STRUCT UBE OF A FLAT BONE 



239 



The shaft, or body, of these bones is not cylindrical, but 

 flattened like a blade. It is this character of the shaft 

 that gives these bones their name. The vertebral end 



Fig. 96 — llibs 



of a rib shows two cartilage-covered surfaces or articula- 

 tions that mark the points where the rib is jointed to the 

 dorsal vertebrae. The rest of the bone is covered with 

 periosteum hke the long bone. 



The internal structure is best examined by sawing the 

 bone crosswise at different points. The structure appears 

 to be tuiiform throughout. The entire interior is filled 

 with the same spongy bone tissue that was present in the 

 heads of the long bone, while the outer walls are composed of 

 layers of hard bone. All flat bones show these layers 

 of periosteum, hard bone, and spongy bone. In the case of 

 the ribs, the spongy layer is usually filled with marrow, 

 but the quantity is necessarily small as compared with 

 that found in the long bones. 



The flat bones of the skull and pelvis form excellent 

 supports and protection to the inclosed parts; while the 

 flatness and thinness of the ribs, and their flexibiUty, 

 permit the covering and movement of the organs of the 

 thorax without making the chest heavy and bulky. 



