STRUCTURE OF THE VERTEBRATES 173 



fusion between the two, for when complete fusion occurs the 

 growth stops. In the human tliis takes place, usually, between 

 fifteen and eighteen years, taller individuals continuing to grow 

 for a longer time than the short ones. Other animals may con- 

 tinue growth throughout life. 



There is also a growth in diameter. This takes place in the 

 periosteum, which lays down concentric layers of bony tissue, 

 the outer layer being the younger. And as the bone grows out- 

 ward, osteoclasts destroy the bone at the center, with the result 

 that the inner marrow cavity becomes larger. 



The growth and structure of bone, however, are not so simple 

 as the above description. A cross section of a long bone will 

 show that the outer layers are concentric, extending entirely 

 around the shaft, but the major portion is composed of micro- 

 scopic groups of concentric .layers, each built around a canal in 

 which lie an artery and vein. These small canals are the 

 Haversian canals, and the canal and the concentric rings sur- 

 rounding it form a Haversian system. As the osteoclasts eat 

 out the peripheral layers, blood vessels with osteoblasts migrate 

 in and lay down a long tube of bone. Other, smaller, layers are 

 formed within the original one, until a new Haversian system is 

 formed. In this case the inner ring is the youngest. Old systems 

 are constantly being destroyed and new ones built. This can be 

 demonstrated experimentally by feeding young animals a stain 

 which colors the bone which is being laid down at the time of 

 feeding. By alternating the stained rings with unstained, the 

 order of destruction and rebuilding can be determined. 



After the fusion of the epiphyses there is little further growth 

 in size, the changes being internal. However, in pathological con- 

 ditions bone may grow or be destroyed. A broken bone releases 

 bone cells and growth proceeds rapidly. Diseases affecting the 

 pituitary gland, which lies in the sella turcica of the skull, will 

 cause bone growth later in life. This usually affects the jaw and 

 the cartilaginous joints of the fingers. 



Destruction is shown in old age by the thinning of the skull 

 bones, and the degeneration of the angles of the jaw bones. The 

 jaw of an old man is usually small and thin. 



The size of the bones is influenced by glandular secretions, 

 and these organs are also a factor in determining height. Proper 



