248 Regeneration. 



hard like ivory and of the compactness of cortical bone, the osteo- 

 blasts in the provisional callus are gradually enclosed in the calci- 

 fying matrix and shrink into the spindle-shaped and stellate form of 

 bone corpuscles ; the newly formed vessels with their contained blood 

 persist in the calcifying osteoplastic tissue, giving an analogy to 

 the svstem of the Haversian canals of bone. There is, however, no 

 lamellated structure in the tissue. After the osseous scar has become 

 hard and the bone is again strong enough to sustain weight and 

 muscular strain, a reduction of the excessive callus sets in, aplana- 

 tion of the callus; the external bulging of the bone diminishes; the 

 periosteum becomes closely adherent; the medullary callus becomes 

 porous, the medullary canal opens and the medullary substance be- 

 comes full of fat. These changes in the callus are brought about 

 by the growth of blood vessels into it from the marrow, and by 

 the influence of multinucleated giant cells of the medulla known as 

 osteoclasts ( KXacnye^v , to break) because of their agency in breaking 

 down osseous tissue (Rindfleisch ). The shrinkage is explained by 

 the biological law that lack of use causes structures to become 

 atrophic, only functionating parts being maintained. Hence in 

 conformity with this rule only those portions of the bony tissue 

 which are required to sustain weight or to oppose strain persist as 

 compact structures. 



It is well known tliat the texture of any bone shows an arrange- 

 ment of its osseous trabecula conforming to the mechanical forces 

 acting upon it; this functional adaptation is operative as well as all 

 pathological conditions which occasion new mechanical features. In 

 the regenerated bone tissue the resumption of function not only 

 assures the maintenance of the osseous scar, but in addition de- 

 termines the precise arrangement of its osseous trabecula by fur- 

 ther thickening of those portions which are most heavily taxed : the 

 direction of the trabecular adjustment develops in correspondence 

 with the lines of stress determined by their functional relations in 

 supporting the body weight and opposing muscular strain. 



Regeneration of Nerves. — Peripheral nerves severed from their 

 connection with the central nervous system by section, contusion 

 or other destructive lesion, degenerate from the point of the lesion 

 centrifugally to their peripheral end fibres, their axis-cylinders and 

 medullary sheaths being destroyed (broken down into myelin and 

 fat droplets) ; but the neurolemma and its nuclei remain intact 

 (Nasse, Waller, Lent, Benecke and others). The degeneration 

 may also extend centrally in the divided nerve for the distance of 

 one segment (to the first node of Ranvier). Conductivity ceases 



