12 THE VERTEBRATE SKELETON. 



perichondrium osteoblasts are developed which also begin to 

 give rise to spongy bone. The perichondrium thus becomes 

 the periosteum, and the bone produced by it, is periosteal or 

 membrane bone. So that while a continuous marrow cavity is 

 gradually being formed in the centre of the shaft, the layer of 

 periosteal bone round the margin is gradually thickening, and 

 becoming more and more compact by the narrowing down of its 

 cavities to the size of Haversian canals. The absorption of 

 endochondral and formation of periosteal bone goes on, till 

 in time it comes about that the whole of the shaft, except 

 its terminations, is of periosteal origin. At the extremities 

 of the shaft, however, and at the epiphyses, each of which 

 is for a long time separated from the shaft by a pad of 

 cartilage, the ossification is mainly endochondral, the peri- 

 osteal bone being represented only by a thin layer. 



Until the adult condition is reached and growth ceases, the 

 pad of cartilage between the epiphysis and the shaft continues 

 to grow, its outer (epiphysial) half growing by the formation of 

 fresh cartilage as fast as its inner half is encroached on by the 

 growth of bone from the shaft. The terminal or articular 

 surfaces of the bone remain throughout life covered by layers 

 of articular cartilage. 



Even after the adult condition is reached the bone is 

 subject to continual change, processes of absorption and fresh 

 formation going on for a time and tending to render the bone 

 more compact. 



METHODS IN WHICH BONES ARE UNITED TO ONE ANOTHER. 



The various bones composing the endoskeleton are united 

 to one another either by sutures or by movable joints. 



When two bones are suturally united, their edges fit 

 closely together and often interlock, being also bound together 

 by the periosteum. 



In many cases this sutural union passes into fusion or 

 ankylosis, ossification extending completely from one bone to 



