50 THE CONNECTIVE TISSUES [CH. V. 



their formation is preceded by a considerable amount of absorption. 

 To carry our simile further, the osteoblasts are not satisfied with the 

 rough constructions that they were first able to make, but having 

 exterminated the cartilage, they destroy (again through the agency 

 of the regiment of giant osteoclasts) their first work, and build regular 

 lamellae, leaving lacunae for the accommodation of those who desire to 

 retire from active warfare. 



About this time, too, the marrow cavity is formed by the absorp- 

 tion of the bony tissue that originally occupied the centre of the 

 shaft. Here the osteoclasts have again to do the work, and, with this 

 final act of destruction, all remains of any calcified cartilage of the 

 foetal bone entirely disappear. 



The formation of a so-called cartilage bone is thus, after all, a 

 formation of bone by subperiosteal tissue, just as it is in the so-called 

 membrane bone. 



After a time the cartilage at the ends of the shaft begins to ossify 

 independently, and the epiphyses are formed. They are not joined 

 on to the shaft till late in life, so that growth of the bone in length 

 can continue till union takes place. 



Bone grows in width by the deposition of layers under the perios- 

 teum, like successive rings formed under the bark of a growing tree. 

 This was shown long before the histological details which we have 

 described were made out by Sharpey. Silver rings were placed by 

 Duhamel around the bones of young pigeons. When killed later, the 

 rings were completely covered in by bone ; and in the animals killed 

 last, were even found in the central cavity. Another series of experi- 

 ments was performed upon pigs. The young animals were fed 

 alternately on ordinary food and food dyed by the red pigment 

 madder. The new bony tissue acts like what dyers called a 

 " mordant " : it fixes the dye, and the rings of bone deposited during 

 the madder periods were distinctly red in colour. 



The importance of the periosteum in bone formation has always 

 been recognised by surgeons. When removing a piece of bone they 

 are careful, if possible, to leave the periosteum behind : this leads to 

 regeneration of the lost bone. If it is absolutely necessary to remove 

 the periosteum, successful cases have occurred in which the living 

 periosteum from an animal has effectively been transplanted. 

 MacEwen has recently shown that minute fragments of living bone 

 are even more efficacious in virtue of the bone-cells they contain. 



The Teeth. 



During the course of his life, man, in common with most other 

 mammals, is provided with two sets of teeth ; the first set, called the 

 temporary or milk teeth, makes its appearance in infancy, and is in 



