MUTILATION AND REGENERATION 399 



venules are formed to meet this requirement. Capil- 

 laries are formed as filamentous offshoots from pre-exist- 

 ing capillaries. These increase in size and gradually 

 come to consist of several endothelial cells which become 

 channeled. Arterioles and venules are formed by en- 

 largement of capillaries whose walls become supported 

 by fibrillar and muscular tissues that extend over them 

 from the larger vessels. Such new vessels may be per- 

 manent or may be of temporary use only and sub- 

 sequently disappear through the pressure exerted upon 

 them by the contracting fibrillar tissue as the repair 

 becomes more and more perfect. 



4. Bone. In all animals fractured bones are per- 

 fectly repaired in uncomplicated cases. As, however, 

 the osseous tissue is inelastic, it is essential that the 

 member to which the bone belongs shall be kept abso- 

 lutely quiet, else instead of a bony union, only a fibrous 

 union will take place and a false joint or pseudarthrosis 

 be formed. In the process of repair, the osteoblasts 

 derived from the periosteum or surrounding membrane 

 are the formative cells. They first elaborate a tem- 

 porary or provisional tissue of a nondescript character, 

 known as callus. It much resembles the hyaline carti- 

 lage with centres of ossification seen in embryonal bone 

 formation, and as it calcifies is, like it, without Haver- 

 sian systems and not distinctly laminated. This tissue 

 is the crude material upon which the bone cells subse- 

 quently work as the callus is reconstructed and rear- 

 ranged so as to bring about complete continuity of the 

 injured bone, after which the surplus is removed. The 

 provisional callus surrounds the ends of the broken bone 

 with a spindle-shaped mass of tissue which acts the part 

 of a splint until the true or definitive callus which forms 

 the permanent bond of union is formed, after which it 

 is absorbed. 



The union of the bones and the restoration of function 

 usually requires but a few weeks, but the final removal 

 of the redundant callus and the restoration of the symme- 

 try of the bone is not perfected for years. 



