NORMAL HISTOLOGY. 



Imprisoned osteo- 



blast becoming a 



bone-cell 



Osteob'as:s 



Bone-cell 



FIG. 58. Portion of developing osseous 

 trabecula and osteoblasts. X 350. 



the newly formed bone as its cells (Fig. 58). The bone-cells are, therefore, 

 imprisoned osteoblasts, which, in turn, are specialized connective tissue ele- 

 ments. The bone-cells occupy minute lenticular or stellate spaces, the 



primary lacuna, at this stage the canaliculi 

 being still unformed. The bone-matrix is 

 at first devoid of calcareous material and 

 is, therefore, soft; very soon, however, the 

 deposit of lime-salts begins and the young 

 bone becomes hard. The increase of the 

 new bone is attended by the gradual disap- 

 pearance of the enclosed calcified cartilage- 

 matrix, the last traces of which, however, 

 persist for some time as irregular deepjy 

 stained patches within the osseous trabec- 

 ulae, at some distance from the zone of 

 calcification (Fig. 57). Many of the newly 

 formed bony trabeculae soon undergo ab- 

 sorption, with corresponding enlargement 

 of the intervening marrow-spaces. The 

 remaining trabeculae increase in thickness 

 by the addition of new lamellae on the surface covered by the osteoblasts 

 and join to form a trabecular network, the primary central spongy bone. 

 In the irregular bones, the primary spongy bone is represented by the 

 cancellated tissue forming the internal framework. v In the long bones, the 

 primary spongy bone undergoes further absorption within the middle of the 

 shaft, simultaneously with its continued development within the cartilage at 

 the ends of the shaft, or diaphyses. As the result of this absorption, a large 

 space, the central mar row -cavity, is formed, the growth of which keeps 

 pace with the general expansion of the bone. So long as a long bone 

 increases in length, new cartilage is added at the ends of the shaft, to be 

 replaced in its turn by the ad- 

 vancing osseous tissue. 



The absorption of the 

 newly formed bone is effected 

 through the agency of large 

 polynucleated cells, the osteo- 

 clasts. These are specialized 

 marrow-elements whose partic- 

 ular role is the breaking down 

 and absorption of the bone- 

 matrix. They are very large 

 (50-100 ii) and lie, singly or in 

 groups, close to the surface of 

 the bone within depressions, 

 the so-called Howship 1 s lacuna, 

 produced by the erosion of the 

 osseous matrix (Fig. 59). The 

 only part of the central spongy 

 bone which persists after the 

 completed development and 

 growth of the long bones, is that constituting the cancellated tissue within 

 their ends. It will be seen, therefore, that in many cases the product of 

 bone-formation within the cartilage, the primary central spongy bone, is to a 







Osteoclast 



Bone-cell 



within 



lacuna 



Howship's 

 lacuna 



Osteoblasts 



FIG. 59. Portion of trabecula of spongy bone undergoing 

 absorption by osteoclast. X 450. 



