THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



179 



deposit layers of calcium salts on the surface of the cartilage in the same 

 manner as around the trabeculae inside the cartilage. 



The transformation of the spongy bone into compact bone is peculiar 

 in that the former is dissolved and then replaced by new bone. This 

 dissolution is brought about by the action of the osteoclasts large mul- 

 tinuclear cells the origin of which is not known. By the process of dis- 

 solution the marrow spaces are increased in size and are known as Haver- 

 sian spaces. Within these spaces new bone is then deposited layer upon layer, 

 under the influence of the osteoblasts, until the Haversian spaces are reduced 

 to narrow channels, the Haversian canals. The layers of bone are the Haver- 

 sian lamellce. The interstitial lamella in compact bone have two possible 

 origins. They may be the remnants of certain lamellae of the original spongy 



Blood vessel 



Bone 



Cartilage 



Bone cell 



Cartilage cell 



Cartilage cell space 



Osteoblasts 



FIG. 156. From same section as Fig. 153; showing bone deposited around one of the 

 trabeculae of cartilage. (Intracartilaginous ossification.) 



bone which were not removed in the enlargement of the primary marrow spaces, 

 or they may be parts of early formed Haversian lamellae which were later more 

 or less replaced by other Haversian lamellae. 



The fact should be emphasized that although it is convenient to describe 

 three types of bone formation, the three do not differ essentially from one 

 another. The similarity of intramembranous and subperiosteal ossification has 

 already been noted (p. 176). In both these types the bone is developed within 

 a membrane of embryonic connective tissue by a transformation of this tissue 

 into osteogenetic tissue and then of the latter into bone. The only way in 

 which intracartilaginous bone formation differs from the other two types is 

 that cartilage is first formed within the membrane in the same general shape as 

 the future bone. But it must be remembered that it is only in this cartilage that 

 bone is developed and not from it, the bone being produced by osteogenetic 



