) closely investing it, and an inner (b') 

 in contact with the marrow. In the fibrous substance of 



FIG. 36. A, transverse section of decalcified Frog's femur under a low power. 



( < 30.) B, portion of the same under a high power. ( x about 150.) 

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

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

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



the lamellae are cell-spaces, corresponding with the 

 lacunae of the 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 impregnated 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 osteoblasts (o), by which new lamellae of bone are 



