40 DEVELOPMENT AND FUNCTION OP MACROPHAGES IN BONE-REPAIR. 



blasts of moribund tissue, are bathed by fluid containing colloidal waste products. 

 Especially in the perisinusoidal spaces (where, as has been many times observed, 

 the phagocytes are often thickly crowded) are they well situated to gather in waste 

 products; for here not only do they have access to the materials coming directly 

 from the dissolving tissues, but, owing to the thinness of the vessel-walls and the 

 slowness of the circulation, diffusion of katabolites from the sinuses into the spaces 

 may easily occur, so that these macrophages are in a position to gain some of their 

 pabulum from the blood-stream. In any event, toxic materials in the blood- 

 sinuses are readily extracted by the phagocytic endothelial cells. 



The enormous thin-walled blood-sinuses of the callus, which have been repeat- 

 edly referred to, are a conspicuous feature of areas of active bone-demolition. They 

 arise, as has been shown, through coalescence of smaller vessels. So striking are 

 they on account of their enormous size and great number, as well as because they 

 are invariably present in areas of active callus destruction, that the conclusion is 

 forced upon us that they must play an important part in the breaking-down and 

 removal of the redundant osseous tissue. Certain it is that the rate of flow in these 

 vessels is very slow and that, judging from the thinness and insecurity of their 

 walls, the pressure is very low. It is probable, too, that the C0 2 tension is high. 



It is significant that a very voluminous blood-current of slow speed and low 

 pressure, flowing through a thin-walled channel, enveloped thickly with phagocytes 

 of high-grade efficiency, is so constant a feature of callus resorption, as it is also 

 of the resorption of young developing bone. It suggests that this is part, at least, 

 of the mechanism of bone erosion. 



DECLINE. 



During the third phase, after the twentieth day, osseous resorption gradually 

 ebbs and the destructive and constructive activities of the callus slowly subside. 

 Reinforcement of the permanent trabeculae is the principal industry of the cells, 

 but, especially in the earlier days, evidence of the work of the wrecking-gang is 

 still seen in the clearing away of the few remaining provisional spicules of bone and 

 the trimming of the rough corners. The result is a firm osseous structure formed 

 of material almost indistinguishable from the bone of the original shaft, containing 

 relatively few (but very large) spaces filled with vascular and marrow tissue. This bony 

 formation occupies the fracture-site and thoroughly immobilizes the shaft of the bone. 



In keeping with this falling away of destructive processes the macrophages 

 gradually decline in size, concentration, and staining activity, and revert to the 

 character of the ordinary bone-marrow reticulum cells. This does not take place 

 uniformly in all stages, however, for in the specimens of the sixtieth day there was 

 still evidence of bone erosion as well as of bone building, whereas in those of the 

 fifty-first and fifty-ninth days these processes had apparently completely ceased. 

 Involution and degenerate forms, like those of the macrophages of the soft parts, 

 were not found in the callus-spaces. 



Cartilage was found almost constantly in the callus of the long bones and seems 

 to be associated with movement of the parts during repair, since they were not 

 splinted. It was not found in any of the skulls, where movement was absent. In 



