172 STRUCTURE OF THE VERTEBRATES 



Dermal bone is limited in its distribution in the mammal, and 

 its origin in the dermal fibers of the skin has been described. 

 The student should recall that the osteoblasts deposit the bony 

 substance around fibers which radiate from definite centers of 

 ossification, growth in size being due to growth of the distal ends 

 of the fibers and peripheral deposition of bone. 



The locomotor elements of the skeleton, with a few minor ex- 

 ceptions, are replacement bone; and with the exception of the 

 dermal bones which have been mentioned, the entire embryonic 

 skeleton is laid down in cartilage which is gradually replaced 

 by bone. However, it is a matter of common observation that 

 growth continues long after embryonic life is ended, and that 

 changes in bony configuration continue throughout life. 



Bone cells are of two types: (1) osteoblasts, or bone building 

 cells which secrete the organic fibers and the inorganic salts 

 which give hardness to the bone; and (2) osteoclasts, or bone 

 destroyers. Both types are at work, even after the limit of 

 growth has been reached. The healing of a broken bone and the 

 smoothing of the jawbone after a tooth extraction are illustra- 

 tions which the student has observed. 



The processes of growth can be demonstrated in a long bone 

 of the arm. The embryonic cartilage is surrounded by a con- 

 nective tissue sheath, the periosteum (called the perichondrium 

 when surrounding cartilage). At about the middle of the shaft 

 of the bone the periosteum breaks and blood vessels migrate 

 into the cartilage, the osteoclasts destroying the cartilage tissue. 

 From this central cavity the destruction proceeds toward either 

 end of the bone, the cartilage being destroyed unevenly, so that 

 finger-like processes are left projecting into the original cavity. 

 Osteoblasts then begin secreting bone which fills the original 

 cavit}^, and also the spaces at either side. As destruction pro- 

 ceeds, the cavities are filled with bone. 



Secondary centers of ossification begin at either end of the 

 cartilage producing the epiphi/ses which form the ends of each 

 long bone. As the shaft continues to grow it soon approaches 

 the terminal epiphyses. The skeleton of a young animal shows 

 this condition, the epiphyses being separated from the shaft by 

 a thin line of cartilage. Growth takes place in this cartilage 

 area. Length of the long bone is determined by the time of 



