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.