98 The Vertebrate Organ Systems 



ally results in producing a marrow cavity surrounded by bony tissue. 

 During growth, much bony tissue is provisional, and is resorbed and 

 replaced. The whole rearrangement is a very complex but orderly 

 process. As the cartilage is replaced in the central region of the bone, 

 it grows toward the ends with the calcification process continuously re- 

 placing it with bone. Thus one process follows the other toward the 

 ends of the bone. The cartilage is not wholly replaced until the adult 

 structure is completed. Even then, portions of hyaline cartilage are 

 present at the articulating ends of the bones. 



EPIPHYSIS 

 GROWTH DISC 



DIAPHVSIS. 



Fig. 29. — Diagrams showing the development of a long bone. Compact bone, 

 black; spongy bone, fine stipple; marrow, coarse stipple; cartilage, white. (Modi- 

 fled from various sources.) 



Even in embryonic bone, it is possible to see that at each end of 

 the shaft or diaphysis is another region, the epiphysis, which is sepa- 

 rated from the diaphysis by a ring of cartilage. Each epiphysis develops 

 its own ossification center at the same time that the diaphysis is under- 

 going its change to bone. Often the epiphysis has its cartilage com- 

 pletely replaced before the diaphysis does. It then becomes an im- 

 portant center for muscle attachment and articulation with other bones. 



Between the epiphysis and each diaphysis, the band of cartilage is 

 the zone in which growth takes place. It is here that the cartilage con- 

 tinues to grow and to be replaced by bone from the epipl'tyses and 

 diaphysis. This band permits the bone to grow in length until the 



