84 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



1. Hemorrhage and clot. The fibrin of the clot 

 tends to hold the broken ends in apposition. The 

 parts are swollen and red, due to the influx of blood. 



2. Organization of the clot. Connective-tissue 

 cells and white corpuscles enter the clot, feed upon 

 it and ultimately replace it, the connective- tissue 

 cells meanwhile producing fibers. The organized 

 clot is a more substantial fabric and more firmly 

 holds the broken ends in apposition. , 



3. Osteogenetic cells enter the organized clot and 

 deposit lime salts, producing a primary callus. The 

 connective fibers shrink, pulling the broken ends 

 firmly together, producing a sensation known as 

 knitting of bone. The primary callus surrounds the 

 bone, and may even fill the marrow cavity. 



4. Haversian systems are formed, uniting the 

 broken ends. These systems appear just as de- 

 scribed under development of bone. 



5. Primary callus is absorbed and marrow cavity 

 excavated. Bone cells called osteoclasts are sup- 

 posed to be active factors in this absorption. 



General Considerations. Bone does not grow in 

 the same sense as other tissues do. Any increase in 

 size is due to apposition of bone lamellae upon those 

 already formed. Accompanying and often pre- 

 ceding bone production we usually find a destructive 

 or excavating process. It is believed two classes of 

 cells bring about these changes: (a) Osteoclasts that 

 cause bone absorption, and (6) osteoblasts that en- 

 gage in bone production. The latter are supposed 

 to be particularly abundant in the osteogenetic 

 layer of the periosteum. 



