The Skeletal System 97 



cate with the marrow cavity. These special canals are easily distin- 

 guished from the Haversian canals by the fact that they contain 

 larger blood vessels and are not surrounded by concentric lamellae. 



LACUNA 

 CANALICULt 



INTEBSTIKAL 

 LAMELLA 



Fig. 28. — Ground section of compact bone. 



BONE GROWTH 



In the embryo, most bones of the skeleton are laid down as carti- 

 lage. The embryo thus has a skeleton which is a miniature of the adult 

 one. Clearly many changes in relationships of parts to one another 

 and to other structures of the body must take place as growth occurs. 

 This involves both destruction of existing materials and construction 

 of new materials. Having the bones in cartilage during the early growth 

 makes for many definite advantages. Cartilage is capable of internal 

 expansion while bone can grow only by addition of materials on the 

 outer surface. If the skeleton were formed originally in bone, articula- 

 tions and muscle attachments could not retain their correct relationships. 



In the embryonic cartilaginous bone, the regions of the adult struc- 

 ture may be clearly seen. The first change that is evident in the replace- 

 ment of cartilage by bone is in the center of the shaft or diaphysis. 

 The cells in this area enlarge and the substance between them becomes 

 calcified. Some of these calcified cells disintegrate and a cavity appears. 

 Blood vessels break in and enter this newly formed cavity which then 

 becomes filled with embryonic bone marrow. Some of the cells in this 

 marrow change to become bone-forming cells or osteoblasts, which sur- 

 round the remaining cartilage with layers of bone. This process gradu- 



