Fig. 64. 



294 DISEASES OF THE BONES AND ARTICULATIONS. 



union of the fractured ends of a bone are very similar to those of 

 wounds, either by first intention (primum intentionem) or by sec- 

 ond intention (secundum intentionem). In simple fractures we 

 generally get union by first intention, and in compound fractures, 

 unless the union be extremely small, we get union by second in- 

 tention. (Fig. 64.) In both cases the union is accomplished by 



means of a callus growing around the 

 ends of the bone — that is to say, a 

 soft cellular tissue which forms an 

 envelope surrounding the bone and 

 gradually becoming hard through the 

 ossific action of the periosteum and 

 the marrow of the bones. The ring- 

 shaped or external callus surrounds 

 the fractured parts. This cellular 

 tissue is formed of osteoblasts. The 

 inner callus is formed by the marrow, 

 forming a peculiar plug-shaped body 

 and filling up the open ends. The 

 periosteum is the true factor in mak- 

 ing union between broken ends of 

 bones. This is especially noticed in 

 fractures where the periosteum is ex- 

 posed, and where that envelope is 

 torn or injured union is almost twice 

 as long as where the periosteum is preserved. The extravasation 

 of blood found in the early stages of a fracture which lies in the 

 surrounding parts does not in any way assist in the actual union, 

 but helps to a certain extent in holding the bones together until the 

 callus is formed. The callus beginning is a spongy mass, especially 

 in bones containing a large quantity of marrow. This gradually 

 changes into a bony cicatrix or callosity. This becomes thinner and 

 denser, lessening in diameter, and finally becomes smooth on its 

 surface, forming what is known as " final callus." Eeabsorption 

 commences at the same time until the bony masses, which are use- 

 less after the bone is united, finally disappear, but there is always 

 a certain amount of enlargement around the fractured ends of a 

 hone at the point of union. Cracks and fissures undergo the same 

 process. 



Diagram of union of fracture in the 

 tibia of the dog : a, outer callus ; b, 

 periosteum ; c, inner callus ; d, in- 

 flammatory deposit. 



