PRINCIPLES OF VETERINARY SURGERY 27 



lary cavity. The blood-vessels also take part in the process 

 and a granulation tissue very rich in cells is formed at the 

 ends of the bony fragments and gradually replaces the blood- 

 clot; in this way a provisional union takes place. This soft 

 tissue eventually becomes a fibrous mass poorly supplied 

 with nuclei and extends some distance above and below the 

 seat of fracture. It is the callus. It does not have a well-de- 

 fined outline and may involve some of the surrounding tis- 

 sues. In a few days the inflamed mass begins to take on 

 much firmer consistency and if examined during the second 

 week of repair, there is found a thick layer of cells embedded 

 in a finely striated intercellular substance somewhat like car- 

 tilage. This is the so-called osteoid tissue, especially com- 

 mon in dogs near the ends of the bones and also in the center 

 of the callus. The intercellular substance has a hyaline ap- 

 pearance and in the early period of repair consists largely of 

 cartilage, 



The cartilage cells now form a kind of hard secretion im- 

 pregnated with calcium salts which gradually surrounds 

 them and incloses their bodies in the ground substance. 

 Presently it is found that trabeculas of bone have formed, 

 and the cartilage cells, now shrunken and spindle shaped, 

 have become typical bone-cells, sending out wavy processes 

 into the bone substance. Between the trabeculse are spaces 

 containing blood vessels. This new spongy bone begins 

 forming some distance from the seat of the fracture, gradu- 

 ally growing thicker as the ends of the bones are approached. 

 As it grows out from each end of the bone, it invades the 

 cartilaginous callus and finally the two developing ends grow 

 together making a bony bridge which unites the fracture. 

 At first the whole mass of callus fills up the medullary cavity 

 and the space between the fractured ends of the bones, and 

 can be differentiated into its periosteal and myelogenic layers. 

 It produces a marked thickening of the bone from a,n exces- 



