The normal patterns of arterial and capillary blood supply of canine diaphyseal cortex and the effect of fracture on these patterns have been described by means of microangiograms. Primary arterioles of medullary origin arborize within the inner two-thirds to three-quarters of the cortex and then anastomose with the scattered blood vessels of the periosteal circulation derived from muscular arterial branches. The medullary circulation thus supplies the greater proportion of cortical bone, but function of the medulloperiosteal vascular anastomoses appears to be essential for blood to flow through the cortex in the vessels derived from the medulla. Interruption of the local periosteal circulation by tight application of a bone plate suppresses even the medullary blood supply of the cortex beneath the plate, whereas loosening of the plate allows ingrowth of new periosteal vessels and leads to filling of cortical vessels again at all depths. This observation supports the contention that blood flow through the cortex is normally centrifugal. However, the periosteal circulation appears to be able to maintain the vascularity of the outer third of the cortex by itself when medullary arterioles are obliterated by an intramedullary rod. Evidently, therefore, the usual direction of flow through the cortex can be reversed, as was also shown by Brookes, and this permits the periosteal circulation to revascularize the cortex when its medullary blood supply has been chronically inhibited.

During the healing of undisplaced closed fractures, the medullary circulation remains dominant throughout. With displaced fractures, however, when the main medullary vessels are disrupted, an enhanced periosteal circulation, derived from torn muscles in the vicinity, becomes at first the primary source of blood for the area of healing. The early external callus receives its blood supply from the periosteal circulation through vessels which are oriented perpendicularly to the cortical surface. As healing progresses, the medullary circulation is re-established across the fracture or osteotomy site. This circulation continues to hypertrophy, while the temporarily increased periosteal circulation recedes, and soon the medullary system provides all the blood for the areas of most effective osseous bridging. Eventually, the medullary circulation comes to be the main supplies of blood even to external callus through arterial branches which traverse the porous cortex, thereby carrying to the extreme the basic centrifugal trend of the blood supply of tubular bone. Finally, as late bone remodeling advances, the normal vascular pattern is reestablished with medullary vessels meeting periosteal vessels in vital anastomoses within the outer cortical layers.

It is idle to argue whether the medullary or periosteal circulation is the more important in the blood supply of tubular bone. While medullary vessels are distributed to the greater portion of the normal cortex, they appear to require their periosteal anastomoses to function properly. The anatomic ascendency of the medullary circulation is even more obvious during the course of bone repair; but here also periosteal connections are probably essential, since, when medullary arterioles are supplying large masses of external callus, the flow of blood surely must still be outward. When the normal outward flow of blood through the cortex is blocked, the periosteal arterioles have more ability than medullary arterioles to function and proliferate; the direction of blood flow therefore reverses and becomes inward. The final conclusion, therefore, is that the medullary supply of the diaphyseal cortex, when available, dominates the anatomic vascular picture, but the periosteal supply has under adverse conditions greater staying power.