DISEASES OF BONES 163 



HEALING OF FRACTURES. 



How do aseptic fractv/res heal ? 



The process of repair is closely related to the healing of 

 an aseptic skin wound. The blood -which has been effused 

 as result of the injury is presently absorbed and has nothing 

 to do with the process of repair. The periosteum and bone 

 marrow and soft, parts undergo an aseptic inflammation, 

 exudation occurs, and a mass of embryonic tissue — that is, 

 granulation tissue — is the result. This granulation tissue, at 

 first soft, subsequently ossified, glues the fragments together, 

 so to speak, and is termed the callus, and the bone once more 

 becomes a solid structure. 



What varieties of callus are concerned in the healing of 

 fractures f 



1. The external callus, developing from the bone-pro- 

 ducing cells (osteoblasts) of the deeper layers of the 

 periosteum. 



2. The internal callus, developing from the bone-pro- 

 ducing cells (osteoblasts) of the marrow. 



3. The intermediate callus. It lies between the fractured 

 ends and is composed partly from the external, partly from 

 the internal callus. 



What is a provisional callus f 



The soft, spongy tissue at first forming around and be- 

 tween the broken ends ; it is the result of the action of the 

 osteoblasts. 

 What is a permanent callus ? 



This is the provisional callus, having undergone ossifica- 

 tion. In other words, it represents a bone scar formed 

 mainly through the agency of the osteoclasts. 



