Intravascular injections of India ink in the femora of adult dogs
revealed the existence of two distinct vascular systems in the cortex. One
courses through the entire thickness of the cortex and is a regular,
longitudinal network of uniform capillaries which is in continuity with the
periosteal and endosteal networks. Once the haversian systems form a second
network appears, radiating from the bone marrow. That system, primarily
transverse, anastomoses in the haversian canals with the capillaries of the
first system. Therefore, the merging of the two networks occurs primarily
in the middle layer of the cortex. The direction of the arterial blood flow
in the mature animal is predominantly centrifugal, while the venous
drainage is centripetal. In the immature animal, the contribution of the
periosteal network is much greater. Because the two cortical systems are
profusely anastomosed with each other and with the periosteal and endosteal
circulatory networks, the blood can flow in either direction, depending on
physiological conditions.