100 STUDIES IN ADVANCED PHYSIOLOGY. 



ally removed piecemeal, and replaced by a new growth of 

 bone extending into it from beneath. To state it again, it 

 is probably the migration of the Haversian systems of the 

 deeper portions of the bone into the outer layers, accomr 

 plished no doubt by a piecemeal absorption of these outer 

 layers and the re-deposition of new bone, showing the 

 Haversian system. It is possible that this change from the 

 stratified appearance of the layers under the periosteum to 

 the normal Haversian systems is to be accounted for on the 

 grounds that the latter system is better adapted for the 

 nourishment of the bone, and that possibly for physical 

 reasons the columnar arrangement of the Haversian systems 

 is stronger than that of the stratified lamellae. Future 

 investigation, however, must solve for us a number of ques- 

 tions still unanswered in this matter. 



While the bones do not grow, as we ordinarily use that 

 term, after the twenty-fifth year, they may change their 

 shapes to some extent ; for throughout the greater part of 

 life there is an absorption of bone in some places where it is 

 no longer needed, and a continued deposition in other places 

 where added strength is desired. When bones are mechan- 

 ically broken they are united by having osteoclasts first 

 remove the dead bone from the broken ends, and then hav- 

 ing the osteoblasts cement the pieces together with new 

 bone, which may in some cases even be preceded by a 

 formation of cartilage. This removal of the dead bone is 

 probably identical in process with the removal of the car- 

 tilage and bone regularly occurring in their growth. 



Such bone absorption occurs in the milk dentation and 

 also explains the occasional loosening of the permanent set 

 of teeth, owing to the removal of the cement, which is really 

 bone. 



While in the long bones there is usually a single center 

 of ossification, near the middle of the shaft, with later ones 

 to form the epiphyses, they may be much more numerous 

 in irregular bones. Thus in the irregular sphenoid bone 



