DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 19 



diminishing evidence of debris and by the extensive scar-formation, but in some 

 areas it is still proceeding actively, as attested by the remaining tissue-waste, 

 engaged by the persisting hordes of active phagocytes. The enlarging callus shows 

 an increase in number of the trypanophilic reticulum cells and some heightening 

 in their staining activity, especially in the older spaces. 



NINTH-DAY STAGE. (S 17-2). 



In the cleared ribs of the ninth day (fig. 4) no diffuse staining of the ends of 

 bone, or the surrounding injured tissues, can be made out. Hence it may be inferred 

 that the clearing away of debris and exudate is at an end. As would be expected, 

 this process is completed sooner in a small bone like the rib than in a larger bone, 

 such as the femur, where the tissue damage is, of course, much more severe. 



The investment of trypanophil cells about the fracture-area in the cleared 

 ribs is still quite obvious (fig. 4), though less conspicuous than in the earlier stages. 

 As will be seen from later specimens, these cells persist at the site of their former 

 labors in the fracture-area for a few days after tinctorial and histological evidence 

 of damaged tissue and exudate has disappeared. They gradually dwindle in num- 

 bers and vital-staining ability. 



The edges of the new callus are much more sharply defined and are distinctly 

 seen throughout. In addition, the callus is considerably denser and is reticular 

 in structure, an appearance which may be interpreted as evidence of extensive 

 ossification. In the rib from which the figure was made this reticular structure is 

 complete throughout, but in the others it is incomplete between the bone ends. 

 Apparently, movement has delayed ossification in these cases. 



In the cleared skidl of the ninth day the bony insert of the left side slightly 

 overlaps one of the edges (fig. 6). This insert consisted of sterile dead bone from 

 another rat. On the right side the space is unfilled. This opening crosses the mid- 

 line and occupies some of the territory of the left parietal bone. A little bluish 

 debris is seen lying about. The edges of the bone are slightly blue. Clouds of 

 macrophages infiltrate the membrane filling the open spaces, as shown in the 

 drawing. The clear spaces in this represent areas of extensive scarring; here the 

 macrophages are much less conspicuous. Some unstained ossified callus radiates 

 from the edges of the openings. 



The sections of this stage are not good. 



The principal features to be noted on the ninth day are the gradual subsidence 

 of the extraosseous macrophages and the occupation of their fields by the scar 

 tissue. In the callus increasing density is evident. 



TENTH-DAY STAGE. (S5-1), (S 6-1), (S6-2). 



In the cleared skull of the tenth day (S5-1) a local excess of macrophages is 

 still evident, as well as a little stained debris. In the sections of this specimen the 

 diffuse staining is found to be due to the presence of a small abscess, the result of 

 an infection. The exudate has taken a diffuse blue color. Macrophages are found 

 in numbers in the periphery of this abscess. Some appear degenerate. Dyestuff 

 was not found in the polymorphonuclear leucocytes of the abscess. 



