Bone as a Skeletal Structure

Bone as structure and mineral reserve 405 ties of the bones become the site of haematopoietic tissue, and in the higher vertebrates bone is known to act as a storehouse of mineral salts. In its finer structure, too, bone shows a progressive evolutionary change, although here the account is far from complete. In the earliest vertebrates the bone is acellular. In the primitive bony fishes the bone is cellular, although the arrangement of the lacunae is very irregular, and the osteocytes do not long survive. In the advanced bony fishes the trend has been once more toward the acellular condition, but terrestrial vertebrates show a trend toward greater regularity, longer survival of the osteocytes, and the appearance of nutrient canals and secondary bone arranged in Haversian systems. These trends reach their culmination in the bone tissue of modern mammals. There is, however, some reason to believe that some of the most ancient fish-like vertebrates had also a very regular type of cellular arrangement. Knowledge is lacking of the histology of bone in many extinct vertebrate groups, but the fact that bone structure is known to vary so greatly between the different groups studied suggests that this knowledge would be well worth having. Even less is known about the reasons underlying this variation. The problem of correlating the evolution of bane structure with that of bone function remains one for the future.